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Steering-induced coherence in decoherence channels
Abstract
We study steering-induced coherence (SIC) in local decoherence channels. Two types of channels are considered in this work, one is the amplitude damping channel under rotating-wave approximation (RWA) and the other the non-RWA channel. Unlike the amplitude damping channel, the system-environment interaction of the non-RWA channel is treated without RWA. We use the hierarchy equation method to calculate the dynamical evolution of SIC and B-side measurement-induced disturbance (MID). For the two channels, SIC decreases with time monotonically or oscillatingly due to the decoherence effect. Moreover, the different behavior of SIC between the two channels are also shown through the numerical results. SIC and MID obviously exhibit more oscillatory behavior in the non-RWA channel compared with the cases in the RWA channel. In some regions of system-environment coupling, the value of SIC (MID) in the non-RWA channel is larger than that in the RWA channel. However, in the strong non-Markovian regions, SIC (MID) drops to zero in the RWA channel, while it does not happen in the non-RWA channels. Additionally, we find that SIC can be generated from the states without entanglement.
... To overcome the difficulty of the experimental demonstration, the nonasymptotic settings of the assisted coherence distillation were proposed [26,27]. Different from the distillation framework above, in [28] the authors introduced a scenario of steering-induced coherence, which is defined on the eigenvectors of the considered system, and has been conveniently used in open systems [29]. ...
We investigate the issue of assisted coherence distillation in the asymptotic limit, by coordinately performing the identical local operations on the auxiliary systems of each copy. When we further restrict the coordinate operations to projective measurements, the distillation process is branched into many sub-processes. Finally, a computable measure of the assisted distillable coherence is derived as the maximal average coherence of the residual states with the maximization taken over all the projective measurements on the auxiliary. The measure can be conveniently used to evaluate the assisted distillable coherence in experiments, especially suitable for the case that the system and its auxiliary are in mixed states. By using the measure, we for the first time study the assisted coherence distillation in multipartite systems. Monogamy-like inequalities are derived to constrain the distribution of the assisted distillable coherence in the subsystems. Taking three-qubit system for example, we experimentally prepare two types of tripartite correlated states, i.e., the W-type and GHZ-type states in a linear optical setup, and experimentally test the assisted distillable coherence. Theoretical and experimental results agree well to verify the distribution inequalities given by us. Three measures of multipartite quantum correlation are also considered. The close relationship between the assisted coherence distillation and the multipartite correlation is revealed.
... To overcome the difficulty of the experimental demonstration, the nonasymptotic settings of the assisted coherence distillation was proposed [26,27]. Different from the distillation framework above, in [28] the authors introduced a scenario of steering-induced coherence, which is defined on the eigenvectors of the considered system, and has been conveniently used in open systems [29]. ...
We investigate the issue of assisted coherence distillation in the asymptotic limit (considering infinite copies of the resource states), by coordinately performing the identical local operations on the auxiliary systems of each copy. When we further restrict the coordinate operations to projective measurements, the distillation process is branched into many sub-processes. Finally, a simple formula is given that the assisted distillable coherence should be the maximal average coherence of the residual states. The formula makes the experimental research of assisted coherence distillation possible and convenient, especially for the case that the system and its auxiliary are in mixed states. By using the formula,\ we for the first time study the assisted coherence distillation in multipartite systems. Monogamy-like inequalities are given to constrain the distribution of the assisted distillable coherence in the subsystems. Taking three-qubit system for example, we experimentally prepare two types of tripartite correlated states, i.e., the $W$-type and GHZ-type states in a linear optical setup, and experimentally explore the assisted coherence distillation. Theoretical and experimental results agree well to verify the distribution inequalities given by us. Three measures of multipartite quantum correlation are also considered. The close relationship between the assisted coherence distillation and the genuine multipartite correlation is revealed.
We introduce and study the l1-norm coherence of assistance both theoretically and operationally. We first provide an upper bound for the l1-norm coherence of assistance and show a necessary and sufficient condition for the saturation of the upper bound. For two- and three-dimensional quantum states, the analytical expression of the l1-norm coherence of assistance is given. Operationally, the mixed quantum coherence can always be increased with the help of another party's local measurement and one way classical communication since the l1-norm coherence of assistance, as well as the relative-entropy coherence of assistance, are shown to be strictly larger than the original coherence. The relation between the l1-norm coherence of assistance and entanglement is revealed. Finally, a comparison between the l1-norm coherence of assistance and the relative-entropy coherence of assistance is made.
Quantum resource theories seek to quantify sources of non-classicality that bestow quantum technologies their operational advantage. Chief among these are studies of quantum correlations and quantum coherence. The former to isolate non-classicality in the correlations between systems, the latter to capture non-classicality of quantum superpositions within a single physical system. Here we present a scheme that cyclically inter-converts between these resources without loss. The first stage converts coherence present in an input system into correlations with an ancilla. The second stage harnesses these correlations to restore coherence on the input system by measurement of the ancilla. We experimentally demonstrate this inter-conversion process using linear optics. Our experiment highlights the connection between non-classicality of correlations and non-classicality within local quantum systems, and provides potential flexibilities in exploiting one resource to perform tasks normally associated with the other.
We experimentally simulate a quantum channel in a linear optical setup, which is modeled by a two-level system (i.e., qubit) interacting with a bosonic bath. Unlike the traditional works, we treat the system–bath interaction without applying the Born approximation, the Markov approximation, or the rotating-wave approximation (RWA). To the best of our knowledge, this is the first experimental simulation of a quantum channel without any of the approximations mentioned above by using linear optical devices. This non-RWA channel provides a more accurate picture of the quantum open-system dynamics. It not only reveals the effect of the counterrotating terms but also enables us to consider arbitrarily strong coupling regimes. With the proposed channel, we further experimentally investigate the dynamics of the quantum temporal steering (TS), i.e., a temporal analog of Einstein–Podolsky–Rosen steering. The experimental and theoretical results are in good agreement and show that the counterrotating terms significantly influence the TS dynamics. The TS in non-RWA and RWA channels presents different dynamics. However, we emphasize that the results without RWA are closer to realistic situations and thus more reliable. Due to the close relationship between TS and the security of the quantum cryptographic protocols, our findings are expected to have useful applications in secure quantum communications. This work also inspires future interest in studying other quantum coherence properties in the non-RWA channels.
The search for a simple description of fundamental physical processes is an important part of quantum theory. One example for such an abstraction can be found in the distance lab paradigm: if two separated parties are connected via a classical channel, it is notoriously difficult to characterize all possible operations these parties can perform. This class of operations is widely known as local operations and classical communication. Surprisingly, the situation becomes comparably simple if the more general class of separable operations is considered, a finding that has been extensively used in quantum information theory for many years. Here, we propose a related approach for the resource theory of quantum coherence, where two distant parties can perform only measurements that do not create coherence and can communicate their outcomes via a classical channel. We call this class local incoherent operations and classical communication. While the characterization of this class is also difficult in general, we show that the larger class of separable incoherent operations has a simple mathematical form, yet still preserves the main features of local incoherent operations and classical communication. We demonstrate the relevance of our approach by applying it to three different tasks: assisted coherence distillation, quantum teleportation, and single-shot quantum state merging. We expect that the results we obtain in this work also transfer to other concepts of coherence that are discussed in recent literature. The approach we present here opens new ways to study the resource theory of coherence in distributed scenarios.
Quantum coherence, which quantifies the superposition properties of a quantum state, plays an indispensable role in quantum resource theory. A recent theoretical work [Phys. Rev. Lett. 116, 070402 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.070402] studied the manipulation of quantum coherence in bipartite or multipartite systems under the protocol local incoherent operation and classical communication (LQICC). Here we present what we believe is the first experimental realization of obtaining maximal coherence in the assisted distillation protocol based on a linear optical system. The results of our work show that the optimal distillable coherence rate can be reached even in one-copy scenario when the overall bipartite qubit state is pure. Moreover, the experiments for mixed states showed that distillable coherence can be increased with less demand than entanglement distillation. Our work might be helpful in remote quantum information processing and quantum control.
As the precious resource for quantum information processing, quantum coherence can be created remotely if the involved two sites are quantum correlated. It can be expected that the amount of coherence created should depend on the quantity of the shared quantum correlation, which is also a resource. Here, we establish an operational connection between coherence induced by steering and the quantum correlation. We find that the steering-induced coherence quantified by such as relative entropy of coherence and trace-norm of coherence is bounded from above by a known quantum correlation measure defined as the one-side measurement-induced disturbance. The condition that the upper bound saturated by the induced coherence varies for different measures of coherence. The tripartite scenario is also studied and similar conclusion can be obtained. Our results provide the operational connections between local and non-local resources in quantum information processing.
We study a quantum heat engine at strong coupling between the system and the thermal reservoirs. Exploiting a collective coordinate mapping, we incorporate system-reservoir correlations into a consistent thermodynamic analysis, thus circumventing the usual restriction to weak coupling and vanishing correlations. We apply our formalism to the example of a quantum Otto cycle, demonstrating that the performance of the engine is diminished in the strong coupling regime with respect to its weakly coupled counterpart, producing a reduced net work output and operating at a lower energy conversion efficiency. We identify costs imposed by sudden decoupling of the system and reservoirs around the cycle as being primarily responsible for the diminished performance, and define an alternative operational procedure which can partially recover the work output and efficiency. More generally, the collective coordinate mapping holds considerable promise for wider studies of thermodynamic systems beyond weak reservoir coupling.
We propose a method to study the thermodynamic behaviour of small systems beyond the weak coupling and Markovian approximation, which is different in spirit from conventional approaches. This method -- based on a reaction coordinate mapping -- is general in the sense that it can be applied to an arbitrary (quantum or classical and even time-dependent) system coupled linearly to an arbitrary number of harmonic oscillator reservoirs. The core of the method relies on an appropriate identification of a part of the environment (the reaction coordinate), which is subsequently included as a part of the system. This supersystem is then treated within the weak coupling and Markovian approximations, ensuring that we obtain a consistent thermodynamic picture. We demonstrate the power of this concept by showing that non-Markovian effects can significantly enhance the steady state efficiency of a three-level-maser heat engine, even in the regime of weak system-bath coupling. Furthermore, we show for a single electron transistor coupled to vibrations that our method allows one to justify the widely used polaron transformation.
We investigate Landau-Zener processes modeled by a two-level quantum system,
with its finite bias energy varied in time and in the presence of a single
broadened cavity mode at zero temperature. By applying the hierarchy equation
method to the Landau-Zener problem, we computationally study the survival
fidelity of adiabatic states without Born, Markov, rotating-wave or other
perturbative approximations. With this treatment it also becomes possible to
investigate cases with very strong system-bath coupling. Different from a
previous study of infinite-time Landau-Zener processes, the fidelity of the
time-evolving state as compared with instantaneous adiabatic states shows
non-monotonic dependence on the system-bath coupling and on the sweep rate of
the bias. We then consider the effect of applying a counter-diabatic driving
field, which is found to be useful in improving the fidelity only for
sufficiently short Landau-Zener processes. Numerically exact results show that
different counter-diabatic driving fields can have much different robustness
against environment effects. Lastly, using a case study we discuss the
possibility of introducing a dynamical decoupling field in order to eliminate
the decoherence effect of the environment and at the same time to retain the
positive role of a counter-diabatic field. Our work indicates that finite-time
Landau-Zener processes with counter-diabatic driving offer a fruitful test bed
to understand controlled adiabatic processes in open systems.
Recent results in quantum information theory characterize quantum coherence
in the context of resource theories. Here we study the relation between quantum
coherence and quantum discord, a kind of quantum correlation which appears even
in non-entangled states. We prove that the creation of quantum discord with
multipartite incoherent operations is bounded by the amount of quantum
coherence consumed in its subsystems during the process. We show how the
interplay between quantum coherence consumption and creation of quantum discord
works in the preparation of multipartite quantum correlated states and in the
model of deterministic quantum computation with one qubit (DQC1).
We introduce and study the task of assisted coherence distillation. This task
arises naturally in bipartite systems where both parties work together to
generate the maximal possible coherence on one of the subsystems. Only
incoherent operations are allowed on the target system while general local
quantum operations are permitted on the other, an operational paradigm that we
call local quantum-incoherent operations and classical communication (LQICC).
We show that the asymptotic rate of assisted coherence distillation for pure
states is equal to the coherence of assistance, a direct analog of the
entanglement of assistance, whose properties we characterize. Our findings
imply a novel interpretation of the von Neumann entropy: it quantifies the
maximum amount of extra quantum coherence a system can gain when receiving
assistance from a collaborative party. Our results are generalized to coherence
localization in a multipartite setting and possible applications are discussed.
We critically assess the problem of extracting work from a coherent
superposition of energy eigenstates of an individual qubit system. By carefully
taking into account all the resources involved in the thermodynamic
transformations in a fully quantum-mechanical treatment, we show that there
exists a thermal machine that can come arbitrarily close to extracting all the
coherence as work. The machine only needs to act on individual copies of a
state and can be reused. On the other hand, we show that for any thermal
machine with finite resources not all the coherence of a state can be extracted
as work. We provide explicit protocols extracting work from coherence when the
resources of a thermal machine are bounded, a scenario potentially relevant for
the thermodynamics at the nanoscale.
We derive an easily computable quantum speed limit (QSL) time bound for open systems whose initial states can be chosen as either pure or mixed states. Moreover, this QSL time is applicable to either Markovian or non-Markovian dynamics. By using of a hierarchy equation method, we numerically study the QSL time bound in a qubit system interacting with a single broadened cavity mode without rotating-wave, Born and Markovian approximation. By comparing with rotating-wave approximation (RWA) results, we show that the counter-rotating terms are helpful to increase evolution speed. The problem of non-Markovianity is also considered. We find that for non-RWA cases, increasing system-bath coupling can not always enhance the non-Markovianity, which is qualitatively different from the results with RWA. When considering the relation between QSL and non-Markovianity, we find that for small broadening widths of the cavity mode, non-Markovianity can increase the evolution speed in either RWA or non-RWA cases, while, for larger broadening widths, it is not true for non-RWA cases.
We study the coherence trapping of a qubit correlated initially with a
non-Markovian bath in a pure dephasing channel. By considering the initial
qubit-bath correlation and the bath spectral density, we find that the initial
qubit-bath correlation can lead to a more efficient coherence trapping than
that of the initially separable qubit-bath state. The stationary coherence in
the long time limit can be maximized by optimizing the parameters of the
initially correlated qubit-bath state and the bath spectral density. In
addition, the effects of this initial correlation on the maximal evolution
speed for the qubit trapped to its stationary coherence state are also
explored.
In recent years it was recognized that properties of physical systems such as
entanglement, athermality, and asymmetry, can be viewed as resources for
important tasks in quantum information, thermodynamics, and other areas of
physics. This recognition followed by the development of specific quantum
resource theories (QRTs), such as entanglement theory, determining how quantum
states that cannot be prepared under certain restrictions may be manipulated
and used to circumvent the restrictions. Here we show that all such QRTs have a
general structure, consisting of three components: free states,
restricted/allowed set of operations, and resource states. We show that under a
few physically motivated assumptions, a QRT is asymptotically reversible if its
set of allowed operations is maximal; that is, if the allowed operations are
the set of all operations that do not generate (asymptotically) a resource. In
this case, the asymptotic conversion rate is given in terms of the regularized
relative entropy of a resource which is the unique measure/quantifier of the
resource in the asymptotic limit of many copies of the state. We also show that
this measure equals the smoothed version of the logarithmic robustness of the
resource.
The first law of thermodynamics imposes not just a constraint on the
energy-content of systems in extreme quantum regimes, but also
symmetry-constraints related to the thermodynamic processing of quantum
coherence. Harmonic analysis allows any density operator to be decomposed in
terms of mode operators which quantify the coherence present in a state in a
natural way. We establish upper and lower bounds for the evolution of such
modes under arbitrary thermal operations, and analyse primitive coherence
manipulations. Well-studied thermo-majorization relations on block-diagonal
quantum states correspond to constraints only on the zero-mode of a state,
governing energy transfers. A complete set of constraints for non-zero modes,
governing coherence transfers, is at present unknown, but these modes are found
to display similar irreversibility, which demands greater study.
We find a new characterization of low-temperature processes, which we call
"cooling processes", incorporating quantum coherence in the model of
thermodynamics for the first time. We derive necessary and sufficient
conditions for the feasibility of state transitions under cooling processes. We
also rigorously confirm the intuitive robustness of coherence against
low-temperature thermal noise. Additionally, we develop the low-temperature
"Gibbs-preserving" model, and by comparing our results on the two models, we
argue that the latter is a poor approximation to physical processes.
We introduce a rigorous framework for the quantification of coherence and
identify intuitive and easily computable measures of coherence. We achieve this
by adopting the viewpoint of coherence as a physical resource. By determining
defining conditions for measures of coherence we identify classes of
functionals that satisfy these conditions and other, at first glance natural
quantities, that do not qualify as coherence measure. We conclude with an
outline of the questions that remain to be answered to complete the theory of
coherence as a resource.
We define a quantitative measure of coherent delocalization; similarly to the concept of entanglement measures, we require that a measure of coherent delocalization may never increase under processes that do not create coherent superpositions. After a complete characterization of such processes, we prove that a set of recently introduced functions that characterize coherent delocalization never grow under such processes and thus are indeed valid measures. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Quantum biology is an emerging field of research that concerns itself with
the experimental and theoretical exploration of non-trivial quantum phenomena
in biological systems. In this tutorial overview we aim to bring out
fundamental assumptions and questions in the field, identify basic design
principles and develop a key underlying theme -- the dynamics of quantum
dynamical networks in the presence of an environment and the fruitful interplay
that the two may enter. At the hand of three biological phenomena whose
understanding is held to require quantum mechanical processes, namely
excitation and charge transfer in photosynthetic complexes, magneto-reception
in birds and the olfactory sense, we demonstrate that this underlying theme
encompasses them all, thus suggesting its wider relevance as an archetypical
framework for quantum biology.
A complete treatment of the entanglement of two-level systems, which evolves
through the contact with a thermal bath, must include the fact that the system
and the bath are not fully separable. Therefore, quantum coherent
superpositions of system and bath states, which are almost never fully included
in theoretical models, are invariably present when an entangled state is
prepared experimentally. We show their importance for the time evolution of the
entanglement of two qubits coupled to independent baths. In addition, our
treatment is able to handle slow and low-temperature thermal baths.
Multiple displaced oscillators coupled to an Ohmic heat bath are used to describe electron transfer (ET) in a dissipative environment. By performing a canonical transformation, the model is reduced to a multilevel system coupled to a heat bath with the Brownian spectral distribution. A reduced hierarchy equations of motion approach is introduced for numerically rigorous simulation of the dynamics of the three-level system with various oscillator configurations, for different nonadiabatic coupling strengths and damping rates, and at different temperatures. The time evolution of the reduced density matrix elements illustrates the interplay of coherences between the electronic and vibrational states. The ET reaction rates, defined as a flux-flux correlation function, are calculated using the linear response of the system to an external perturbation as a function of activation energy. The results exhibit an asymmetric inverted parabolic profile in a small activation regime due to the presence of the intermediate state between the reactant and product states and a slowly decaying profile in a large activation energy regime, which arises from the quantum coherent transitions.
We investigate how the classical correlations between successive actions of a colored dephasing channel affect the decoherence of a two-qubit system. Our calculation shows that the l 1 norm of coherence, the relative entropy of coherence, and the robustness of coherence can be noticeably enhanced by increasing the strength of classical correlations in the whole time interval during which the two qubits pass through the channel, while the coherence weight is enhanced only when exceeds a critical value. Moreover, in the infinite-time limit, all the coherence measures reach their steady values which are determined only by .
The precision of measurements for two incompatible observables in a physical system can be improved with the assistance of quantum memory. In this paper, we investigate the quantum-memory-assisted entropic uncertainty relation for a spin-1 Heisenberg model in the presence of external magnetic fields, the systemic quantum entanglement (characterized by the negativity) is analyzed as contrast. Our results show that for the XY spin chain in thermal equilibrium, the entropic uncertainty can be reduced by reinforcing the coupling between the two particles or decreasing the temperature of the environment. At zero-temperature, the strong magnetic field can result in the growth of the entropic uncertainty. Moreover, in the Ising case, the variation trends of the uncertainty are relied on the choices of anisotropic parameters. Taking the influence of intrinsic decoherence into account, we find that the strong coupling accelerates the inflation of the uncertainty over time, whereas the high magnetic field contributes to its reduction during the temporal evolution. Furthermore, we also verify that the evolution behavior of the entropic uncertainty is roughly anti-correlated with that of the entanglement in the whole dynamical process. Our results could offer new insights into quantum precision measurement for the high spin solid-state systems.
The coherent superposition of states, in combination with the quantization of observables, represents one of the most fundamental features that mark the departure of quantum mechanics from the classical realm. Quantum coherence in many-body systems embodies the essence of entanglement and is an essential ingredient for a plethora of physical phenomena in quantum optics, quantum information, solid state physics, and nanoscale thermodynamics. In recent years, research on the presence and functional role of quantum coherence in biological systems has also attracted a considerable interest. Despite the fundamental importance of quantum coherence, the development of a rigorous theory of quantum coherence as a physical resource has only been initiated recently. In this Colloquium we discuss and review the development of this rapidly growing research field that encompasses the characterization, quantification, manipulation, dynamical evolution, and operational application of quantum coherence.
Considerable work has recently been directed toward developing resource theories of quantum coherence. In this Letter, we establish a criterion of physical consistency for any resource theory. This criterion requires that all free operations in a given resource theory be implementable by a unitary evolution and projective measurement that are both free operations in an extended resource theory. We show that all currently proposed basis-dependent theories of coherence fail to satisfy this criterion. We further characterize the physically consistent resource theory of coherence and find its operational power to be quite limited. After relaxing the condition of physical consistency, we introduce the class of dephasing-covariant incoherent operations as a natural generalization of the physically consistent operations. Necessary and sufficient conditions are derived for the convertibility of qubit states using dephasing-covariant operations, and we show that these conditions also hold for other well-known classes of incoherent operations.
We establish an operational theory of coherence (or of superposition) in
quantum systems. Namely, we introduce the two basic concepts --- "coherence
distillation" and "coherence cost" in the processing quantum states under
so-called \emph{incoherent operations} [Baumgratz \emph{et al.}, Phys. Rev.
Lett. 113:140401 (2014)]. We then show that in the asymptotic limit of many
copies of a state, both are given by simple single-letter formulas: the
distillable coherence is given by the relative entropy of coherence (in other
words, we give the relative entropy of coherence its operational
interpretation), and the coherence cost by the coherence of formation, which is
an optimization over convex decompositions of the state. An immediate corollary
is that there exists no bound coherent state in the sense that one would need
to consume coherence to create the state but no coherence could be distilled
from it. Further we demonstrate that the coherence theory is generically an
irreversible theory by a simple criterion that completely characterizes all the
reversible coherent states.
We study the interplay between coherence trapping, information back-flow and
the form of the reservoir spectral density for dephasing qubits. We show that
stationary coherence is maximized when the qubit undergoes non-Markovian
dynamics, and we elucidate the different roles played by the low and high
frequency parts of the environmental spectrum. We show that the low frequencies
fully determine the presence or absence of information back-flow while the high
frequencies dictate the maximal amount of coherence trapping.
This paper reviews the role of quantum mechanics in biological systems, and shows how the interplay between coherence and decoherence can strongly enhance quantum transport in photosynthesis.
In the study of open quantum systems, the polaron transformation has recently attracted a renewed interest as it offers the possibility to explore the strong system-bath coupling regime. Despite this interest, a clear and unambiguous analysis of the regimes of validity of the polaron transformation is still lacking. Here we provide such a benchmark, comparing second order perturbation theory results in the original untransformed frame, the polaron frame, and the variational extension with numerically exact path integral calculations of the equilibrium reduced density matrix. Equilibrium quantities allow a direct comparison of the three methods without invoking any further approximations as is usually required in deriving master equations. It is found that the second order results in the original frame are accurate for weak system-bath coupling; the results deteriorate when the bath cut-off frequency decreases. The full polaron results are accurate for the entire range of coupling for a fast bath but only in the strong coupling regime for a slow bath. The variational method is capable of interpolating between these two methods and is valid over a much broader range of parameters.
The role of quantum coherence in the dynamics of photosynthetic energy transfer in chromophoric complexes is not fully understood. In this work, we quantify the biological importance of fundamental physical processes, such as the excitonic Hamiltonian evolution and phonon-induced decoherence, by their contribution to the efficiency of the primary photosynthetic event. We study the effect of spatial correlations in the phonon bath and slow protein scaffold movements on the efficiency and the contributing processes. To this end, we develop two theoretical approaches based on a Green's function method and energy transfer susceptibilities. We investigate the Fenna-Matthews-Olson protein complex, in which we find a contribution of coherent dynamics of about 10% in the presence of uncorrelated phonons and about 30% in the presence of realistically correlated ones.
We derive transport equations for fermions and bosons in spatially or temporally varying backgrounds with special symmetries, by use of the Schwinger-Keldysh formalism. In a noninteracting theory the coherence information is shown to be encoded in new singular shells for the 2-point function. Imposing this phase space structure to the interacting theory leads to a a self-consistent equation of motion for a physcial density matrix, including coherence and a well defined collision integral. The method is applied e.g. to demonstrate how an initially coherent out-of-equlibrium state approaches equlibrium through decoherence and thermalization. Comment: 4 pages. To appear in the proceedings of the 8th Conference on Strong and Electroweak Matter (SEWM08), Amsterdam, the Netherlands, 26-29 August 2008
The concept of steering was introduced by Schrödinger in 1935 as a generalization of the Einstein-Podolsky-Rosen paradox for arbitrary pure bipartite entangled states and arbitrary measurements by one party. Until now, it has never been rigorously defined, so it has not been known (for example) what mixed states are steerable (that is, can be used to exhibit steering). We provide an operational definition, from which we prove (by considering Werner states and isotropic states) that steerable states are a strict subset of the entangled states, and a strict superset of the states that can exhibit Bell nonlocality. For arbitrary bipartite Gaussian states we derive a linear matrix inequality that decides the question of steerability via Gaussian measurements, and we relate this to the original Einstein-Podolsky-Rosen paradox.
Reduced equation of motion for a multimode system coupled to multiple heat baths is constructed by extending the quantum Fokker-Planck equation with low-temperature correction terms (J. Phys. Soc. Jpn. 2005, 74, 3131). Unlike such common approaches used to describe intramolecular multimode vibration as a Bloch-Redfield theory and a stochastic theory, the present formalism is defined by the molecular coordinates. To explore the correlation among different modes through baths, we consider two cases of system-bath couplings. One is a correlated case in which two modes are coupled to a single bath, and the other is an uncorrelated case in which each mode is coupled to a different bath. We further classify the correlated case into two cases, the plus- and minus-correlated cases, according to distinct correlation manners. For these, one-dimensional and two-dimensional infrared (2D-IR) spectra are calculated numerically by solving the equation of motion. It is demonstrated that 2D-IR spectroscopy has the ability to analyze the correlation of fluctuation-dissipation processes among different modes.
The entanglement of a pure state of a pair of quantum systems is defined as the entropy of either member of the pair. The entanglement of formation of a mixed state is defined as the minimum average entanglement of an ensemble of pure states that represents the given mixed state. An earlier paper [Phys. Rev. Lett. 78, 5022 (1997)] conjectured an explicit formula for the entanglement of formation of a pair of binary quantum objects (qubits) as a function of their density matrix, and proved the formula to be true for a special class of mixed states. The present paper extends the proof to arbitrary states of this system and shows how to construct entanglement-minimizing pure-state decompositions.
- J Aberg
Aberg J 2006 (arXiv:quant-ph/0612146)
- S Du
- Z Bai
- X Qi
Du S, Bai Z and Qi X 2015 Quantum Inf. Comput. 15 1307