arXiv subjects

Sheng-Wen Li

Publications and source records attributed to Sheng-Wen Li.

36 records · Page 2Linked to original sources

arXiv2018

Entropy dynamics of a dephasing model in a squeezed thermal bath

We study the entropy dynamics of a dephasing model, where a two-level system (TLS) is coupled with a squeezed thermal bath via non-demolition interaction. This model is exactly solvable, and the time dependent states of both the TLS and its bath can be obtained exactly. Based on these states, we calculate the entropy dynamics of both the TLS and the bath, and find that the dephasing rate of the system relies on the squeezing phase of the bath. In zero temperature and high temperature limits, we prove that both the system and bath entropy increases monotonically. Moreover, we find that the dephasing rate of the system relies on the squeezing phase of the bath, and this phase dependence cannot be precisely derived from the Born-Markovian approximation which is widely adopted in open quantum systems.

quant-ph↗

arXiv2017

Quantum statistics of a single-atom Scovil-Schulz-DuBois heat engine

We study the statistics of the lasing output from a single atom quantum heat engine, which was originally proposed by Scovil and Schulz-DuBois (SSDB). In this heat engine model, a single three-level atom is strongly coupled with an optical cavity, and contacted with a hot and a cold heat bath together. We derive a fully quantum laser equation for this heat engine model, and obtain the photon number distribution for both below and above the lasing threshold. With the increase of the hot bath temperature, the population is inverted and lasing light comes out. However, we notice that if the hot bath temperature keeps increasing, the atomic decay rate is also enhanced, which weakens the lasing gain. As a result, another critical point appears at a very high temperature of the hot bath, after which the output light become thermal radiation again. To avoid this double-threshold behavior, we introduce a four-level heat engine model, where the atomic decay rate does not depend on the hot bath temperature. In this case, the lasing threshold is much easier to achieve, and the double-threshold behavior disappears.

quant-ph↗

arXiv2017

The production rate of the system-bath mutual information

When an open system is contacted with several thermal baths, the entropy produced by the irreversible processes ($dS_{\mathrm{i}}=dS-\sum_α\textđQ_α/T_α$) keeps increasing, and this entropy production rate is always non-negative. But when the system is contacted with some non-thermal baths containing quantum coherence or squeezing, this entropy production formula does not apply. In this paper, we study the increasing rate of the mutual information between the open system and its environment. When the baths are canonical thermal ones, we prove that this mutual information production rate could exactly return to the previous entropy production rate. Further, we study an example of a single boson mode contacted with multiple squeezed thermal baths, where the conventional entropy production rate does not apply, and we find that this mutual information production rate still keeps non-negative, which means the monotonic increasing of the correlation between the system and its environment.

quant-ph↗

arXiv2017

Photon-blockade induced photon anti-bunching in photosynthetic Antennas with cyclic structures

One of the important nonclassical effects in quantum optics is the anti-bunching, which has been observed in a large class of physical systems - including light-harvesting antennas with cyclic structures. The units of the ring couple with adjacent ones through dipole-dipole interactions. We show how this strong dipole-dipole interaction leads to photon-blockade resulting in the suppression of double excitation pathway and anti-bunching in photosynthesis systems. The robustness of the photon blockade is demonstrated against the disorder in the ring structures. We hypothesis that the effect may be utilized by light-harvesting systems to avoid damages from excess energy.

physics.chem-ph↗

arXiv2016

Non-Markovianity in a non-thermal bath

We study the dynamics of an open quantum system interacting with a non-thermal bath. Here, "non-thermal" means that the bath modes do not need to have the same temperature, but they have an effective temperature distribution. We find that, when a quantum system is interacting with such a non-thermal bath far from thermal equilibrium, it is no longer proper to use any coarse-grained Markovian description for the system, even when their coupling strength is quite weak. Especially, when there is coherent transition with strong interference strength in the quantum system, the Markovian master equation would bring in a serious problem of negative probability. After we consider some proper non-Markovian corrections, the problem can be naturally resolved.

quant-ph↗

arXiv2016

Photoelectric converters with quantum coherence

Photon impingement is capable of liberating electrons in electronic devices and driving the electron flux from the lower chemical potential to higher chemical potential. Previous studies hinted that the thermodynamic efficiency of a nano-sized photoelectric converter at maximum power is bounded by the Curzon-Ahlborn efficiency. In this study, we apply quantum effects to design a photoelectric converter based on a three-level quantum dot (QD) interacting with fermionic baths and photons. We show that, by adopting a pair of suitable degenerate states, quantum coherences induced by the couplings of quantum dots (QDs) to sunlight and fermion baths can coexist steadily in nano-electronic systems. Our analysis indicates that the efficiency at maximum power is no more limited to Curzon-Ahlborn efficiency through manipulation of carefully controlled quantum coherences.

quant-ph↗

arXiv2015

Quantum optics in a non-inertial reference frame: the Rabi splitting in a rotating ring cavity

We study quantum optics with the atoms coupled to the quantized electromagnetic (EM) field in a non-inertial reference frame by making use of quantum field theory in curved spacetime. We rigorously establish the microscopic model for a two-level atom interacting with the quantized EM field in a rotating ring cavity by deriving a Jaynes-Cummings (JC) type Hamiltonian. Due to the two fold degeneracy of the ring cavity modes, the Rabi splitting exhibits three rather than two resonant frequency peaks. We find that the heights of the two side peaks show a sensitive linear dependence on the rotating velocity. This high sensitivity can be utilized to detect the angular velocity of the whole system.

quant-ph↗

arXiv2015

Steady quantum coherence in non-equilibrium environment

We study the steady state of a three-level system in contact with a non-equilibrium environment, which is composed of two independent heat baths at different temperatures. We derive a master equation to describe the non-equilibrium process of the system. For the three level systems with two dipole transitions, i.e., the $Λ$-type and V-type, we find that the interferences of two transitions in a non-equilibrium environment can give rise to non-vanishing steady quantum coherence, namely, there exist non-zero off-diagonal terms in the steady state density matrix (in the energy representation). Moreover, the non-vanishing off-diagonal terms increase with the temperature difference of the two heat baths. Such interferences of the transitions were usually omitted by secular approximation, for it was usually believed that they only take effect in short time behavior and do not affect the steady state. Here we show that, in non-equilibrium systems, such omission would lead to the neglect of the steady quantum coherence.

quant-ph↗

arXiv2014

Entropy Production of Open Quantum System in Multi-Bath Environment

We study the entropy production of an open quantum system surrounded by a complex environment consisting of several heat baths at different temperatures. The detailed balance is elaborated in view of the distinguishable channels provided by the couplings to different heat baths, and a refined entropy production rate is derived accordingly. It is demonstrated that the entropy production rates can characterize the quantum statistical property of the baths: the bosonic and fermionic baths display different behaviors in the high-temperature limit while they have the same asymptotic behavior at low temperature.

cond-mat.stat-mech↗

arXiv2014

Synchro-thermalization of composite quantum system

We study the thermalization of a composite quantum system consisting of several subsystems, where only a small one of the subsystem contacts with a heat bath in equilibrium, while the rest of the composite system is contact free. We show that the whole composite system still can be thermalized after a relaxation time long enough, if the energy level structure of the composite system is connected, which means any two energy levels of the composite system can be connected by direct or indirect quantum transitions. With an example where an multi-level system interacts with a set of harmonic oscillators via non-demolition coupling, we find that the speed of relaxation to the global thermal state is suppressed by the multi-Franck-Condon factor due to the displacements of the Fock states when the degrees of freedom is large.

quant-ph↗

arXiv2014

Non-canonical statistics of finite quantum system

The canonical statistics describes the statistical properties of an open system by assuming its coupling with the heat bath infinitesimal in comparison with the total energy in thermodynamic limit. In this paper, we generally derive a non-canonical distribution for the open system with a finite coupling to the heat bath, which deforms the energy shell to effectively modify the conventional canonical way. The obtained non-canonical distribution reflects the back action of system on the bath, and thus depicts the statistical correlations through energy fluctuations.

cond-mat.stat-mech↗

arXiv2014

Long-term effect of inter-mode transitions in quantum Markovian process

We study the Markovian process of a multi-mode open system connecting with a non-equilibrium environment, which consists of several heat baths with different temperatures. As an illustration, we study the steady state of three linearly coupled harmonic oscillators in long time evolution, two of which contact with two independent bosonic heat baths with different temperatures respectively. We show that the inter-mode transitions mediated by the environment is responsible for the long time behaviour of the dynamics evolution, which is usually considered to take effect only in short time dynamics of the system immersed in a equilibrium heat bath with a single temperature. In non-equilibrium systems, the ignorance of the inter-mode transitions may give rise to some counter-intuitive result, thus they cannot be neglected.

quant-ph↗

arXiv2013

Probing zero-modes of defect in Kitaev quantum wire

The Kitaev quantum wire (KQW) model with open boundary possesses two Majorana edge modes. When the local chemical potential on a defect site is much higher than that on other sites and than the hopping energy, the electron hopping is blocked at this site. We show that the existence of such a defect on a closed KQW also gives rise to two low-energy modes, which can simulate the edge modes. The energies of the defect modes vanish to zero as the local chemical potential of the defect increase to infinity. We develop a quantum Langevin equation to study the transport of KQW for both open and closed cases. We find that when the lead is contacted with the site beside the defect, we can observe two splitted peaks around the zero-bias voltage in the differential conductance spectrum. While if the lead is contacted with the bulk of the quantum wire far from the the defect or the open edges, we cannot observe any zero-bias peak.

quant-ph↗

arXiv2013

Fidelity of remote state preparation can be enhanced by local operation

Remote state preparation (RSP) is a quantum information protocol which allows preparing a quantum state at a distant location with the help of a preshared nonclassical resource state and a classical channel. The efficiency of successfully doing this task can be represented by the RSP-fidelity of the resource state. In this paper, we study the influence on the RSP-fidelity by applying certain local operations on the resource state. We prove that RSP-fidelity does not increase for any unital local operation. However, for nonunital local operation, such as local amplitude damping channel, we find that some resource states can be enhanced to increase the RSP-fidelity. We give the optimal parameter of symmetric local amplitude damping channel for enhancing Bell-diagonal resource states. In addition, we show RSP-fidelity can suddenly change or even vanish at instant under local decoherence.

quant-ph↗

arXiv2011

Quantum refrigerator driven by current noise

We proposed a scheme to implement a self-contained quantum refrigerator system composed of three rf-SQUID qubits, or rather, flux-biased phase qubits. The three qubits play the roles of the target, the refrigerator and the heat engine respectively. We provide different effective temperatures for the three qubits, by imposing external current noises of different strengths. The differences of effective temperatures give rise to the flow of free energy and that drives the refrigerator system to cool down the target. We also show that the efficiency of the system approaches the Carnot efficiency.

quant-ph↗

arXiv2010

Quantum correlations in a cluster-like system

We discuss a cluster-like 1D system with triplet interaction. We study the topological properties of this system. We find that the degeneracy depends on the topology of the system, and well protected against external local perturbations. All these facts show that the system is topologically ordered. We also find a string order parameter to characterize the quantum phase transition. Besides, we investigate two-site correlations including entanglement, quantum discord and mutual information. We study the different divergency behaviour of the correlations. The quantum correlation decays exponentially in both topological and magnetic phases, and diverges in reversed power law at the critical point. And we find that in TQPT systems, the global difference of topology induced by dimension can be reflected in local quantum correlations.

quant-ph↗

arXiv2010

Tunneling Qubit Operation on a Protected Josephson Junction Array

We discuss a protected quantum computation process based on a hexagon Josephson junction array. Qubits are encoded in the punctured array, which is topologically protected. The degeneracy is related to the number of holes. The topological degeneracy is lightly shifted by tuning the flux through specific hexagons. We also show how to perform single qubit operation and basic quantum gate operations in this system.

quant-ph↗

arXiv2009

Quantum correlations in topological quantum phase transitions

We study the quantum correlations in a 2D system that possesses a topological quantum phase transition. The quantumness of two-body correlations is measured by quantum discord. We calculate both the correlation of two local spins and that of an arbitrary spin with the rest of the lattice. It is notable that local spins are classically correlated, while the quantum correlation is hidden in the global lattice. This is different from other systems which are not topologically orderd. Moreover, the mutual information and global quantum discord show critical behavior in the topological quantum phase transition.

quant-ph↗