Physics

Single-photon limit of dispersive readout of a qubit with a photodetector

PRA: Quantum information - 10 hours 37 min ago

Author(s): Andrii M. Sokolov and Eugene V. Stolyarov

We study the dispersive readout of a qubit in the ultimate limit of a single-photon probe. The use of a single-photon probe avoids the errors due to nonorthogonality of coherent states. A photodetector is used in the scheme we consider. The dynamics of the system is studied using the Heisenberg-Lang...


[Phys. Rev. A 101, 042306] Published Fri Apr 03, 2020

Categories: Journals, Physics

Anomalous Diffusion in Dipole- and Higher-Moment Conserving Systems. (arXiv:2004.00635v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - 10 hours 52 min ago

The presence of global conserved quantities in interacting systems generically leads to diffusive transport at late times. Here, we show that systems conserving the dipole moment of an associated global charge, or even higher moment generalizations thereof, escape this scenario, displaying subdiffusive decay instead. Modelling the time evolution as cellular automata for specific cases of dipole- and quadrupole-conservation, we numerically find distinct anomalous exponents of the late time relaxation. We explain these findings by analytically constructing a general hydrodynamic model that results in a series of exponents depending on the number of conserved moments, yielding an accurate description of the scaling form of charge correlation functions. We analyze the spatial profile of the correlations and discuss potential experimentally relevant signatures of higher moment conservation.

Categories: Journals, Physics

Giant Magnetoresistance in Boundary Driven Spin Chains. (arXiv:2004.00639v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - 10 hours 52 min ago

In solid state physics, giant magnetoresistance is the large change in electrical resistance due to an external magnetic field. Here we show that giant magnetoresistance is possible in a spin chain composed of weakly interacting layers of strongly coupled spins. This is found for all system sizes even down to a minimal system of four spins. The mechanism driving the effect is a mismatch in the energy spectrum resulting in spin excitations being reflected at the boundaries between layers. This mismatch, and thus the current, can be controlled by external magnetic fields resulting in giant magnetoresistance. A simple rule for determining the behavior of the spin transport under the influence of a magnetic field is presented based on the energy levels of the strongly coupled spins.

Categories: Journals, Physics

The powerfulness of classical correlations. (arXiv:2004.00695v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Determination of classical and quantum values of bipartite Bell inequalities plays a central role in quantum nonlocality. In this work, we characterize in a simple way bipartite Bell inequalities, free of marginal terms, for which the quantum value can be achieved by considering a classical strategy, for any number of measurement settings and outcomes. These findings naturally generalize known results about nonlocal computation and quantum XOR games. Additionally, our technique allows us to determine the classical value for a wide class of Bell inequalities, having quantum advantage or not, in any bipartite scenario.

Categories: Journals, Physics

Quantum computing using continuous-time evolution. (arXiv:2004.00704v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Computational methods are the most effective tools we have besides scientific experiments to explore the properties of complex biological systems. Progress is slowing because digital silicon computers have reached their limits in terms of speed. Other types of computation using radically different architectures, including neuromorphic and quantum, promise breakthroughs in both speed and efficiency. Quantum computing exploits the coherence and superposition properties of quantum systems to explore many possible computational paths in parallel. This provides a fundamentally more efficient route to solving some types of computational problems, including several of relevance to biological simulations. In particular, optimisation problems, both convex and non-convex, feature in many biological models, including protein folding and molecular dynamics. Early quantum computers will be small, reminiscent of the early days of digital silicon computing. Understanding how to exploit the first generation of quantum hardware is crucial for making progress in both biological simulation and the development of the next generations of quantum computers. This review outlines the current state-of-the-art and future prospects for quantum computing, and provides some indications of how and where to apply it to speed up bottlenecks in biological simulation.

Categories: Journals, Physics

Multiparameter quantum estimation under dephasing noise. (arXiv:2004.00720v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Simultaneous quantum estimation of multiple parameters has recently become essential in quantum metrology. Although the ultimate sensitivity of a multiparameter quantum estimation in noiseless environments can beat the standard quantum limit that every classical sensor is bounded by, it is unclear whether the quantum sensor has an advantage over the classical one under realistic noise. In this work, we present a framework of the simultaneous estimation of multiple parameters with quantum sensors in a certain noisy environment. Our multiple parameters to be estimated are three components of an external magnetic field, and we consider the noise that causes only dephasing. We show that there is an optimal sensing time in the noisy environment and the sensitivity can beat the standard quantum limit when the noisy environment is non-Markovian.

Categories: Journals, Physics

Particle detectors as witnesses for quantum gravity. (arXiv:2004.00724v1 [gr-qc])

arXiv.org: Quantum Physics - 10 hours 52 min ago

We present a model for the coupling of non-relativistic quantum systems with a linearized gravitational field from a Lagrangian formulation. The coupling strongly resembles the light-matter interaction models that are known to be well approximated by the Unruh-DeWitt detector model for interactions with quantum fields. We then apply our model to linearized quantum gravity, which allows us to propose a detector based setup that can in principle probe the quantum nature of the gravitational field.

Categories: Journals, Physics

Creating big time crystals with ultracold atoms. (arXiv:2004.00755v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - 10 hours 52 min ago

We investigate the size of discrete time crystals in the range s = 10 - 100 (ratio of response period to driving period) that can be created for a Bose-Einstein condensate (BEC) bouncing resonantly on an oscillating mirror. We consider the effects of having a realistic soft Gaussian potential mirror for the bouncing BEC, such as that produced by a repulsive light-sheet, which is found to have a significant effect on the dynamics of the system. Finally, we discuss the choice of atomic system for creating time crystals based on a bouncing BEC and present an experimental protocol for realizing big time crystals. Such a system provides a platform for investigating a broad range of non-trivial condensed matter phenomena in the time domain.

Categories: Journals, Physics

Note on Product-Form Monogamy Relations for Nonlocality and Other Correlation Measures. (arXiv:2004.00757v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

The monogamy relations satisfied by quantum correlation measures play important roles in quantum information processing. Generally they are given in summation form. In this note, we study monogamy relations in product form. We present product-form monogamy relations for Bell nonlocality for three-qubit and multi-qubit quantum systems. We then extend our studies to other quantum correlations such as concurrence.

Categories: Journals, Physics

Single Quantum Deletion Error-Correcting Codes. (arXiv:2004.00814v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

In this paper, we discuss a construction method of quantum deletion error-correcting codes. First of all, we define deletion errors for quantum states, an encoder, a decoder, and two conditions which is expressed by only the combinatorial language. Then, we prove that quantum deletion error-correcting codes can be constructed by two sets that satisfy the conditions. In other words, problems that correct the deletion errors for quantum states are reduced to problems that find the sets satisfying the condition by this paper. Also, we performed experiment of the codes over IBM Quantum Experience.

Categories: Journals, Physics

Planar Maximally Entangled States. (arXiv:2004.00906v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

We construct a large family of Planar Maximally Entangled (PME) states which are a wider class of multi-partite entangled states than Absolutely Maximally Entangled (AME) states. We show that in contrast to AMEs, PMEs are easier to find and there are various PMEs for any even number of qudits. In particular, while it is known that no AME state of four qubits exists, we show that there are two distinct multi-parameter classes of four qubit PMEs. We also give explicit families of PMEs for any even number of particles and for any dimension.

Categories: Journals, Physics

Coexistence of localized and extended phases: Many-body localization in a harmonic trap. (arXiv:2004.00954v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - 10 hours 52 min ago

We show that the presence of a harmonic trap may in itself lead to many-body localization for cold atoms confined in that trap in a quasi-one-dimensional geometry. Specifically, the coexistence of delocalized phase in the center of the trap with localized region closer to the edges is predicted with the borderline dependent on the curvature of the trap. The phenomenon, similar in its origin to Stark localization, should be directly observed with cold atomic species. We discuss both the spinless and the spinful fermions, for the latter we address Stark localization at the same time as it has not been analyzed up till now.

Categories: Journals, Physics

A Heuristic Quantum-Classical Algorithm for Modeling Substitutionally Disordered Binary Crystalline Materials. (arXiv:2004.00957v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Improving the efficiency and accuracy of energy calculations has been of significant and continued interest in the area of materials informatics, a field that applies machine learning techniques to computational materials data. Here, we present a heuristic quantum-classical algorithm to efficiently model the energy of substitutionally disordered binary crystalline materials. Specifically, a quantum circuit, that scales linearly in its parameters, is designed to predict the energies of quantum chemical simulations in an exponentially-scaling feature space. This circuit is trained by classical supervised-learning using data obtained from classically-computed quantum chemical simulations. The algorithm is able to detect and rectify anomalies in the data. The feasibility of the algorithm is demonstrated on the layer-structured Li-cobaltate system, a widely-used Li-ion battery cathode material component. The result shows that our quantum circuit model presents a suitable choice for modelling the energies obtained from a quantum mechanical system. Analysis of the anomalous data provides insights into the thermodynamic properties of the system studied.

Categories: Journals, Physics

Quantum state tomography via sequential uses of the same informationally incomplete measuring apparatus. (arXiv:2004.00966v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

State of a $d$-dimensional quantum system can only be inferred by performing an informationally complete measurement with $m\geqslant d^2$ outcomes. However, an experimentally accessible measurement can be informationally incomplete. Here we show that a single informationally incomplete measuring apparatus is still able to provide all the information about the quantum system if applied several times in a row. We derive a necessary and sufficient condition for such a measuring apparatus and give illustrative examples for qubits, qutrits, general $d$-level systems, and composite systems of $n$ qubits, where such a measuring apparatus exists. We show that projective measurements and L\"{u}ders measurements with 2 outcomes are useless in the considered scenario.

Categories: Journals, Physics

Drive-noise tolerant optical switching inspired by composite pulses. (arXiv:2004.00986v1 [physics.app-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Electro-optic modulators within Mach--Zehnder interferometers are a common construction for optical switches in integrated photonics. A challenge faced when operating at high switching speeds is that noise from the electronic drive signals will effect switching performance. Inspired by the Mach--Zehnder lattice switching devices of Van Campenhout et al. [Opt. Express, 17, 23793 (2009)] and techniques from the field of Nuclear Magnetic Resonance known as composite pulses, we present switches which offer protection against drive-noise in both the on and off state of the switch for both the phase and intensity information encoded in the switched optical mode.

Categories: Journals, Physics

Interplay between shell structure and trap deformation in dipolar fermi gases. (arXiv:2004.01006v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Finite fermion systems are known to exhibit shell structure in the weakly-interacting regime, as well known from atoms, nuclei, metallic clusters or even quantum dots in two dimensions. All these systems have in common that the particle interactions between electrons or nucleons are spatially isotropic. Dipolar quantum systems as they have been realized with ultra-cold gases, however, are governed by an intrinsic anisotropy of the two-body interaction that depends on the orientation of the dipoles relative to each other. Here we investigate how this interaction anisotropy modifies the shell structure in a weakly interacting two-dimensional anisotropic harmonic trap. Going beyond Hartree-Fock by applying the so-called "importance-truncated" configuration interaction (CI) method as well as quadratic CI with single- and double-substitutions, we show how the magnetostriction in the system may be counteracted upon by a deformation of the isotropic confinement, restoring the symmetry.

Categories: Journals, Physics

Quantum Incompatibility of a Physical Context. (arXiv:2004.01008v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Pivotal within quantum physics, the concept of quantum incompatibility is generally related to algebraic aspects of the formalism, such as commutation relations and unbiasedness of bases. Recently, the concept was identified as a resource in tasks involving quantum state discrimination and quantum programmability. Here we link quantum incompatibility with the amount of information that can be extracted from a system upon successive measurements of noncommuting observables, a scenario related to communication tasks. This approach leads us to characterize incompatibility as a resource encoded in a physical context, which involves both the quantum state and observables. Moreover, starting with a measure of context incompatibility we derive a measurement-incompatibility quantifier that is easily computable, admits a geometrical interpretation, and is maximum only if the eigenbases of the involved observables are mutually unbiased.

Categories: Journals, Physics

Geometry of variational methods: dynamics of closed quantum systems. (arXiv:2004.01015v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

We present a systematic geometric framework to study closed quantum systems based on suitably chosen variational families. For the purpose of (A) real time evolution, (B) excitation spectra, (C) spectral functions and (D) imaginary time evolution, we show how the geometric approach highlights the necessity to distinguish between two classes of manifolds: K\"ahler and non-K\"ahler. Traditional variational methods typically require the variational family to be a K\"ahler manifold, where multiplication by the imaginary unit preserves the tangent spaces. This covers the vast majority of cases studied in the literature. However, recently proposed classes of generalized Gaussian states make it necessary to also include the non-K\"ahler case, which has already been encountered occasionally. We illustrate our approach in detail with a range of concrete examples where the geometric structures of the considered manifolds are particularly relevant. These go from Gaussian states and group theoretic coherent states to generalized Gaussian states.

Categories: Journals, Physics

Triplet-Triplet Decoherence in Singlet Fission. (arXiv:2004.01058v1 [physics.chem-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

Singlet fission is commonly defined to involve a process by which an overall singlet state with local triplet structure spin-decoheres into two triplet states, thereby completing the fission process. This process, often defined in loose terms involving the multiplicity of the overall state, is investigated here using a uniform Heisenberg spin-chain subject to a dephasing environmental interaction. We introduce new results from quantum information theory which enables the quantification of coherence and entanglement in a bi- and multipartite system. The calculated measures of these quantum effects can be linked to observables, such as magnetisation and total spin, with simulations of the model and using theoretical methods. We demonstrate that these observables can act as a proxy for the coherence and entanglement measures. The decay of both of these between the two local triplets can be monitored, enabling a clear definition of the spin-decoherence process in singlet fission.

Categories: Journals, Physics

Proceedings 9th International Conference on Quantum Simulation and Quantum Walks. (arXiv:2004.01061v1 [quant-ph])

arXiv.org: Quantum Physics - 10 hours 52 min ago

This volume contains a selection of papers presented at the 9th in a series of international conferences on Quantum Simulation and Quantum Walks (QSQW). During this event, we worked on the development of theories based upon quantum walks and quantum simulation models, in order to solve interrelated problems concerning the simulation of standard quantum field theory, quantum gravity and cosmological models, dissipative quantum computing, searching on complex quantum networks, and the topological classification of multi-particle quantum walks.

Categories: Journals, Physics
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