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System Characterization of Dispersive Readout in Superconducting Qubits: Designing quantum systems with the measurement speed and accuracy needed for
quantum error correction using superconducting qubits requires iterative design
and test informed by accurate models and characterization tools. We introduce a
single protocol, with few prerequisite calibrations, which measures the
dispersive shift, resonator linewidth, and drive power used in the dispersive
readout of superconducting qubits. We find that the resonator linewidth is
poorly controlled with a factor of 2 between the maximum and minimum measured
values, and is likely to require focused attention in future quantum error
correction experiments. We also introduce a protocol for measuring the readout
system efficiency using the same power levels as are used in typical qubit
readout, and without the need to measure the qubit coherence. We routinely run
these protocols on chips with tens of qubits, driven by automation software
with little human interaction. Using the extracted system parameters, we find
that a model based on those parameters predicts the readout signal to noise
ratio to within 10% over a device with 54 qubits.
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quant-ph
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Semispectral measures as convolutions and their moment operators: The moment operators of a semispectral measure having the structure of the
convolution of a positive measure and a semispectral measure are studied, with
paying attention to the natural domains of these unbounded operators. The
results are then applied to conveniently determine the moment operators of the
Cartesian margins of the phase space observables.
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quant-ph
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Quantum k-means algorithm based on Trusted server in Quantum Cloud
Computing: We propose a quantum k-means algorithm based on quantum cloud computing that
effectively solves the problem that the client can not afford to execute the
same quantum subroutine repeatedly in the face of large training samples. In
the quantum k-means algorithm, the core subroutine is the Quantum minimization
algorithm (GroverOptim), the client needs to repeat several Grover searches to
find the minimum value in each iteration to find a new clustering center, so we
use quantum homomorphic encryption scheme (QHE) to encrypt the data and upload
it to the cloud for computing. After calculation, the server returns the
calculation result to the client. The client uses the key to decrypt to get the
plaintext result. It reduces the computing pressure for the client to repeat
the same operation. In addition, when executing in the cloud, the key update of
T-gate in the server is inevitable and complex. Therefore, this paper also
proposes a T-gate update scheme based on trusted server in quantum ciphertext
environment. In this scheme, the server is divided into trusted server and
semi-trusted server. The semi-trusted server completes the calculation
operation, and when the T-gate is executed in the circuit, the trusted server
assists the semi-trusted server to calculate the T-gate, and then randomly
generates a key and uploads it to the semi-trusted server. The trusted server
assists the client to complete the key update operation, which once again
reduces the pressure on the client and improves the efficiency of the quantum
homomorphic encryption scheme. And on the basis of this scheme, the experiment
is given by using IBM Qiskit to give the subroutine of quantum k-means. The
experimental results show that the scheme can realize the corresponding
computing function on the premise of ensuring security.
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quant-ph
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On a Density-of-States Approach to Bohmian Mechanics: We propose the idea that in Bohmian mechanics the wavefunction is related to
a density of states and explore some of its consequences. Specifically, it
allows a maximum-entropy interpretation of quantum probabilities, which creates
a stronger link between it and statistical mechanics. The proposed approach
also allows a range of extensions of the guidance condition in Bohmian
mechanics.
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quant-ph
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Quadratic Models for Engineered Control of Open Quantum Systems: We introduce a framework to model the evolution of a class of open quantum
systems whose environments periodically undergo an instantaneous non-unitary
evolution stage. For the special case of quadratic models, we show how this
approach can generalise the formalism of repeated interactions to allow for the
preservation of system-environment correlations. Furthermore, its continuous
zero-period limit provides a natural description of the evolution of small
systems coupled to large environments in negligibly perturbed steady states. We
explore the advantages and limitations of this approach in illustrative
applications to thermalisation in a simple hopping ring and to the problem of
initialising a qubit chain via environmental engineering.
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quant-ph
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On radiation reaction and the [x,p ] commutator for an accelerating
charge: We formally state the connection between the relativistic part of the
radiation reaction and the Poynting Robertson force term, $-Rv/c^2$, where $R$
is power radiated. Then we address the question, does $[x,p]=i \hb$ for an
accelerating charge ? The full radiation reaction term is used, which includes
the relativistic term (von Laue vector.). We show that the full relativistic
radiation reaction term must be taken into account if a commutation relation
between $x$ and $p$ is to hold for an electron under uniform acceleration,
consistent with the expectation values of $x^2$ and $p^2$.
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quant-ph
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Teleportation between distant qudits via scattering of mobile qubits: We consider a one-dimensional (1D) structure where non-interacting spin-$s$
scattering centers, such as quantum impurities or multi-level atoms, are
embedded at given positions. We show that the injection into the structure of
unpolarized flying qubits, such as electrons or photons, along with {path}
detection suffice to accomplish spin-state teleportation between two centers
via a third ancillary one. {No action over the internal quantum state of both
the spin-$s$ particles and the flying qubits is required. The protocol enables
the transfer of quantum information between well-seperated static entities in
nanostructures by exploiting a very low-control mechanism, namely scattering.
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quant-ph
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How to make qubits speak: This is a story about making quantum computers speak, and doing so in a
quantum-native, compositional and meaning-aware manner. Recently we did
question-answering with an actual quantum computer. We explain what we did,
stress that this was all done in terms of pictures, and provide many pointers
to the related literature. In fact, besides natural language, many other things
can be implemented in a quantum-native, compositional and meaning-aware manner,
and we provide the reader with some indications of that broader pictorial
landscape, including our account on the notion of compositionality. We also
provide some guidance for the actual execution, so that the reader can give it
a go as well.
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quant-ph
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XpookyNet: Advancement in Quantum System Analysis through Convolutional
Neural Networks for Detection of Entanglement: The application of machine learning models in quantum information theory has
surged in recent years, driven by the recognition of entanglement and quantum
states, which are the essence of this field. However, most of these studies
rely on existing prefabricated models, leading to inadequate accuracy. This
work aims to bridge this gap by introducing a custom deep convolutional neural
network (CNN) model explicitly tailored to quantum systems. Our proposed CNN
model, the so-called XpookyNet, effectively overcomes the challenge of handling
complex numbers data inherent to quantum systems and achieves an accuracy of
98.5%. Developing this custom model enhances our ability to analyze and
understand quantum states. However, first and foremost, quantum states should
be classified more precisely to examine fully and partially entangled states,
which is one of the cases we are currently studying. As machine learning and
quantum information theory are integrated into quantum systems analysis,
various perspectives, and approaches emerge, paving the way for innovative
insights and breakthroughs in this field.
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quant-ph
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Entanglement resource theory of quantum channel: Quantum channels can represent dynamic resources, which are indispensable
elements in many physical scenarios. To describe certain facets of
nonclassicality of the channels, it is necessary to quantify their properties.
In the framework of resource theory of quantum channel, we show two general
ways of constructing entanglement measure of channels. We also present several
entanglement measures of channels based on the Choi relative entropy of
channels, concurrence and $k$-ME concurrence and give some specific examples.
These entanglement measures of channels can deepen the cognizing about channel
and advance the research on the transformation between coherent resources and
entangled resources. In addition, we prove that these measures satisfy the
properties including nonnegativity, monotonicity, convexity and so on.
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quant-ph
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Error Sensitivity to Environmental Noise in Quantum Circuits for
Chemical State Preparation: Calculating molecular energies is likely to be one of the first useful
applications to achieve quantum supremacy, performing faster on a quantum than
a classical computer. However, if future quantum devices are to produce
accurate calculations, errors due to environmental noise and algorithmic
approximations need to be characterized and reduced. In this study, we use the
high performance qHiPSTER software to investigate the effects of environmental
noise on the preparation of quantum chemistry states. We simulated eighteen
16-qubit quantum circuits under environmental noise, each corresponding to a
unitary coupled cluster state preparation of a different molecule or molecular
configuration. Additionally, we analyze the nature of simple gate errors in
noise-free circuits of up to 40 qubits. We find that the Jordan-Wigner (JW)
encoding produces consistently smaller errors under a noisy environment as
compared to the Bravyi-Kitaev (BK) encoding. For the JW encoding,
pure-dephasing noise is shown to produce substantially smaller errors than pure
relaxation noise of the same magnitude. We report error trends in both
molecular energy and electron particle number within a unitary coupled cluster
state preparation scheme, against changes in nuclear charge, bond length,
number of electrons, noise types, and noise magnitude. These trends may prove
to be useful in making algorithmic and hardware-related choices for quantum
simulation of molecular energies.
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quant-ph
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Landauer vs. Nernst: What is the True Cost of Cooling a Quantum System?: Thermodynamics connects our knowledge of the world to our capability to
manipulate and thus to control it. This crucial role of control is exemplified
by the third law of thermodynamics, Nernst's unattainability principle, which
states that infinite resources are required to cool a system to absolute zero
temperature. But what are these resources and how should they be utilized? And
how does this relate to Landauer's principle that famously connects information
and thermodynamics? We answer these questions by providing a framework for
identifying the resources that enable the creation of pure quantum states. We
show that perfect cooling is possible with Landauer energy cost given infinite
time or control complexity. However, such optimal protocols require complex
unitaries generated by an external work source. Restricting to unitaries that
can be run solely via a heat engine, we derive a novel Carnot-Landauer limit,
along with protocols for its saturation. This generalizes Landauer's principle
to a fully thermodynamic setting, leading to a unification with the third law
and emphasizes the importance of control in quantum thermodynamics.
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quant-ph
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Approximate simulation of quantum channels: In Ref. [1], we proved a duality between two optimizations problems. The
primary one is, given two quantum channels M and N, to find a quantum channel R
such that RN is optimally close to M as measured by the worst-case entanglement
fidelity. The dual problem involves the information obtained by the environment
through the so-called complementary channels M* and N*, and consists in finding
a quantum channel R' such that R'M* is optimally close to N*. It turns out to
be easier to find an approximate solution to the dual problem in certain
important situations, notably when M is the identity channel---the problem of
quantum error correction---yielding a good near-optimal worst-case entanglement
fidelity as well as the corresponding near-optimal correcting channel. Here we
provide more detailed proofs of these results. In addition, we generalize the
main theorem to the case where there are certain constraints on the
implementation of R, namely on the number of Kraus operators. We also offer a
simple algebraic form for the near-optimal correction channel in the case M=id.
For approximate error correction, we show that any epsilon-correctable channel
is, up to appending an ancilla, epsilon-close to an exactly correctable one. We
also demonstrate an application of our theorem to the problem of minimax state
discrimination.
|
quant-ph
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Quantum $\varphi$-synchronization in coupled optomechanical system with
periodic modulation: Based on the concepts of quantum synchronization and quantum phase
synchronization proposed by A. Mari \textit{et al.} in Phys. Rev. Lett. 111,
103605 (2013), we introduce and characterize the measure of a more generalized
quantum synchronization called quantum $\varphi$-synchronization under which
the pairs of variables have the same amplitude and possess the same $\varphi$
phase shift. Naturally, quantum synchronization and quantum
anti-synchronization become special cases of quantum $\varphi$-synchronization.
Their relations and differences are also discussed. To illustrate these
theories, we investigate the quantum $\varphi$-synchronization and quantum
phase synchronization phenomena of two coupled optomechanical systems with
periodic modulation and show that quantum $\varphi$-synchronization is more
general as a measure of synchronization. We also show the phenomenon of quantum
anti-synchronization when $\varphi=\pi$.
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quant-ph
|
Some Non-Perturbative and Non-Linear Effects in Laser-Atom Interaction: We show that if the laser is intense enough, it may always ionize an atom or
induce transitions between discrete energy levels of the atom, no matter what
is its frequency. It means in the quantum transition of an atom interacting
with an intense laser of circular frequency $\omega$, the energy difference
between the initial and the final states of the atom is not necessarily being
an integer multiple of the quantum energy $\hbar\omega$. The absorption spectra
become continuous. The Bohr condition is violated. The energy of photoelectrons
becomes light intensity dependent in the intense laser photoelectric effect.
The transition probabilities and cross sections of photo-excitations and
photo-ionizations are laser intensity dependent, showing that these processes
cannot be reduced to the results of interactions between the atom and separate
individual photons, they are rather the processes of the atom interacting with
the laser as a whole. The interaction of photons on atoms are not simply
additive. The effects are non-perturbative and non-linear. Some numerical
results for processes between hydrogen atom and intense circularly polarized
laser, illustrating the non-perturbative and non-linear character of the
atom-laser interaction, are given.
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quant-ph
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Removing correlations in signals transmitted over a quantum memory
channel: We consider a model of bosonic memory channel, which induces correlations
among the transmitted signals. The application of suitable unitary
transformations at encoding and decoding stages allows the complete removal of
correlations, mapping the memory channel into a memoryless one. However, such
transformations, being global over an arbitrary large number of bosonic modes,
are not realistically implementable. We then introduce a family of efficiently
realizable transformations which can be used to partially remove correlations
among errors, and we quantify the reduction of the gap with memoryless
channels.
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quant-ph
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Unconditionally secure quantum commitments with preprocessing: We demonstrate how to build computationally secure commitment schemes with
the aid of quantum auxiliary inputs without unproven complexity assumptions.
Furthermore, the quantum auxiliary input can be prepared either (1) efficiently
through a trusted setup similar to the classical common random string model, or
(2) strictly between the two involved parties in uniform exponential time.
Classically this remains impossible without first proving $\mathsf{P} \neq
\mathsf{NP}$.
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quant-ph
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BROTOCs and Quantum Information Scrambling at Finite Temperature: Out-of-time-ordered correlators (OTOCs) have been extensively studied in
recent years as a diagnostic of quantum information scrambling. In this paper,
we study quantum information-theoretic aspects of the regularized
finite-temperature OTOC. We introduce analytical results for the bipartite
regularized OTOC (BROTOC): the regularized OTOC averaged over random unitaries
supported over a bipartition. We show that the BROTOC has several interesting
properties, for example, it quantifies the purity of the associated thermofield
double state and the operator purity of the analytically continued
time-evolution operator. At infinite-temperature, it reduces to one minus the
operator entanglement of the time-evolution operator. In the zero-temperature
limit and for nondegenerate Hamiltonians, the BROTOC probes the groundstate
entanglement. By computing long-time averages, we show that the equilibration
value of the BROTOC is intimately related to eigenstate entanglement. Finally,
we numerically study the equilibration value of the BROTOC for various
physically relevant Hamiltonian models and comment on its ability to
distinguish integrable and chaotic dynamics.
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quant-ph
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The Chiral Qubit: quantum computing with chiral anomaly: The quantum chiral anomaly enables a nearly dissipationless current in the
presence of chirality imbalance and magnetic field -- this is the Chiral
Magnetic Effect (CME), observed recently in Dirac and Weyl semimetals. Here we
propose to utilize the CME for the design of qubits potentially capable of
operating at THz frequency, room temperature, and the coherence time to gate
time ratio of about $10^4$. The proposed "Chiral Qubit" is a micron-scale ring
made of a Weyl or Dirac semimetal, with the $|0\rangle$ and $|1\rangle$ quantum
states corresponding to the symmetric and antisymmetric superpositions of
quantum states describing chiral fermions circulating along the ring clockwise
and counter-clockwise. A fractional magnetic flux through the ring induces a
quantum superposition of the $|0\rangle$ and $|1\rangle$ quantum states. The
entanglement of qubits can be implemented through the near-field THz frequency
electromagnetic fields.
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quant-ph
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Symmetries of the free Schrodinger Equation: An algorithm is proposed for research into the symmetrical properties of
theoretical and mathematical physics equations. The application of this
algorithm to the free Schrodinger equation permited us to establish that in
addition to the known Galilei symmetry, the free Schrodinger equation possesses
also the relativistic symmetry in some generalized sense. This property of the
free Schrodinger equation permits the equation to be extended into the
relativistic area of movements of a particle being studied.
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quant-ph
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Time and Quantum Clocks: a review of recent developments: In this review we present the problem of time in quantum physics, including a
short history of the problem and the known objections about considering time a
quantum observable. The need to deal with time as an observable is elaborated
through some unresolved problems. The lack of a consistent theory of time is
currently hindering the formulation of a full-fledged theory of quantum
gravity. It is argued that the proposal set forth by several authors of
considering an intrinsic measurement of quantum time, besides having the
conventional external time, is compelling. Recently several suggestions have
been put forward to revive the proposal of Page and Wootters (1983),
elaborating and resolving some of the main ambiguities of the original proposal
and opening new scope for understanding its content. The approach followed in
these new contributions exposes the need to go beyond the limitations enforced
by the conventional approach of quantum physics. The attitude of covariant loop
quantum gravity, in which it is called to completely ignore time, is also
discussed. This review could be a step forward in an endeavour to reform our
outlook of the unification of the theory of relativity and quantum physics by
furnishing the conceptual ground needed for this goal. Intentionally, some
technical details are avoided since we aim to present the approaches to resolve
the problem in a simple way with the clearest possible outlook. These can be
looked up in the original references provided.
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quant-ph
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Nonclasscial interference between independent intrinsically pure single
photons at telecom wavelength: We demonstrate a Hong-Ou-Mandel interference between two independent,
intrinsically pure, heralded single photons from spontaneous parametric down
conversion (SPDC) at telecom wavelength. A visibility of $85.5\pm8.3%$ was
achieved without using any bandpass filter. Thanks to the
group-velocity-matched SPDC and superconducting nanowire single photon
detectors (SNSPDs), the 4-fold coincidence counts are one order higher than
that in the previous experiments. The combination of bright single photon
sources and SNSPDs is a crucial step for future practical quantum
info-communication systems at telecom wavelength.
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quant-ph
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Enhanced Estimation of a Noisy Quantum Channel Using Entanglement: We discuss the estimation of channel parameters for a noisy quantum channel -
the so-called Pauli channel - using finite resources. It turns out that prior
entanglement considerably enhances the fidelity of the estimation when we
compare it to an estimation scheme based on separable quantum states.
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quant-ph
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Electron beams of cylindrically symmetric spin polarization: Cylindrically symmetric electron beams in spin polarization are reported for
the first time. They are shown to be the eigen states of total angular momentum
in the $z$ direction. But they are neither the eigen states of spin nor the
eigen states of orbital angular momentum in that direction.
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quant-ph
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Optimizing performance of quantum operations with non-Markovian
decoherence: the tortoise or the hare?: The interaction between a quantum system and its environment limits our
ability to control it and perform quantum operations on it. We present an
efficient method to find optimal controls for quantum systems coupled to
non-Markovian environments, by using the process tensor to compute the gradient
of an objective function. We consider state transfer for a driven two-level
system coupled to a bosonic environment, and characterize performance in terms
of speed and fidelity. We thus determine the best achievable fidelity as a
function of process duration. We show there is a trade-off between speed and
fidelity, and that slower processes can have higher fidelity by exploiting
non-Markovian effects.
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quant-ph
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Electric Current Induced by Microwave Stark Effect of Electrons on
Liquid Helium: We propose a frequency-mixed effect of Terahertz (THz) and Gigahertz (GHz)
electromagnetic waves in the cryogenic system of electrons floating on liquid
helium surface. The THz wave is near-resonant with the transition frequency
between the lowest two levels of surface state electrons. The GHz wave does not
excite the transitions but generates a GHz-varying Stark effect with the
symmetry-breaking eigenstates of electrons on liquid helium. We show an
effective coupling between the inputting THz and GHz waves, which appears at
the critical point that the detuning between electrons and THz wave is equal to
the frequency of GHz wave. By this coupling, the THz and GHz waves
cooperatively excite electrons and generate the low-frequency ac currents along
the perpendicular direction of liquid helium surface to be experimentally
detected by the image-charge approach [Phys. Rev. Lett. 123, 086801 (2019)].
This offers an alternative approach for THz detections.
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quant-ph
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Violation of Leggett-type inequalities in the spin-orbit degrees of
freedom of a single photon: We report the experimental violation of Leggett-type inequalities for a
hybrid entangled state of spin and orbital angular momentum of a single photon.
These inequalities give a physical criterion to verify the possible validity of
a class of hidden-variable theories, originally named "crypto non-local", that
are not excluded by the violation of Bell-type inequalities. In our case, the
tested theories assume the existence of hidden variables associated with
independent degrees of freedom of the same particle, while admitting the
possibility of an influence between the two measurements, i.e. the so-called
contextuality of observables. We observe a violation the Leggett inequalities
for a range of experimental inputs, with a maximum violation of seven standard
deviations, thus ruling out this class of hidden variable models with a high
confidence.
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quant-ph
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Quantum entropy and polarization measurements of the two-photon system: We consider the bipartite state of a two-photon polarization system and
obtain the exact analytical expression for the von Neumann entropy in the
particular case of a 5-parameter polarization density matrix. We investigate
and graphically illustrate the dependence of the entropy on these five
parameters, in particular, the existence of exotic, transition from exotic to
non-exotic, and non-exotic states, where the quantum conditional entropy is
negative, both positive and negative, and positive, respectively. We study the
"cooling" or "heating" effect that follows from the reduced density of photon 2
when a measurement is performed on photon 1.
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quant-ph
|
Comment on "Generating a perfect quantum optical vortex": In a recent article, Banerji et al. introduced a novel quantum state of
light, coined as the perfect quantum optical vortex state [Phys. Rev. A 94,
053838 (2016)] due to its mathematical similarity with the classical perfect
vortex beam. This state is obtained by means of the Fourier transform of a
Bessel-Gaussian vortex state, and the authors claim that this can be
accomplished by means of a simple lens. Here, we will show that this statement
is wrong since a lens cannot modify the quantum noise distribution related to
the input optical quantum state and this has to be exchanged by an "effective
lens".
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quant-ph
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Comment on "On the temperature dependence of the Casimir effect": Recently, Brevik et al. [Phys. Rev. E 71, 056101 (2005)] adduced arguments
against the traditional approach to the thermal Casimir force between real
metals and in favor of one of the alternative approaches. The latter assumes
zero contribution from the transverse electric mode at zero frequency in
qualitative disagreement with unity as given by the thermal quantum field
theory for ideal metals. Those authors claim that their approach is consistent
with experiments as well as with thermodynamics. We demonstrate that these
conclusions are incorrect. We show specifically that their results are
contradicted by four recent experiments and also violate the third law of
thermodynamics (the Nernst heat theorem).
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quant-ph
|
Diverse tunable dynamics of two quantum random walkers: Quantum walk research has mainly focused on evolutions due to repeated
applications of time-independent unitary coin operators. However, the idea of
controlling the single particle evolution using time-dependent unitary coins
has still been a subject of multiple studies as it not only hosts interesting
possibilities for quantum information processing but also opens a much richer
array of phenomena including static and dynamic localizations. So far, such
studies have been performed only for single quantum walkers. In case of
multi-walker systems, time-dependent coins may generate measurable phenomena
not described by the single-particle model, due to entanglement and interaction
among the walkers. In this context, we present here a thorough numerical study
of an one dimensional system of two quantum walkers exhibiting rich collective
dynamics controlled by simple time-dependent unitary coins proposed in [Phys.
Rev. A \textbf{80}, 042332(2009)] and [Phys. Rev. A \textbf{73},062304(2006)].
We study how the interplay of coin time-dependence, simple interaction schemes,
entanglement and the relative phase between the coin states of the particles
influences the evolution of the quantum walk. The results show that the system
offers a rich variety of collective dynamical behavior while being controlled
by time dependent coins. In particular, we find and characterize fascinating
two-body localization phenomena with tunable quasiperiodic dynamics of
correlations and entanglements which are quantities of quantum origin.
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quant-ph
|
The Schwarz-Hora effect: present-day situation: The electron-diffraction pattern at a nonfluorescent target was observed by
Schwarz under attempts to modulate an electron beam by laser light. The pattern
was of the same color as the laser light. The analysis of the literature shows
there are the unresolved up to now significant contradictions between the
theory and the Schwarz experiments. To resolve these contradictions, the
interpretation of the Schwarz-Hora effect is considered, which is a development
of the idea formulated by Schwarz and Hora. It is supposed that the interaction
of electrons with the laser field inside a thin dielectric film is accompanied
not only by the processes of absorption and stimulated emission of photons but
also by formation of some metastable electron states in which the captured
photons can be transferred with a following emission at the target.
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quant-ph
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Quantum correlations and fluctuations in the pulsed light produced by a
synchronously pumped optical parametric oscillator below its oscillation
threshold: We present a simple quantum theory for the pulsed light generated by a
synchronously pumped optical parametric oscillator (SPOPO) in the degenerate
case where the signal and idler trains of pulses coincide, below threshold and
neglecting all dispersion effects. Our main goal is to precise in the obtained
quantum effects, which ones are identical to the c.w. case and which ones are
specific to the SPOPO. We demonstrate in particular that the temporal
correlations have interesting peculiarities: the quantum fluctuations at
different times within the same pulse turn out to be totally not correlated,
whereas they are correlated between nearby pulses at times that are placed in
the same position relative to the centre of the pulses. The number of
significantly correlated pulses is of the order of cavity finesse. We show also
that there is perfect squeezing at noise frequencies multiple of the pulse
repetition frequency when one approaches the threshold from below on the signal
field quadrature measured by a balanced homodyne detection with a local
oscillator of very short duration compared to the SPOPO pulse length.
|
quant-ph
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Characterization and Reduction of Capacitive Loss Induced by Sub-Micron
Josephson Junction Fabrication in Superconducting Qubits: Josephson junctions form the essential non-linearity for almost all
superconducting qubits. The junction is formed when two superconducting
electrodes come within $\sim$1 nm of each other. Although the capacitance of
these electrodes is a small fraction of the total qubit capacitance, the nearby
electric fields are more concentrated in dielectric surfaces and can contribute
substantially to the total dissipation. We have developed a technique to
experimentally investigate the effect of these electrodes on the quality of
superconducting devices. We use $\lambda$/4 coplanar waveguide resonators to
emulate lumped qubit capacitors. We add a variable number of these electrodes
to the capacitive end of these resonators and measure how the additional loss
scales with number of electrodes. We then reduce this loss with fabrication
techniques that limit the amount of lossy dielectrics. We then apply these
techniques to the fabrication of Xmon qubits on a silicon substrate to improve
their energy relaxation times by a factor of 5.
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quant-ph
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Collective radiative dynamics of an ensemble of cold atoms coupled to an
optical waveguide: We experimentally and theoretically investigate collective radiative effects
in an ensemble of cold atoms coupled to a single-mode optical nanofiber. Our
analysis unveils the microscopic dynamics of the system, showing that
collective interactions between the atoms and a single guided photon gradually
build-up along the atomic array in the direction of propagation of light. These
results are supported by time-resolved measurements of the light transmitted
and reflected by the ensemble after excitation via nanofiber-guided laser
pulses, whose rise and fall times are shorter than the atomic lifetime.
Superradiant decays more than one order of magnitude faster than the
single-atom free-space decay rate are observed for emission in the
forward-propagating guided mode, while at the same time no speed-up of the
decay rate are measured in the backward direction. In addition,
position-resolved measurements of the light that is transmitted past the atoms
are performed by inserting the nanofiber-coupled atomic array in a 45-m long
fiber ring-resonator, which allow us to experimentally reveal the progressive
growth of the collective response of the atomic ensemble. Our results highlight
the unique opportunities offered by nanophotonic cold atom systems for the
experimental investigation of collective light-matter interaction.
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quant-ph
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Quantum Metropolis Solver: A Quantum Walks Approach to Optimization
Problems: The efficient resolution of optimization problems is one of the key issues in
today's industry. This task relies mainly on classical algorithms that present
scalability problems and processing limitations. Quantum computing has emerged
to challenge these types of problems. In this paper, we focus on the
Metropolis-Hastings quantum algorithm that is based on quantum walks. We use
this algorithm to build a quantum software tool called Quantum Metropolis
Solver (QMS). We validate QMS with the N-Queen problem to show a potential
quantum advantage in an example that can be easily extrapolated to an
Artificial Intelligence domain. We carry out different simulations to validate
the performance of QMS and its configuration.
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quant-ph
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Generation of a coherent superposition state on demand: We propose an experimentally feasible scheme to generate a superposition of
travelling field coherent states using extremely small Kerr effect and an
ancilla which could be a single photon or two entangled twin photons. The
scheme contains ingredients which are all within the current state of the art
and is robust against the main sources of errors which can be identified in our
setups.
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quant-ph
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The mixed Schur transform: efficient quantum circuit and applications: The Schur transform, which block-diagonalizes the tensor representation
$U^{\otimes n}$ of the unitary group $\mathbf{U}_d$ on $n$ qudits, is an
important primitive in quantum information and theoretical physics. We give a
generalization of its quantum circuit implementation due to Bacon, Chuang, and
Harrow (SODA 2007) to the case of mixed tensor $U^{\otimes n} \otimes
\bar{U}^{\otimes m}$, where $\bar{U}$ is the dual representation. This
representation is the symmetry of unitary-equivariant channels, which find
various applications in quantum majority vote, multiport-based teleportation,
asymmetric state cloning, black-box unitary transformations, etc. The "mixed"
Schur transform contains several natural extensions of the representation
theory used in the Schur transform, in which the main ingredient is a duality
between the mixed tensor representations and the walled Brauer algebra. Another
element is an efficient implementation of a "dual" Clebsch-Gordan transform for
$\bar{U}$. The overall circuit has complexity $\widetilde{O} ((n+m)d^4)$.
Finally, we show how the mixed Schur transform enables efficient implementation
of unitary-equivariant channels in various settings and discuss other potential
applications, including an extension of permutational quantum computing that
includes partial transposes.
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quant-ph
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Min-entropy and quantum key distribution: non-zero key rates for "small"
numbers of signals: We calculate an achievable secret key rate for quantum key distribution with
a finite number of signals, by evaluating the min-entropy explicitly. The
min-entropy can be expressed in terms of the guessing probability, which we
calculate for d-dimensional systems. We compare these key rates to previous
approaches using the von Neumann entropy and find non-zero key rates for a
smaller number of signals. Furthermore, we improve the secret key rates by
modifying the parameter estimation step. Both improvements taken together lead
to non-zero key rates for only 10^4-10^5 signals. An interesting conclusion can
also be drawn from the additivity of the min-entropy and its relation to the
guessing probability: for a set of symmetric tensor product states the optimal
minimum-error discrimination (MED) measurement is the optimal MED measurement
on each subsystem.
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quant-ph
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Generation of Long-Lived Isomeric States via Bremsstrahlung Irradiation: A method to generate long-lived isomeric states effectively for Mossbauer
applications is reported. We demonstrate that this method is better and easier
to provide highly sensitive Mossbauer effect of long-lived isomers (>1ms) such
as 103Rh. Excitation of (gamma,gamma) process by synchrotron radiation is
painful due mainly to their limited linewidth. Instead,(gamma,gamma') process
of bremsstrahlung excitation is applied to create these long-lived isomers.
Isomers of 45Sc, 107Ag, 109Ag, and 103Rh have been generated from this method.
Among them, 103Rh is the only one that we have obtained the gravitational
effect at room temperature.
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quant-ph
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Degradation of entanglement in moving frames: The distillability of bipartite entangled state as seen by moving observers
has been investigated. It is found that the same initial entanglement for a
state parameter $\alpha$ and its "normalized partner" $\sqrt{1-\alpha^2}$ will
be degraded as seen by moving observer. It is shown that in the ultra
relativistic limit, the state does not have distillable entanglement for any
$\alpha$.
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quant-ph
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Entanglement witnessing by arbitrarily many independent observers
recycling a local quantum shared state: We investigate the scenario where an observer, Alice, shares a two-qubit
state with an arbitrary number of observers, Bobs, via sequentially and
independently recycling the qubit in possession of the first Bob. It is known
that there exist entangled states which can be used to have an arbitrarily long
sequence of Bobs who can violate the Clauser-Horne-Shimony-Holt (CHSH) Bell
inequality with the single Alice. We show that there exist entangled states
that do not violate the Bell inequality and whose entanglement can be detected
by an arbitrary number of Bobs by suitably choosing the entanglement witness
operator and the unsharp measurement settings by the Bobs. This proves that the
set of states that can be used to witness entanglement sequentially is larger
than those that can witness sequential violation of local realism. There exist,
therefore, two-party quantum correlations that are Bell "classical", but whose
entanglement "nonclassicality" can be witnessed sequentially and independently
by an arbitrarily large number of observers at one end of the shared state with
the single observer at the other end.
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quant-ph
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Measuring effective temperatures of qubits using correlations: Initialization of a qubit in a pure state is a prerequisite for quantum
computer operation. Qubits are commonly initialized by cooling to their ground
states through passive thermalization or by using active reset protocols. To
accurately quantify the initialization one requires a tool to measure the
excited state population with sufficient accuracy given that the spurious
excited state population may not exceed a fraction of a percent. In this Letter
we propose a new technique of finding the excited state population of a qubit
using correlations between two sequential measurements. We experimentally
implement the proposed technique using a circuit QED platform and compare its
performance with previously developed techniques. Unlike other techniques, our
method does not require high-fidelity readout and does not involve the excited
levels of the system outside of the qubit subspace. We experimentally
demonstrated measurement of the spurious qubit population with accuracy of up
to $0.01\%$. This accuracy enabled us to perform "temperature spectroscopy" of
the qubit which helps to shed light on sources of the decoherence.
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quant-ph
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Nondeterministic quantum computation via ground state cooling and
ultrafast Grover algorithm: Over the last decades, there have been many proposals for quantum
computation. One of the promising candidates is adiabatic quantum computation
(AQC). The central idea of AQC is about finding the ground state of a system
with a problem Hamiltonian via particular adiabatic passages, starting from an
initialized ground state of a simple Hamiltonian. One disadvantage of AQC is
the significant growth of necessary runtime, in particular when there are
quantum phase transitions during the AQC passages. Here we propose a
nondeterministic ground state cooling quantum computation model based on
selective projection measurements on an ancilla coupled to the system with the
problem Hamiltonian previously cooled by conventional techniques. We illustrate
the model by Grover search problem and show that our nondeterministic model
requires a constant or at most logarithmic runtime and can also get rid of
possible difficulties in preparing the ground state of the simple Hamiltonian.
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quant-ph
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Quantum holography with undetected light: Holography exploits the interference of light fields to obtain a systematic
reconstruction of the light fields wavefronts. Classical holography techniques
have been very successful in diverse areas such as microscopy, manufacturing
technology, and basic science. Extending holographic methods to the level of
single photons has been proven challenging, since applying classical holography
techniques to this regime pose technical problems. Recently the retrieval of
the spatial structure of a single photon, using another photon under
experimental control with a well-characterized spatial shape as reference, was
demonstrated using the intrinsically non-classical Hong-Ou-Mandel interference
on a beam splitter. Here we present a method for recording a hologram of single
photons without detecting the photons themselves, and importantly, with no need
to use a well-characterized companion reference photon. Our approach is based
on quantum interference between two-photon probability amplitudes in a
nonlinear interferometer. As in classical holography, the hologram of a single
photon allows retrieving the complete information about the "shape" of the
photon (amplitude and phase) despite the fact that the photon is never
detected.
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quant-ph
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Analysis of assumptions of recent tests of local realism: Local realism in recent experiments is excluded on condition of freedom or
randomness of choice combined with no signaling between observers by
implementations of simple quantum models. Both no-signaling and the underlying
quantum model can be directly checked by analysis of experimental data. For
particular tests performed on the data, it is shown that two of these
experiments give the probability of the data under no-signaling (or choice
independence in one of them) hypothesis at the level of 5%, accounting for the
look-elsewhere-effect, moderately suggesting that no-signaling is violated with
95% confidence. On the other hand the data from the two other experiments
violate the assumption of the simple quantum model. Further experiments are
necessary to clarify these issues and freedom and randomness of choice.
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quant-ph
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Classical Complexity of Unitary Transformations: We discuss a classical complexity of finite-dimensional unitary
transformations, which can been seen as a computable approximation of classical
descriptional complexity of a unitary transformation acting on a set of qubits.
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quant-ph
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Rotational Correction on the Morse Potential Through the Pekeris
Approximation and Nikiforov-Uvarov Method: The Nikiforov-Uvarov method is employed to calculate the the Schrodinger
equation with a rotation Morse potential. The bound state energy eigenvalues
and the corresponding eigenfunction are obtained. All of these calculation
present an effective and clear method under a Pekeris approximation to solve a
rotation Morse model. Meanwhile the results got here are in a good agreement
with ones before.
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quant-ph
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Quantum walks on regular graphs with realizations in a system of anyons: We build interacting Fock spaces from association schemes and set up quantum
walks on the resulting regular graphs (distance-regular and
distance-transitive). The construction is valid for growing graphs and the
interacting Fock space is well defined asymptotically for the growing graph. To
realize the quantum walks defined on the graphs in terms of anyons we switch to
the dual view of the association schemes and identify the corresponding modular
tensor categories from the Bose-Mesner algebra. Informally, the fusion ring
induced by the association scheme and a topological twist can be the basis for
developing a modular tensor category and thus a system of anyons. Finally, we
demonstrate the framework in the case of Grover quantum walk on
distance-regular graph in terms of anyon systems for the graphs considered. In
the dual perspective interacting Fock spaces gather a new meaning in terms of
any
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quant-ph
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Sparse-Hamiltonian approach to the time evolution of molecules on
quantum computers: Quantum chemistry has been viewed as one of the potential early applications
of quantum computing. Two techniques have been proposed for electronic
structure calculations: (i) the variational quantum eigensolver and (ii) the
phase-estimation algorithm. In both cases, the complexity of the problem
increases for basis sets where either the Hamiltonian is not sparse, or it is
sparse, but many orbitals are required to accurately describe the molecule of
interest. In this work, we explore the possibility of mapping the molecular
problem onto a sparse Hubbard-like Hamiltonian, which allows a
Green's-function-based approach to electronic structure via a hybrid
quantum-classical algorithm. We illustrate the time-evolution aspect of this
methodology with a simple four-site hydrogen ring.
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quant-ph
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On the irreversibility of entanglement distillation: We investigate the irreversibility of entanglement distillation for a
symmetric d-1 parameter family of mixed bipartite quantum states acting on
Hilbert spaces of arbitrary dimension d x d. We prove that in this family the
entanglement cost is generically strictly larger than the distillable
entanglement, such that the set of states for which the distillation process is
asymptotically reversible is of measure zero. This remains true even if the
distillation process is catalytically assisted by pure state entanglement and
every operation is allowed, which preserves the positivity of the partial
transpose. It is shown, that reversibility occurs only in cases where the state
is quasi-pure in the sense that all its pure state entanglement can be revealed
by a simple operation on a single copy. The reversible cases are shown to be
completely characterized by minimal uncertainty vectors for entropic
uncertainty relations.
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quant-ph
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Quantum Advantages in Hypercube Game: We introduce a novel generalization of the Clauser-Horne-Shimony-Holt (CHSH)
game to a multiplayer setting, i.e., Hypercube game, where all $m$ players are
required to assign values to vertices on corresponding facets of an
$m$-dimensional hypercube. The players win if and only if their answers satisfy
both parity and consistency conditions. We completely characterize the maximum
winning probabilities (game value) under classical, quantum and no-signalling
strategies, respectively. In contrast to the original CHSH game designed to
demonstrate the superiority of quantumness, we find that the quantum advantages
in the Hypercube game significantly decrease as the number of players increase.
Notably, the quantum value decays exponentially fast to the classical value as
$m$ increases, while the no-signalling value always remains to be one.
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quant-ph
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Multipartite quantum correlations in a two-mode Dicke model: We analyze multipartite correlations in a generalized Dicke model involving
two optical modes interacting with an ensemble of two-level atoms. In
particular, we examine correlations beyond the standard bipartite entanglement
and derive exact results in the thermodynamic limit. The model presents two
superradiant phases involving the spontaneous breaking of either a
$\mathbb{Z}_2$ or $\mathrm{U}(1)$ symmetry. The latter is characterized by the
emergence of a Goldstone excitation, found to significantly affect the
correlation profiles. Focusing on the correlations between macroscopic
subsystems, we analyze both the mutual information as well as the entanglement
of formation for all possible bipartitions among the optical and matter degrees
of freedom. It is found that while each mode entangles with the atoms, the
bipartite entanglement between the modes is zero, and they share only classical
correlations and quantum discord. We also study the monogamy of multipartite
entanglement and show that there exists genuine tripartite entanglement, i.e.
quantum correlations that the atoms share with the two modes but that are not
shared with them individually, only in the vicinity of the critical lines. Our
results elucidate the intricate correlation structures underlying superradiant
phase transitions in multimode systems.
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quant-ph
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Demonstrating a superconducting dual-rail cavity qubit with
erasure-detected logical measurements: A critical challenge in developing scalable error-corrected quantum systems
is the accumulation of errors while performing operations and measurements. One
promising approach is to design a system where errors can be detected and
converted into erasures. Such a system utilizing erasure qubits are known to
have relaxed requirements for quantum error correction. A recent proposal aims
to do this using a dual-rail encoding with superconducting cavities. However,
experimental characterization and demonstration of a dual-rail cavity qubit has
not yet been realized. In this work, we implement such a dual-rail cavity
qubit; we demonstrate a projective logical measurement with integrated erasure
detection and use it to measure dual-rail qubit idling errors. We measure
logical state preparation and measurement errors at the $0.01\%$-level and
detect over $99\%$ of cavity decay events as erasures. We use the precision of
this new measurement protocol to distinguish different types of errors in this
system, finding that while decay errors occur with probability $\sim 0.2\%$ per
microsecond, phase errors occur 6 times less frequently and bit flips occur at
least 140 times less frequently. These findings represent the first
confirmation of the expected error hierarchy necessary to concatenate dual-rail
erasure qubits into a highly efficient erasure code.
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quant-ph
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Conflict-free joint sampling for preference satisfaction through quantum
interference: Collective decision-making is vital for recent information and communications
technologies. In our previous research, we mathematically derived conflict-free
joint decision-making that optimally satisfies players' probabilistic
preference profiles. However, two problems exist regarding the optimal joint
decision-making method. First, as the number of choices increases, the
computational cost of calculating the optimal joint selection probability
matrix explodes. Second, to derive the optimal joint selection probability
matrix, all players must disclose their probabilistic preferences. Now, it is
noteworthy that explicit calculation of the joint probability distribution is
not necessarily needed; what is necessary for collective decisions is sampling.
This study examines several sampling methods that converge to heuristic joint
selection probability matrices that satisfy players' preferences. We show that
they can significantly reduce the above problems of computational cost and
confidentiality. We analyze the probability distribution each of the sampling
methods converges to, as well as the computational cost required and the
confidentiality secured. In particular, we introduce two conflict-free joint
sampling methods through quantum interference of photons. The first system
allows the players to hide their choices while satisfying the players'
preferences almost perfectly when they have the same preferences. The second
system, where the physical nature of light replaces the expensive computational
cost, also conceals their choices under the assumption that they have a trusted
third party. This paper has been published in Phys. Rev. Applied 18, 064018
(2022) (DOI: 10.1103/PhysRevApplied.18.064018).
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quant-ph
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Decoherence and purity of a driven solid-state qubit in Ohmic bath: In this paper we study the decoherence and purity of a driven solid-state
qubit in the Ohmic bath by using the method based on the master equation. At
first, instead of solving the master equation we investigate the coefficients
of the equation which describe the shift in frequency, diffusive, decoherence,
and so on. It is shown that one of the coefficients (we called it decoherence
coefficient) is crucial to the decoherence of the qubit in the model. Then we
investigate the evolution of the purity of the state in the model. From the
analysis of the purity we see that the decoherence time of the qubit decrease
with the increase of the amplitude of the driven fields and it is increase with
the increase of the frequency of the driven fields.
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quant-ph
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Lateral Casimir force between sinusoidally corrugated surfaces:
Asymmetric profiles, deviations from the proximity force approximation and
comparison with exact theory: The lateral Casimir force, which arises between aligned sinusoidally
corrugated surfaces of a sphere and a plate, was measured for the case of a
small corrugation period beyond the applicability region of the proximity force
approximation. The increased amplitudes of the corrugations on both the sphere
and the plate allowed observation of an asymmetry of the lateral Casimir force,
i.e., deviation of its profile from a perfect sine function. The dependences of
the lateral force on the phase shift between the corrugations on both test
bodies were measured at different separations in two sets of measurements with
different amplitudes of corrugations on the sphere. The maximum magnitude of
the lateral force as a function of separation was also measured in two
successive experiments. All measurement data were compared with the theoretical
approach using the proximity force approximation and with the exact theory
based on Rayleigh expansions with no fitting parameters. In both cases real
material properties of the test bodies and nonzero temperature were taken into
account. The data were found to be in a good agreement with the exact theory
but deviate significantly from the predictions of the proximity force
approximation approach. This provides the quantitative confirmation for the
observation of diffraction-type effects that are disregarded within the PFA
approach. Possible applications of the phenomenon of the lateral Casimir force
in nanotechnology for the operation of micromachines are discussed.
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quant-ph
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Semiclassical Propagator in the Generalized Coherent-State
Representation: A detailed derivation of the semiclassical propagator in the generalized
coherent-state representation is performed by applying the saddle-point method
to a path integral over the classical phase space. With the purpose of
providing greater accessibility and applicability to the developed formalism, a
brief review of the generalized concept of coherent states is presented, in
which three examples of coherent-state sets are examined, namely, the
canonical, spin, and SU(n) bosonic coherent states.
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quant-ph
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Graphical rule of transforming continuous-variable graph states by local
homodyne detection: Graphical rule, describing that any single-mode homodyne detection turns a
given continuous-variable (CV) graph state into a new one, is presented.
Employing two simple graphical rules: local complement operation and vertex
deletion (single quadrature-amplitude $\hat{x}$ measurement), the graphical
rule for any single-mode quadrature component measurement can be obtained. The
shape of CV weighted graph state may be designed and constructed easily from a
given larger graph state by applying this graphical rule.
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quant-ph
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A Factorization Law for Entanglement Decay: We present a simple and general factorization law for quantum systems shared
by two parties, which describes the time evolution of entanglement upon passage
of either component through an arbitrary noisy channel. The robustness of
entanglement-based quantum information processing protocols is thus easily and
fully characterized by a single quantity.
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quant-ph
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Quantum phase transitions of the Dirac oscillator in a minimal length
scenario: We obtain exact solutions of the (2+1) dimensional Dirac oscillator in a
homogeneous magnetic field within a minimal length ($\Delta x_0=\hbar
\sqrt{\beta}$), or generalised uncertainty principle (GUP) scenario. This
system in ordinary quantum mechanics has a single left-right chiral quantum
phase transition (QPT). We show that a non zero minimal length turns on a
infinite number of quantum phase transitions which accumulate towards the known
QPT when $\beta \to 0$. It is also shown that the presence of the minimal
length modifies the degeneracy of the states and that in this case there exist
a new class of states which do not survive in the ordinary quantum mechanics
limit $\beta \to 0$.
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quant-ph
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A qubit-ADAPT Implementation for H$_2$ Molecules using an Explicitly
Correlated Basis: With the recent advances in the development of devices capable of performing
quantum computations, a growing interest in finding near-term applications has
emerged in many areas of science. In the era of non-fault tolerant quantum
devices, algorithms that only require comparably short circuits accompanied by
high repetition rates are considered to be a promising approach for assisting
classical machines with finding solution on computationally hard problems. The
ADAPT approach previously introduced in Nat. Commun. 10, 3007 (2019) extends
the class of variational quantum eigensolver (VQE) algorithms with dynamically
growing ans\"atze in order to find approximations to ground and excited state
energies of molecules. In this work, the ADAPT algorithm has been combined with
a first-quantized formulation for the hydrogen molecule in the Born-Oppenheimer
approximation, employing the explicitly correlated basis functions introduced
in J. Chem. Phys. 43, 2429 (1965). By the virtue of their explicit electronic
correlation properties, it is shown in classically performed simulations that
relatively short circuits yield chemical accuracy ($< 1.6$ mHa) for ground and
excited state potential curves that can compete with second quantized
approaches such as Unitary Coupled Cluster.
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quant-ph
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Local certification of unitary operations and von Neumann measurements: In this work, we analyze the local certification of unitary quantum channels
and von Neumann measurements, which is a natural extension of quantum
hypothesis testing. A particular case of a quantum channel and von Neumann
measurement, operating on two systems corresponding to product states at the
input, is considered. The goal is to minimize the probability of the type II
error, given a specified maximum probability of the type I error, considering
assistance through entanglement. We introduce a new mathematical structure
q-product numerical range, which is a natural generalization of the q-numerical
range, used to obtain result, when dealing with one system. In our findings, we
employ the q-product numerical range as a pivotal tool, leveraging its
properties to derive our results and minimize the probability of type II error
under the constraint of type I error probability. We show a fundamental
dependency: for local certification, the tensor product structure inherently
manifests, necessitating the transition from q-numerical range to q-product
numerical range.
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quant-ph
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On the exact solutions of the Lipkin-Meshkov-Glick model: We present the many-particle Hamiltonian model of Lipkin, Meshkov and Glick
in the context of deformed polynomial algebras and show that its exact
solutions can be easily and naturally obtained within this formalism. The
Hamiltonian matrix of each $j$ multiplet can be split into two submatrices
associated to two distinct irreps of the deformed algebra. Their invariant
subspaces correspond to even and odd numbers of particle-hole excitations.
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quant-ph
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Single-photon nonreciprocal transport in one-dimensional
coupled-resonator waveguides: We study the transport of a single photon in two coupled one-dimensional
semi-infinite coupled-resonator waveguides (CRWs), in which both end sides are
coupled to a dissipative cavity. We demonstrate that a single photon can
transfer from one semi-infinite CRW to the other nonreciprocally. Based on such
nonreciprocity, we further construct a three-port single-photon circulator by a
T-shaped waveguide, in which three semi-infinite CRWs are pairwise mutually
coupled to each other. The single-photon nonreciprocal transport is induced by
the breaking of the time-reversal symmetry and the optimal conditions for these
phenomena are obtained analytically. The CRWs with broken time-reversal
symmetry will open up a kind of quantum devices with versatile applications in
quantum networks.
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quant-ph
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Generalized partial measurements: We introduce a type of measurements that generalize the so-called "partial
measurements" performed in recent years with phase qubits. While in the case of
partial measurements it has been demonstrated that one could undo the effect of
the measurement only for non-switching events, we show here that generalized
partial measurements can be reversed probabilistically for both switching and
non-switching events. We calculate the associated Fisher information and
discuss the estimation sensitivity for quantum tomography. Two ways of
implementing this type of measurements with superconducting qubits are
proposed.
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quant-ph
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Time reversal symmetry of generalized quantum measurements with past and
future boundary conditions: We expand the time reversal symmetry arguments of quantum mechanics,
originally proposed by Wigner in the context of unitary dynamics, to contain
situations including generalized measurements for monitored quantum systems. We
propose a scheme to derive the time reversed measurement operators by
considering the Schr\"{o}dinger picture dynamics of a qubit coupled to a
measuring device, and show that the time reversed measurement operators form a
Positive Operator Valued Measure (POVM) set. We present three particular
examples to illustrate time reversal of measurement operators: (1) the Gaussian
spin measurement, (2) a dichotomous POVM for spin, and (3) the measurement of
qubit fluorescence. We then propose a general rule to unravel any rank two
qubit measurement, and show that the backward dynamics obeys the retrodicted
equations of the forward dynamics starting from the time reversed final state.
We demonstrate the time reversal invariance of dynamical equations using the
example of qubit fluorescence. We also generalize the discussion of a
statistical arrow of time for continuous quantum measurements introduced by
Dressel et al. [Phys. Rev. Lett. 119, 220507 (2017)]: we show that the backward
probabilities can be computed from a process similar to retrodiction from the
time reversed final state, and extend the definition of an arrow of time to
ensembles prepared with pre- and post-selections, where we obtain a
non-vanishing arrow of time in general. We discuss sufficient conditions for
when time's arrow vanishes and show our method also captures the contributions
to time's arrow due to natural physical processes like relaxation of an atom to
its ground state. As a special case, we recover the time reversibility of the
weak value as its complex conjugate using our method, and discuss how our
conclusions differ from the time-symmetry argument of
Aharonov-Bergmann-Lebowitz (ABL) rule.
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quant-ph
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Spin tomography: We propose a tomographic reconstruction scheme for spin states. The
experimental setup, which is a modification of the Stern-Gerlach scheme, can be
easily performed with currently available technology. The method is generalized
to multi-particle states, analyzing the spin 1/2 case for indistinguishable
particles. Some Monte Carlo numerical simulations are given to illustrate the
technique.
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quant-ph
|
A geometric comparison of entanglement and quantum nonlocality in
discrete systems: We compare entanglement with quantum nonlocality employing a geometric
structure of the state space of bipartite qudits. Central object is a regular
simplex spanned by generalized Bell states. The
Collins-Gisin-Linden-Massar-Popescu-Bell inequality is used to reveal states of
this set that cannot be described by local-realistic theories. Optimal
measurement settings necessary to ascertain nonlocality are determined by means
of a recently proposed parameterization of the unitary group U(d) combined with
robust numerical methods. The main results of this paper are descriptive
geometric illustrations of the state space that emphasize the difference
between entanglement and quantum nonlocality. Namely, it is found that the
shape of the boundaries of separability and Bell inequality violation are
essentially different. Moreover, it is shown that also for mixtures of states
sharing the same amount of entanglement, Bell inequality violations and
entanglement measures are non-monotonically related.
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quant-ph
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$n$-dimensional PDM-damped harmonic oscillators: Linearizability, and
exact solvability: We consider position-dependent mass (PDM) Lagrangians/Hamiltonians in their
standard textbook form, where the long-standing \emph{gain-loss balance}
between the kinetic and potential energies is kept intact to allow conservation
of total energy (i.e., $L=T-V$, $H=T+V$, and $dH/dt=dE/dt=0$). Under such
standard settings, we discuss and report on $n$-dimensional PDM damped harmonic
oscillators (DHO). We use some $n$-dimensional point canonical transformation
to facilitate the linearizability of their $n$-PDM dynamical equations into
some $n$-linear DHOs' dynamical equations for constant mass setting.
Consequently, the well know exact solutions for the linear DHOs are mapped,
with ease, onto the exact solutions for PDM DHOs. A set of one-dimensional and
a set of $n$-dimensional PDM-DHO illustrative examples are reported along with
their phase-space trajectories.
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quant-ph
|
Shortcut to adiabatic gate teleportation: We introduce a shortcut to the adiabatic gate teleportation model of quantum
computation. More specifically, we determine fast local counterdiabatic
Hamiltonians able to implement teleportation as a universal computational
primitive. In this scenario, we provide the counterdiabatic driving for
arbitrary n-qubit gates, which allows to achieve universality through a variety
of gate sets. Remarkably, our approach maps the superadiabatic Hamiltonian for
an arbitrary n-qubit gate teleportation into the implementation of a rotated
superadiabatic dynamics of an n-qubit state teleportation. This result is
rather general, with the speed of the evolution only dictated by the quantum
speed limit. In particular, we analyze the energetic cost for different
Hamiltonian interpolations in the context of the energy-time complementarity.
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quant-ph
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Locally curved quantum layers: We consider a quantum particle constrained to a curved layer of a constant
width built over an infinite smooth surface. We suppose that the latter is a
locally deformed plane and that the layer has the hard-wall boundary. Under
this assumptions we prove that the particle Hamiltonian possesses geometrically
induced bound states.
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quant-ph
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Practical implications of SFQ-based two-qubit gates: Scalability of today's superconducting quantum computers is limited due to
the huge costs of generating/routing microwave control pulses per qubit from
room temperature. One active research area in both industry and academia is to
push the classical controllers to the dilution refrigerator in order to
increase the scalability of quantum computers. Superconducting Single Flux
Quantum (SFQ) is a classical logic technology with low power consumption and
ultra-high speed, and thus is a promising candidate for in-fridge classical
controllers with maximized scalability. Prior work has demonstrated
high-fidelity SFQ-based single-qubit gates. However, little research has been
done on SFQ-based multi-qubit gates, which are necessary to realize SFQ-based
universal quantum computing.
In this paper, we present the first thorough analysis of SFQ-based two-qubit
gates. Our observations show that SFQ-based two-qubit gates tend to have high
leakage to qubit non-computational subspace, which presents severe design
challenges. We show that despite these challenges, we can realize gates with
high fidelity by carefully designing optimal control methods and qubit
architectures. We develop optimal control methods that suppress leakage, and
also investigate various qubit architectures that reduce the leakage. After
carefully engineering our SFQ-friendly quantum system, we show that it can
achieve similar gate fidelity and gate time to microwave-based quantum systems.
The promising results of this paper show that (1) SFQ-based universal quantum
computation is both feasible and effective; and (2) SFQ is a promising approach
in designing classical controller for quantum machines because it can increase
the scalability while preserving gate fidelity and performance.
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quant-ph
|
Comment on: "On the Dirac oscillator subject to a Coulomb-type central
potential induced by the Lorentz symmetry violation": We analyze recent results on a Dirac oscillator. We show that the truncation
of the Frobenius series does not yield all the eigenvalues and eigenfunctions
of the radial equation. For this reason the eigenvalues reported by the authors
are useless and the prediction of allowed oscillator frequencies meaningless.
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quant-ph
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Artificial Neural Network Syndrome Decoding on IBM Quantum Processors: Syndrome decoding is an integral but computationally demanding step in the
implementation of quantum error correction for fault-tolerant quantum
computing. Here, we report the development and benchmarking of Artificial
Neural Network (ANN) decoding on IBM Quantum Processors. We demonstrate that
ANNs can efficiently decode syndrome measurement data from heavy-hexagonal code
architecture and apply appropriate corrections to facilitate error protection.
The current physical error rates of IBM devices are above the code's threshold
and restrict the scope of our ANN decoder for logical error rate suppression.
However, our work confirms the applicability of ANN decoding methods of
syndrome data retrieved from experimental devices and establishes machine
learning as a promising pathway for quantum error correction when quantum
devices with below threshold error rates become available in the near future.
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quant-ph
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Partitioning Quantum Chemistry Simulations with Clifford Circuits: Current quantum computing hardware is restricted by the availability of only
few, noisy qubits which limits the investigation of larger, more complex
molecules in quantum chemistry calculations on quantum computers in the
near-term. In this work, we investigate the limits of their classical and
near-classical treatment while staying within the framework of quantum circuits
and the variational quantum eigensolver. To this end, we consider naive and
physically motivated, classically efficient product ansatz for the parametrized
wavefunction adapting the separable pair ansatz form. We combine it with
post-treatment to account for interactions between subsystems originating from
this ansatz. The classical treatment is given by another quantum circuit that
has support between the enforced subsystems and is folded into the Hamiltonian.
To avoid an exponential increase in the number of Hamiltonian terms, the
entangling operations are constructed from purely Clifford or near-Clifford
circuits. While Clifford circuits can be simulated efficiently classically,
they are not universal. In order to account for missing expressibility,
near-Clifford circuits with only few, selected non-Clifford gates are employed.
The exact circuit structure to achieve this objective is molecule-dependent and
is constructed using simulated annealing and genetic algorithms. We demonstrate
our approach on a set of molecules of interest and investigate the extent of
our methodology's reach. Empirical validation of our approach using numerical
simulations shows a reduction of the qubit count of up to a 50\% at a similar
accuracy as compared to the separable-pair ansatz.
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quant-ph
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Controlling Polar Molecules in Optical Lattices: We investigate theoretically the interaction of polar molecules with optical
lattices and microwave fields. We demonstrate the existence of frequency
windows in the optical domain where the complex internal structure of the
molecule does not influence the trapping potential of the lattice. In such
frequency windows the Franck-Condon factors are so small that near-resonant
interaction of vibrational levels of the molecule with the lattice fields have
a negligible contribution to the polarizability and light-induced decoherences
are kept to a minimum. In addition, we show that microwave fields can induce a
tunable dipole-dipole interaction between ground-state rotationally symmetric
(J=0) molecules. A combination of a carefully chosen lattice frequency and
microwave-controlled interaction between molecules will enable trapping of
polar molecules in a lattice and possibly realize molecular quantum logic
gates. Our results are based on ab initio relativistic electronic structure
calculations of the polar KRb and RbCs molecules combined with calculations of
their rovibrational motion.
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quant-ph
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Quantum State Transfer Characterized by Mode Entanglement: We study the quantum state transfer (QST) of a class of tight-bonding Bloch
electron systems with mirror symmetry by considering the mode entanglement.
Some rigorous results are obtained to reveal the intrinsic relationship between
the fidelity of QST and the mirror mode concurrence (MMC), which is defined to
measure the mode entanglement with a certain spatial symmetry and is just the
overlap of a proper wave function with its mirror image. A complementarity is
discovered as the maximum fidelity is accompanied by a minimum of MMC. And at
the instant, which is just half of the characteristic time required to
accomplish a perfect QST, the MMC can reach its maximum value one. A large
class of perfect QST models with a certain spectrum structure are discovered to
support our analytical results.
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quant-ph
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Quantum tricriticality of chiral-coherent phase in quantum Rabi triangle: The interplay of interactions, symmetries and gauge fields usually leads to
intriguing quantum many-body phases. To explore the nature of emerging phases,
we study a quantum Rabi triangle system as an elementary building block for
synthesizing an artificial magnetic field. We develop an analytical approach to
study the rich phase diagram and the associated quantum criticality. Of
particular interest is the emergence of a chiral-coherent phase, which breaks
both the $\mathbb{Z}_2$ and the chiral symmetry. In this chiral phase, photons
flow unidirectionally and the chirality can be tuned by the artificial gauge
field, exhibiting a signature of broken time-reversal symmetry. The
finite-frequency scaling analysis further confirms the associated phase
transition to be in the universality class of the Dicke model. This model can
simulate a broad range of physical phenomena of light-matter coupling systems,
and may have an application in future developments of various quantum
information technologies.
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quant-ph
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Semiclassical theory of weak values: Aharonov-Albert-Vaidman's weak values are investigated by a semiclassical
method. Examples of the semiclassical calculation that reproduces "anomalous"
weak values are shown. Furthermore, a complex extension of Ehrenfest's
quantum-classical correspondence between quantum expectation values of the
states with small quantum fluctuation, and classical dynamics, is shown.
|
quant-ph
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Quantum Bicyclic Hyperbolic Codes: Bicyclic codes are a generalization of the one dimensional (1D) cyclic codes
to two dimensions (2D). Similar to the 1D case, in some cases, 2D cyclic codes
can also be constructed to guarantee a specified minimum distance. Many aspects
of these codes are yet unexplored. Motivated by the problem of constructing
quantum codes, in this paper, we study some structural properties of certain
bicyclic codes. We show that a primitive narrow-sense bicyclic hyperbolic code
of length $n^2$ contains its dual if and only if its design distance is lower
than $n-\Delta$, where $\Delta=\mathcal{O}(\sqrt{n})$. We extend the
sufficiency condition to the non-primitive case as well. We also show that over
quadratic extension fields, a primitive bicyclic hyperbolic code of length
$n^2$ contains Hermitian dual if and only if its design distance is lower than
$n-\Delta_h$, where $\Delta_h=\mathcal{O}(\sqrt{n})$. Our results are analogous
to some structural results known for BCH and Reed-Solomon codes. They further
our understanding of bicyclic codes. We also give an application of these
results by showing that we can construct two classes of quantum bicyclic codes
based on our results.
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quant-ph
|
Vestiges of quantum oscillations in the open evolution of semiclassical
states: A single wave component of a quantum particle can in principle be detected by
the way that it interferes with itself, that is, through the local wave
function correlation. The interpretation as the expectation of a local
translation operator allows this measure of quantum wavyness to be followed
through the process of decoherence in open quantum systems. This is here
assumed to be Markovian, determined by Lindblad operators that are linear in
position and momentum. The limitation of small averaging windows and even
smaller correlation lengths simplifies the semiclassical theory for the
evolving local correlation. Its spectrum has a peak for each classical
momentum, subjected to Gaussian broadening with decoherence. These spectral
lines can be clearly resolved even after the Wigner function has become
positive: The correlations located far from caustics seem to be the last
vestige of quantum oscilations.
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quant-ph
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Limit cycles in periodically driven open quantum systems: We investigate the long-time behavior of quantum N-level systems that are
coupled to a Markovian environment and subject to periodic driving. As our main
result, we obtain a general algebraic condition ensuring that all solutions of
a periodic quantum master equation with Lindblad form approach a unique limit
cycle. Quite intuitively, this criterion requires that the dissipative terms of
the master equation connect all subspaces of the system Hilbert space during an
arbitrarily small fraction of the cycle time. Our results provide a natural
extension of Spohn's algebraic condition for the approach to equilibrium to
systems with external driving.
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quant-ph
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Experimental investigation of initial system-environment correlations
via trace distance evolution: The trace distance between two states of an open quantum system quantifies
their distinguishability, and for a fixed environmental state can increase
above its initial value only in the presence of initial system-environment
correlations. We provide for the first time experimental evidence of such a
behavior. In our all-optical apparatus we exploit spontaneous parametric down
conversion as a source of polarization entangled states, and a spatial light
modulator to introduce in a general fashion correlations between the
polarization and the momentum degrees of freedom, which act as environment.
|
quant-ph
|
Exact State Revival in a Spin Chain with Next-To-Nearest Neighbour
Interactions: An extension with next-to-nearest neighbour interactions of the simplest XX
spin chain with perfect state transfer (PST) is presented. The conditions for
PST and entanglement generation (balanced fractional revival) can be obtained
exactly and are discussed.
|
quant-ph
|
Quantum coding theorem from privacy and distinguishability: We prove direct quantum coding theorem for random quantum codes. The problem
is separated into two parts: proof of distinguishability of codewords by
receiver, and that of indistinguishability of codewords by environment
(privacy). For a large class of codes, only privacy has to be checked.
|
quant-ph
|
Experimental Data from a Quantum Computer Verifies the Generalized Pauli
Exclusion Principle: "What are the consequences ... that Fermi particles cannot get into the same
state ... " R. P. Feynman wrote of the Pauli exclusion principle, "In fact,
almost all the peculiarities of the material world hinge on this wonderful
fact." In 1972 Borland and Dennis showed that there exist powerful constraints
beyond the Pauli exclusion principle on the orbital occupations of Fermi
particles, providing important restrictions on quantum correlation and
entanglement. Here we use computations on quantum computers to experimentally
verify the existence of these additional constraints. Quantum many-fermion
states are randomly prepared on the quantum computer and tested for constraint
violations. Measurements show no violation and confirm the generalized Pauli
exclusion principle with an error of one part in one quintillion.
|
quant-ph
|
Quantum theory within the probability calculus: a there-you-go theorem
and partially exchangeable models: "Ever since the advent of modern quantum mechanics in the late 1920's, the
idea has been prevalent that the classical laws of probability cease, in some
sense, to be valid in the new theory. [...] The primary object of this
presentation is to show that the thesis in question is entirely without
validity and is the product of a confused view of the laws of probability"
(Koopman, 1957). The secondary objects are: to show that quantum inferences are
cases of partially exchangeable statistical models with particular prior
constraints; to wonder about such constraints; and to plead for a dialogue
between quantum theory and the theory of exchangeable models.
|
quant-ph
|
Observation of high coherence in Josephson junction qubits measured in a
three-dimensional circuit QED architecture: Superconducting quantum circuits based on Josephson junctions have made rapid
progress in demonstrating quantum behavior and scalability. However, the future
prospects ultimately depend upon the intrinsic coherence of Josephson
junctions, and whether superconducting qubits can be adequately isolated from
their environment. We introduce a new architecture for superconducting quantum
circuits employing a three dimensional resonator that suppresses qubit
decoherence while maintaining sufficient coupling to the control signal. With
the new architecture, we demonstrate that Josephson junction qubits are highly
coherent, with $T_2 \sim 10 \mu$s to $20 \mu$s without the use of spin echo,
and highly stable, showing no evidence for $1/f$ critical current noise. These
results suggest that the overall quality of Josephson junctions in these qubits
will allow error rates of a few $10^{-4}$, approaching the error correction
threshold.
|
quant-ph
|
Homodyne-detector-blinding attack in continuous-variable quantum key
distribution: We propose an efficient strategy to attack a continuous-variable quantum key
distribution (CV-QKD) system, that we call homodyne detector blinding. This
attack strategy takes advantage of a generic vulnerability of homodyne
receivers: a bright light pulse sent on the signal port can lead to a
saturation of the detector electronics. While detector saturation has already
been proposed to attack CV-QKD, the attack we study in this paper has the
additional advantage of not requiring an eavesdropper to be phase locked with
the homodyne receiver. We show that under certain conditions, an attacker can
use a simple laser, incoherent with the homodyne receiver, to generate bright
pulses and bias the excess noise to arbitrary small values, fully comprising
CV-QKD security. These results highlight the feasibility and the impact of the
detector blinding attack. We finally discuss how to design countermeasures in
order to protect against this attack.
|
quant-ph
|
Supersensitive sensing of quantum reservoirs via breaking antisymmetric
coupling: We investigate the utilization of a single generalized dephasing qubit for
sensing a quantum reservoir, where the antisymmetric coupling between the qubit
and its reservoir is broken. It is found that in addition to the decay factor
encoding channel, the antisymmetric coupling breaking gives rise to another
phase factor encoding channel. We introduce an optimal measurement for the
generalized dephasing qubit which enables the practical measurement precision
to reach the theoretical ultimate precision quantified by the quantum
signal-to-noise ratio (QSNR). As an example, the generalized dephasing qubit is
employed to estimate the $s$-wave scattering length of an atomic Bose-Einstein
condensate. It is found that the phase-induced QSNR caused by the antisymmetric
coupling breaking is at least two orders of magnitude higher than the
decay-induced QSNR at the millisecond timescale and the optimal relative error
can achieve a scaling $\propto 1/t$ with $t$ being the encoding time in
long-term encoding. Our work opens a way for supersensitive sensing of quantum
reservoirs.
|
quant-ph
|
Electromagnetically induced transparency in circuit QED with nested
polariton states: Electromagnetically induced transparency (EIT) is a signature of quantum
interference in an atomic three-level system. By driving the dressed
cavity-qubit states of a two-dimensional circuit QED system, we generate a set
of polariton states in the nesting regime. The lowest three energy levels are
utilized to form the $\Lambda$-type system. EIT is observed and verified by
Akaike's information criterion based testing. Negative group velocities up to
$-0.52\pm0.09$ km/s are obtained based on the dispersion relation in the EIT
transmission spectrum.
|
quant-ph
|
Noise-Resilient Quantum Machine Learning for Stability Assessment of
Power Systems: Transient stability assessment (TSA) is a cornerstone for resilient
operations of today's interconnected power grids. This paper is a confluence of
quantum computing, data science and machine learning to potentially address the
power system TSA challenge. We devise a quantum TSA (qTSA) method to enable
scalable and efficient data-driven transient stability prediction for bulk
power systems, which is the first attempt to tackle the TSA issue with quantum
computing. Our contributions are three-fold: 1) A low-depth, high
expressibility quantum neural network for accurate and noise-resilient TSA; 2)
A quantum natural gradient descent algorithm for efficient qTSA training; 3) A
systematical analysis on qTSA's performance under various quantum factors. qTSA
underpins a foundation of quantum-enabled and data-driven power grid stability
analytics. It renders the intractable TSA straightforward and effortless in the
Hilbert space, and therefore provides stability information for power system
operations. Extensive experiments on quantum simulators and real quantum
computers verify the accuracy, noise-resilience, scalability and universality
of qTSA.
|
quant-ph
|
Characterization of Suspended Membrane Waveguides towards a Photonic
Atom Trap Integrated Platform: We demonstrate an optical waveguide device, capable of supporting the high,
in-vacuum, optical power necessary for trapping a single atom or a cold atom
ensemble with evanescent fields. Our photonic integrated platforms, with
suspended membrane waveguides, successfully manages optical powers of 6 mW (500
um span) to nearly 30 mW (125 um span) over an un-tethered waveguide span. This
platform is compatible with laser cooling and magneto-optical traps (MOTs) in
the vicinity of the suspended waveguide, called the membrane MOT and the needle
MOT, a key ingredient for efficient trap loading. We evaluate two novel designs
that explore critical thermal management features that enable this large power
handling. This work represents a significant step toward an integrated platform
for coupling neutral atom quantum systems to photonic and electronic integrated
circuits on silicon.
|
quant-ph
|
Exact solvability of the quantum Rabi models within Bogoliubov operators: The quantum Rabi model can be solved exactly by the Bargmann transformation
from real coordinate to complex variable recently [Phys. Rev. Lett.
\textbf{107}, 100401 (2011)]. By the extended coherent states, we recover this
solution in an alternative simpler and perhaps more physical way without uses
of any extra conditions, like Bargmann conditions. In the same framework, the
two-photon Rabi model are solved exactly by extended squeeze states.
Transcendental functions have been derived with the similar form as those in
one-photon model. Both extended coherent states and squeeze states are
essentially Fock states in the space of the corresponding Bogoliubov operators.
The present approach could be easily extended to study the exact solvability or
integrability of various spin-boson systems with multi-level, even multi-mode.
|
quant-ph
|
Bidirectional quantum teleportation and secure direct communication via
entanglement swapping: In this paper, a bidirectional quantum teleportation protocol based on
Einstein-Podolsky-Rosen (EPR) pairs and entanglement swapping is proposed. In
this scheme, two users can simultaneously transmit an unknown single-qubit
state to each other. The implementation of the proposed scheme is easier in
experiment as compared to previous work. By utilizing this bidirectional
quantum teleportation protocol, a bidirectional quantum secure direct
communication scheme without carrying secret message is presented. Therefore,
in the case of using perfect quantum channel, the protocol is completely
secure. Finally, security analyses are investigated.
|
quant-ph
|
One-Way Deficit of Two Qubit X States: Quantum deficit originates in questions regarding work extraction from
quantum systems coupled to a heat bath [Phys. Rev. Lett. 89, 180402 (2002)]. It
links quantum correlations with quantum thermodynamics and provides a new
standpoint for understanding quantum non-locality. In this paper, we propose a
new method to evaluate the one-way deficit for a class of two-qubit states. The
dynamic behavior of the one-way deficit under decoherence channel is
investigated and it is shown that the one-way deficit of the X states with five
parameters is more robust against the decoherence than the entanglement.
|
quant-ph
|
Generalized binomial state: Nonclassical features observed through
various witnesses and a measure of nonclassicality: Experimental realization of various quantum states of interest has become
possible in the recent past due to the rapid developments in the field of
quantum state engineering. Nonclassical properties of such states have led to
various exciting applications, specifically in the area of quantum information
processing. The present article aims to study lower- and higher-order
nonclassical features of such an engineered quantum state (a generalized
binomial state based on Abel's formula). Present study has revealed that the
state studied here is highly nonclassical. Specifically, higher-order
nonclassical properties of this state are reported using a set of witnesses,
like higher-order antibunching, higher-order sub-Poissonian photon statistics,
higher-order squeezing (both Hong Mandel type and Hillery type). A set of other
witnesses for lower- and higher-order nonclassicality (e.g., Vogel's criterion
and Agarwal's A parameter) have also been explored. Further, an analytic
expression for the Wigner function of the generalized binomial state is
reported and the same is used to witness nonclassicality and to quantify the
amount of nonclassicality present in the system by computing the nonclassical
volume (volume of the negative part of the Wigner function). Optical tomogram
of the generalized binomial state is also computed for various conditions as
Wigner function cannot be measured directly in an experiment in general, but
the same can be obtained from the optical tomogram with the help of Radon
transform.
|
quant-ph
|
Stabilizing qubit coherence via tracking-control: We consider the problem of stabilizing the coherence of a single qubit
subject to Markovian decoherence, via the application of a control Hamiltonian,
without any additional resources. In this case neither quantum error
correction/avoidance, nor dynamical decoupling applies. We show that using
tracking-control, i.e., the conditioning of the control field on the state of
the qubit, it is possible to maintain coherence for finite time durations,
until the control field diverges.
|
quant-ph
|
Combinatorial optimization solving by coherent Ising machines based on
spiking neural networks: Spiking neural network is a kind of neuromorphic computing that is believed
to improve the level of intelligence and provide advantages for quantum
computing. In this work, we address this issue by designing an optical spiking
neural network and find that it can be used to accelerate the speed of
computation, especially on combinatorial optimization problems. Here the
spiking neural network is constructed by the antisymmetrically coupled
degenerate optical parametric oscillator pulses and dissipative pulses. A
nonlinear transfer function is chosen to mitigate amplitude inhomogeneities and
destabilize the resulting local minima according to the dynamical behavior of
spiking neurons. It is numerically shown that the spiking neural
network-coherent Ising machines have excellent performance on combinatorial
optimization problems, which is expected to offer new applications for neural
computing and optical computing.
|
quant-ph
|
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