Presence Of Coherence Research Articles

Heat currents in qubit systems

There is a current interest in quantum thermodynamics in the context of open quantum systems. An important issue is the consistency of quantum thermodynamics, in particular the second law of thermodynamics, i.e. the flow of heat from a hot reservoir to a cold reservoir. Here recent emphasis has been on composite system and in particular the issue regarding the application of local or global master equations. In order to contribute to this discussion we discuss two cases, namely as an example a single qubit and as a simple composite system two coupled qubits driven by two heat reservoirs at different temperatures, respectively. Applying a global Lindblad master equation approach we present explicit expressions for the heat currents in agreement with the second law of thermodynamics. The analysis is carried out in the Born–Markov approximation. We also discuss issues regarding the possible presence of coherences in the steady state.

Quantum coherence enables hybrid multitask and multisource regimes in autonomous thermal machines

Nonequilibrium effects may have a profound impact on the performance of thermal devices performing thermodynamic tasks such as refrigeration or heat pumping. The possibility of enhancing the performance of thermodynamic operations by means of quantum coherence is of particular interest but requires an adequate characterization of heat and work at the quantum level. In this work, we demonstrate that the presence of even small amounts of coherence in the thermal reservoirs powering a three-terminal machine, enables the appearance of combined, and hybrid modes of operation, where either different resources are combined to perform a single thermodynamic task, or more than one task is performed at the same time. We determine the performance of such coherence-enabled modes of operation obtaining their power and efficiency. In the case of hybrid regimes, the presence of coherence in the hot bath allows for an increase in power while maintaining high efficiencies. On the other hand, in combined regimes, a contrasting behavior emerges whereby coherence has a detrimental impact on power output and efficiency. Published by the American Physical Society 2024

Improving Entanglement Generation Using the Coherence of a Nonthermal Reservoir

AbstractThe effect of environmental coherence on bipartite and tripartite quantum entanglements using quantum collision models is studied. In the collision model, an open quantum system of two qubits indirectly interacts with the reservoir via auxiliary qubits. The reservoir is modeled as a collection of initially uncorrelated qubits prepared in an identically nonthermal state. It is shown that the coherence of the nonthermal reservoir contributes to the generation and protection of entanglement. It is found that adding coherence to the initial state of the reservoir qubit can significantly enhance the steady‐state entanglement of the system, and the presence of coherence in the reservoir allows steady‐state entanglement to be obtained in a higher temperature region.

QUANTUM STATISTICAL PERSPECTIVE TO EXAMINE THE SOURCE CHAOS FRACTION THROUGH BOSON FEMTOSCOPY

Bose–Einstein correlations of identical particles reveal the shape and size of the particle emitting source of the given boson. These correlations are pivotal for a better understanding of the source dynamics and developing techniques to examine the propagation of quantum chaos in the presence of coherence at a certain temperature and momentum. Femtoscopy is of the utmost importance during this transformation for data processing and coherence-chaos analysis discrepancies. We introduced an evolving source to describe chaotic information inside quantum entanglement and the impact of the coherence order is discussed in this research. We also investigate the role of source size parameter and particle number in the dynamics of the temperature profiles with the function of momentum variation. The influences of the modeling factors on the correlations with their normalized correlator in the perspective of source coherence have been evaluated and the orientation of the source has an effect on the diffusivity of the fluid under consideration. Such consequences can cause an increase or decrease the genuine correlations due to the distributions of velocity and temperature. The main findings of the paper have been illustrated using the graphical representations of the considered correlations according to the geometry of the expanding source. Such results reveal the lucrativeness in the field of engineering applications.

Collective effects and quantum coherence in dissipative charging of quantum batteries

We consider the dissipative charging process of quantum batteries in terms of a collisional model, where the batteries are coupled to a heat bath using non-energy preserving interactions. First, we show that for low temperatures the collective process can attain a charging power that increases polynomically with the number of batteries. The scaling we find is $N^3$ that, while being grater than the bound obtained for unitary processes, it has a lower efficiency. Then, we study the dissipative charging process of single battery using a time dependent Hamiltonian that generates coherences in the energy basis. In this case we find that the presence of coherence could enhance the charging power and also its efficiency. Finally, we show how this process can be used in a quantum heat engine that contains the charging process as one of its open strokes.

Suppressing coherence effects in quantum-measurement-based engines

The recent advances in the study of thermodynamics of microscopic processes have driven the search for new developments in energy converters utilizing quantum effects. We here propose a universal framework to describe the thermodynamics of a quantum engine fueled by quantum projective measurements. Standard quantum thermal machines operating in a finite-time regime with a driven Hamiltonian that does not commute in different times have the performance decreased by the presence of coherence, which is associated with a larger entropy production and irreversibility degree. However, we show that replacing the standard hot thermal reservoir by a projective measurement operation with general basis in the Bloch sphere and controlling the basis angles suitably could improve the performance of the quantum engine as well as decrease the entropy change during the measurement process. Our results go in direction of a generalization of quantum thermal machine models where the fuel comes from general sources beyond the standard thermal reservoir.

Weak and Ultrastrong Coupling Limits of the Quantum Mean Force Gibbs State.

The Gibbs state is widely taken to be the equilibrium state of a system in contact with an environment at temperature T. However, non-negligible interactions between system and environment can give rise to an altered state. Here, we derive general expressions for this mean force Gibbs state, valid for any system that interacts with a bosonic reservoir. First, we derive the state in the weak coupling limit and find that, in general, it maintains coherences with respect to the bare system Hamiltonian. Second, we develop a new expansion method suited to investigate the ultrastrong coupling regime. This allows us to derive the explicit form for the mean force Gibbs state, and we find that it becomes diagonal in the basis set by the system-reservoir interaction instead of the system Hamiltonian. Several examples are discussed including a single qubit, a three-level V-system, and two coupled qubits all interacting with bosonic reservoirs. The results shed light on the presence of coherences in the strong coupling regime, and provide key tools for nanoscale thermodynamics investigations.

Discrimination of thermal baths by single-qubit probes

Non-equilibrium states of quantum systems in contact with thermal baths help telling environments with different temperatures or different statistics apart. We extend these studies to a more generic problem that consists in discriminating between two baths with disparate constituents at unequal temperatures. Notably there exist temperature regimes in which the presence of coherence in the initial state preparation is beneficial for the discrimination capability. We also find that non-equilibrium states are not universally optimal, and detail the conditions in which it becomes convenient to wait for complete thermalisation of the probe. These concepts are illustrated in a linear optical simulation.

Phase Time for the Tunneling of Ultracold V-Type Atoms Through a Mazer Cavity**Supported by the National Natural Science Foundation of China under Grant Nos. 11750110411 and 11274132

We study the tunneling time of ultracold V-type atoms interacting a high quality microwave cavity. Here atomic coherence is introduced in the system by a strong driving field which couples the two lower states of the three-level atom. It is found that in the presence of coherence, mazer action or the scattering like nature of the interaction may be examined for extended energies of the incident cold atoms. Our results show that position and amplitudes of the peak values of the phase time (traversal time) may be very effectively controlled by the coherent driving field. Further, here we obtained superclassical values of the phase time corresponding to much higher values of the transmission amplitudes of the tunneling atoms which may be advantageous in the possible experimental realization of the superclassical tunneling time of the traversing cold atoms. In addition, we examine a mirror reflection type symmetry in the phase time curve for a judicious choice of the external driving field.

Coherent fluctuation relations: from the abstract to the concrete

Recent studies using the quantum information theoretic approach to thermodynamics show that the presence of coherence in quantum systems generates corrections to classical fluctuation theorems. To explicate the physical origins and implications of such corrections, we here convert an abstract framework of an autonomous quantum Crooks relation into quantum Crooks equalities for well-known coherent, squeezed and cat states. We further provide a proposal for a concrete experimental scenario to test these equalities. Our scheme consists of the autonomous evolution of a trapped ion and uses a position dependent AC Stark shift.

Nonequilibrium fluctuations of a driven quantum heat engine via machine learning.

We propose a machine-learning approach based on artificial neural network to efficiently obtain new insights on the role of geometric contributions to the nonequilibrium fluctuations of an adiabatically temperature-driven quantum heat engine coupled to a cavity. Using the artificial neural network we have explored the interplay between bunched and antibunched photon exchange statistics for different engine parameters. We report that beyond a pivotal cavity temperature, the Fano factor oscillates between giant and low values as a function of phase difference between the driving protocols. We further observe that the standard thermodynamic uncertainty relation is not valid when there are finite geometric contributions to the fluctuations but holds true for zero phase difference even in the presence of coherences.

Hallmarking quantum states: unified framework for coherence and correlations

Quantum coherence and distributed correlations among subparties are often considered as separate, although operationally linked to each other, properties of a quantum state. Here, we propose a measure able to quantify the contributions derived by both the tensor structure of the multipartite Hilbert space and the presence of coherence inside each of the subparties. Our results hold for any number of partitions of the Hilbert space. Within this unified framework, global coherence of the state is identified as the ingredient responsible for the presence of distributed quantum correlations, while local coherence also contributes to the quantumness of the state. A new quantifier, the "hookup", is introduced within such a framework. We also provide a simple physical interpretation, in terms of coherence, of the difference between total correlations and the sum of classical and quantum correlations obtained using relative-entropy-based quantifiers.

Coherence and non-classicality of quantum Markov processes

Although quantum coherence is a basic trait of quantum mechanics, the presence of coherences in the quantum description of a certain phenomenon does not rule out the possibility to give an alternative description of the same phenomenon in purely classical terms. Here, we give definite criteria to determine when and to what extent quantum coherence is equivalent to non-classicality. We prove that a Markovian multi-time statistics obtained from repeated measurements of a non-degenerate observable cannot be traced back to a classical statistics if and only if the dynamics generates coherences and subsequently turns them into populations. Furthermore, we show with simple examples that such connection between quantum coherence and non-classicality is generally absent if the statistics is non-Markovian.

Quantum Simulation of Single-Qubit Thermometry Using Linear Optics.

Standard thermometry employs the thermalization of a probe with the system of interest. This approach can be extended by incorporating the possibility of using the nonequilibrium states of the probe and the presence of coherence. Here, we illustrate how these concepts apply to the single-qubit thermometer introduced by Jevtic etal. [Phys. Rev. A 91, 012331 (2015)PLRAAN1050-294710.1103/PhysRevA.91.012331] by performing a simulation of the qubit-environment interaction in a linear-optical device. We discuss the role of the coherence and how this affects the usefulness of nonequilibrium conditions. The origin of the observed behavior is traced back to how the coherence affects the propensity to thermalization. We discuss this aspect by considering the availability function.

Strong quantum memory at resonant Fermi edges revealed by shot noise.

Studies of non-equilibrium current fluctuations enable assessing correlations involved in quantum transport through nanoscale conductors. They provide additional information to the mean current on charge statistics and the presence of coherence, dissipation, disorder, or entanglement. Shot noise, being a temporal integral of the current autocorrelation function, reveals dynamical information. In particular, it detects presence of non-Markovian dynamics, i.e., memory, within open systems, which has been subject of many current theoretical studies. We report on low-temperature shot noise measurements of electronic transport through InAs quantum dots in the Fermi-edge singularity regime and show that it exhibits strong memory effects caused by quantum correlations between the dot and fermionic reservoirs. Our work, apart from addressing noise in archetypical strongly correlated system of prime interest, discloses generic quantum dynamical mechanism occurring at interacting resonant Fermi edges.

Moving toward “laboratory‐supported” criteria for psychogenic tremor

A confident clinical diagnosis of psychogenic tremor is often possible, but, in some cases, a “laboratory-supported” level of certainty would aid in early positive diagnosis. Various electrophysiological tests have been suggested to identify patients with psychogenic tremor, but their diagnostic reliability has never been assessed “head to head” nor compared to forms of organic tremor other than essential tremor or PD. We compared baseline tremor characteristics (e.g., frequency and amplitude) as well as electrophysiological tests previously reported to distinguish psychogenic and organic tremor in a cohort of 13 patients with psychogenic tremor and 25 patients with organic tremor, the latter including PD, essential-, dystonic-, and neuropathic tremors. We assessed between-group differences and calculated sensitivity and specificity for each test. A number of tests, including entrainment or frequency changes with tapping, pause of tremor during contralateral ballistic movements, increase in tremor amplitude with loading, presence of coherence, and tonic coactivation at tremor onset, revealed significant differences on a group level, but there was no single test with adequate sensitivity and specificity for separating the groups (33%–77% and 84%–100%, respectively). However, a combination of electrophysiological tests was able to distinguish psychogenic and organic tremor with excellent sensitivity and specificity. A laboratory-supported level of diagnostic certainty in psychogenic tremor is likely to require a battery of electrophysiological tests to provide sufficient specificity and sensitivity. Our data suggest such a battery that, if supported in a prospective study, may form the basis of laboratory-supported criteria for the diagnosis of psychogenic tremor. © 2011 Movement Disorder Society

JONATHAN ALLISON (ed.). Letters of Louis MacNeice.

In November 1940, having decided after much vacillation to spend the war in England, Louis MacNeice was on the verge of making the hazardous journey back across the Atlantic from New York. Writing from the evocative address of 7 Middagh Street, Brooklyn Heights, home to W. H. Auden and Gypsy Rose Lee (and the eponymous subject of a fine Paul Muldoon poem), MacNeice instructed his friend, the Irish classicist E. R. Dodds, about what to do in the event of his death with his literary remains. In case ‘any mug’ wants to publish his correspondence: ‘I do not want any letters to my father or stepmother to be published as they nearly always contain some falsity. I also regret most of my undergraduate letters (esp. to Anthony Blunt) which are nearly always v. affected & forced but I suppose they might be interesting to social historians’ (p. 415). In this generous selection of the poet’s letters, Jonathan Allison has thankfully ignored his wartime wishes (as well as braving his pre-emptive insult). However, MacNeice’s judgement on the inauthentic nature of these letters points to a more fundamental elusiveness. In contrast to several vivid volumes of twentieth-century literary correspondence recently published—such as those by Ted Hughes and Samuel Beckett—the voice in these letters lacks a certain coherent presence; MacNeice remains, as ever, aloof. But to view this quality as a failure—the poet somehow neglecting to provide the requisite mix of psychological revelation, intellectual muscle-flexing and literary sensibility—would be to misunderstand MacNeice’s literary achievements, pass over the unsettling darkness lurking below so much of the shimmering surface of his life and work, and to neglect the considerable insights this volume offers readers and scholars beyond the distorting category of character.

Propagation and Beam Geometry Effects on Two-Dimensional Fourier Transform Spectra of Multilevel Systems

Four-level two-dimensional (2D) Fourier transform relaxation spectra are simulated with response functions for a chromophore pair in the exponential relaxation (optical Bloch model) limit. The parameters in this study are chosen to model coupled carbonyl stretching vibrations. As long as coherence persists, every peak in the real 2D spectra has a partially mixed absorptive/dispersive ("phase-twisted") shape because the nonlinear signals are not symmetric with respect to interchange of the first two pulses. This asymmetry in 2D relaxation spectra arises from coherence between singly excited states and a red shift of the doubly excited state. Coherence between the singly excited states causes oscillation of the 2D spectra and the associated spectrally resolved pump-probe (SRPP) transients at the quantum beat frequency. Projecting the phase-twisted nature of the 2D peaks onto the detection frequency axis, the SRPP peaks are also asymmetric about their maximum when not at maximum or minimum amplitude. Three-dimensional Fourier transform (3DFT) methods are used to simulate absorption/dispersion and beam geometry distortions of the multilevel 2D spectra with cross peaks. The distortions can be understood by consideration of their effects on individual coherence pathways that contribute to peaks in the 2D spectra. The beam geometry distortion explains some unequal cross peak amplitudes previously observed experimentally by Khalil et al. (J. Chem. Phys. 2004, 121, 362). A representation of 2D spectra that reduces beam geometry distortion is presented. If the transformation to correct for beam geometry distortion is combined with the transformations that correct absorptive/dispersive propagation distortions (J. Chem. Phys. 2007, 126, 044511), the recovered 2D spectrum matches the ideal 2D spectrum after all coherence is destroyed. In the presence of coherence, the new representation reduces the error in the distorted 2D spectrum by a factor of 4 for practical 2D-IR experimental conditions.

Back scattering from nano-sized ZnO colloids

We report enhanced back scattering in nanometer-sized ZnO colloids prepared in two different media, by different methods. The FWHM of the back scattered cone and hence the mean free path varied with concentration of ZnO as well as particle size. The Lorentzian profile of backscattered cone indicates the presence of coherence.

Cumulants, coherence, and contamination in multiparticle Bose-Einstein interferometry.

We examine the formalism of multiparticle correlations used in Bose-Einstein interferometry with pions produced in ultrarelativistic heavy ion collisions. We include incoherent and quantum optics coherent contributions as well as the effect of contamination from particles included in the correlation that are not pions. We give expressions for the correlation functions and normalized cumulants for orders 2{endash}5 in the presence of these effects. We show that in the presence of coherence the normalized cumulants include an additional contribution besides that usually called the {open_quote}{open_quote}true{close_quote}{close_quote} multiparticle correlation. We also consider the {ital Q}=0 intercepts of the correlation functions and normalized cumulants in the presence of coherence and of contamination and show that values of the intercept of the normalized cumulant as a function of order can distinguish these two effects. {copyright} {ital 1996 The American Physical Society.}