Bin Shao

Quantum coherence rather than quantum correlations reflect the effects of a reservoir on a system's work capability.

We consider a model of an optical cavity with a nonequilibrium reservoir consisting of a beam of identical two-level atom pairs (TLAPs) in the general X state. We find that coherence of multiparticle nonequilibrium reservoir plays a central role on the potential work capability of the cavity. We show that no matter whether there are quantum correlations in each TLAP (including quantum entanglement and quantum discord) or not, the coherence of the TLAPs has an effect on the work capability of the cavity. Additionally, constructive and destructive interferences could be induced to influence the work capability of the cavity by adjusting only the relative phase, with which quantum correlations have nothing to do. In this paper, the coherence of the reservoir, rather than the quantum correlations, effectively reflecting the effects of the reservoir on the systems work capability is demonstrated clearly.

Quantum discord for a central two-qubit system coupled to an XY-spin-chain environment

We investigate the dynamic behaviors of quantum discord for a central two-qubit system coupled to an XY-spin-chain environment. In the weak-coupling regime, we show that the quantum discord for the two central qubits can become minimized rapidly close to the critical point of a quantum phase transition. By considering the two qubits that are initially prepared in the Werner state, we study the evolution of the quantum discord and that of entanglement under the same conditions. Our results imply that entanglement can disappear completely after a finite time, while the quantum discord decreases and tends to be a stable value according to the initial-state parameter for a very-long-time interval. In this sense, the quantum discord is more robust than entanglement for the quantum system exposed to the environment. The relation between the quantum correlations and the classical correlation is also shown for two particular cases.

The decoherence dynamics of multipartite entanglement in a non-Markovian environment

We consider four two-level atoms interacting with independent non-Markovian reservoirs with detuning. We mainly investigate the effects of the detuning and the length of the reservoir correlation time on the decoherence dynamics of multipartite entanglement. We find that the time evolution of the entanglement of atomic and reservoir subsystems is determined by a parameter which is a function of the detuning and the reservoir correlation time. We also find that the decay and revival of the entanglement of the atomic and reservoir subsystems are closely related to the sign of the decay rate. We also show that the cluster state is the most robust to decoherence compared with Dicke, GHZ and W states for this decoherence channel.

Feed-forward control for quantum state protection against decoherence

We propose a novel scheme of feed-forward control and its reversal for protecting quantum state against decoherence. Before the noise channel our pre-weak measurement and feed-forward are just to change the protected state into the state almost immune to the noise channel, and after the channel our reversed operations and post-weak measurements are just to restore the protected state. Unlike most previous state protection schemes, ours only concerns the noise channel and does not care about the protected state. We show that our scheme can effectively protect unknown states, nonorthogonal states and entangled states against amplitude damping noise. Our scheme has dramatic merits of protecting quantum states against heavy amplitude damping noise, and can perfectly protect some specific nonorthogonal states in an almost deterministic way, which might be found some applications in current quantum communication technology. And it is most important that our scheme is experimentally available with current technology.

Estimating the hyperfine coupling parameters of the avian compass by comprehensively considering the available experimental results.

Migratory birds can utilize the geomagnetic field for orientation and navigation through a widely accepted radical-pair mechanism. Although many theoretical works have been done, the available experimental results have not been fully considered, especially the temporary disorientation induced by the field which is increased by 30% of the geomagnetic field and the disorientation of the very weak resonant field of 15 nT. In this paper, we consider the monotonicity of the singlet yield angular profile as the prerequisite of direction sensitivity, and find that for some optimal values of the hyperfine coupling parameters (that is, the order of 10^{-7}∼10^{-6} meV) the experimental results available so far can be satisfied. We also investigate the effects of two decoherence environments and demonstrate that, in order to satisfy the available experimental results, the decoherence rate should be lower than the recombination rate. Finally, we investigate the effects of the fluctuating magnetic noises and find that the vertical noise destroys the monotonicity of the profile completely, but the parallel noise preserves the monotonicity perfectly and even can enhance the direction sensitivity.

Quantum speed limit and a signal of quantum criticality

We study the quantum speed limit time (QSLT) of a coupled system consisting of a central spin and its surrounding environment, and the environment is described by a general XY spin-chain model. For initial pure state, we find that the local anomalous enhancement of the QSLT occurs near the critical point. In addition, we investigate the QSLT for arbitrary time-evolution state in the whole dynamics process and find that the QSLT will decay monotonously and rapidly at a large size of environment near the quantum critical point. These anomalous behaviors in the critical vicinity of XY spin-chain environment can be used to indicate the quantum phase transition point. Especially for the XX spin-chain environment, we find that the QSLT displays a sudden transition from discontinuous segmented values to a steady value at the critical point. In this case, the non-Makovianity and the Loschmidt echo are incapable of signaling the critical value of the transverse field, while the QSLT can still witness the quantum phase transition. So, the QSLT provides a further insight and sharper identification of quantum criticality.

Quantum speed limit in a qubit-spin-bath system

We investigate the behavior of quantum speed limit (QSL) time for a typical non-Markovian system, a central spin coupled to a spin star configuration. We connect the QSL time with an external control and show that the effectiveness of the external magnetic field, as well as the coupling strength, is related to the fundamental bounds that affect the maximum speed at which a quantum system can evolve in its state space. We also demonstrate that a spin bath with larger size may shorten the QSL time, while the upper state population plays an important role for the acceleration of quantum evolution in the memory surrounding.

Non-classical effects from coupled field-mesoscopic Josephson junction system

Abstract An effective bosonic Hamiltonian describing the interaction of a mesoscopic Josephson junction with a quantized radiation field is studied. We find that when the field was initially prepared in a coherent state and the junction initially in its lowest energy level state, the state of the coupled system may evolve into a squeezed state. Our results show that as time evolves both the field and Josephson junction subsystems can exhibit nonclassical behaviour.

Improved thermometry of low-temperature quantum systems by a ring-structure probe

The thermometry precision of a sample is a question of both fundamental and technological importance. In this paper, we consider a ring-structure system as our probe to estimate the temperature of a bath. Based on the Markovian master equation of the probe, we calculate the quantum Fisher information (QFI) of the probe at any time. We find that for the thermal equilibrium thermometry, the ferromagnetic structure can measure a lower temperature of the bath with a higher precision compared with the nonstructure probe, while for the dynamical thermometry, the antiferromagnetic structure can make the QFI of the probe in the dynamical process much larger than that in equilibrium with the bath, which is somewhat counterintuitive. Moreover, the best accuracy for the thermometry achieved in the antiferromagnetic structure case can be much higher than that in the nonstructure case. The physical mechanisms of the above phenomena are given in this paper.

Controlling entanglement between two separated atoms by quantum-jump-based feedback

We consider a model consisting of two (two-level) atoms which are placed in two separated heavily damped cavities respectively. We show that the quantum-jump-based feedback based on joint measurement can be used to generate a steady entangled state between two atoms against decoherence. We analyse the effects of general local control Hamiltonians in the performance of entanglement production and show that by choosing a proper control Hamiltonian the steady maximal entanglement between two separated atoms can be protected.