Yun-Jie Xia

Nonclassical properties of even and odd coherent states

Abstract Squeezing and antibunching effects of even and odd coherent states are discussed. We also studied the change of these two effects and the amplitude-squared squeezing when the two states are displaced. The displaced even coherent state can exhibit amplitude-squared squeezing.

Cavity-based architecture to preserve quantum coherence and entanglement

Quantum technology relies on the utilization of resources, like quantum coherence and entanglement, which allow quantum information and computation processing. This achievement is however jeopardized by the detrimental effects of the environment surrounding any quantum system, so that finding strategies to protect quantum resources is essential. Non-Markovian and structured environments are useful tools to this aim. Here we show how a simple environmental architecture made of two coupled lossy cavities enables a switch between Markovian and non-Markovian regimes for the dynamics of a qubit embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely preserved if the cavity without qubit is perfect. We then focus on entanglement control of two independent qubits locally subject to such an engineered environment and discuss its feasibility in the framework of circuit quantum electrodynamics. With up-to-date experimental parameters, we show that our architecture allows entanglement lifetimes orders of magnitude longer than the spontaneous lifetime without local cavity couplings. This cavity-based architecture is straightforwardly extendable to many qubits for scalability.

Genuine multiqubit entanglement and controlled teleportation

We construct a genuine (2N+1)-qubit entangled state to perform controlled teleportation of an arbitrary N-qubit state. The constructed state is a complementarity to the genuine 2N-qubit entangled state constructed by Yeo and Chua for N=2 [Phys. Rev. Lett. 96, 060502 (2006)] and by Chen, Zhu, and Guo for any N [Phys. Rev. A 74, 032324 (2006)]. We also quantify the entanglement of the state and classify it with the well-known GHZ and W states by means of the recently proposed generalized global entanglement and the associated auxiliary measures [Phys. Rev. A 74, 022314 (2006)]. Our study is of general importance with respect to exploring and exploiting the genuine multiqubit entanglement.

Quantum speed limit for arbitrary initial states

The minimal time a system needs to evolve from an initial state to its one orthogonal state is defined as the quantum speed limit time, which can be used to characterize the maximal speed of evolution of a quantum system. This is a fundamental question of quantum physics. We investigate the generic bound on the minimal evolution time of the open dynamical quantum system. This quantum speed limit time is applicable to both mixed and pure initial states. We then apply this result to the damped Jaynes-Cummings model and the Ohimc-like dephasing model starting from a general time-evolution state. The bound of this time-dependent state at any point in time can be found. For the damped Jaynes-Cummings model, when the system starts from the excited state, the corresponding bound first decreases and then increases in the Markovian dynamics. While in the non-Markovian regime, the speed limit time shows an interesting periodic oscillatory behavior. For the case of Ohimc-like dephasing model, this bound would be gradually trapped to a fixed value. In addition, the roles of the relativistic effects on the speed limit time for the observer in non-inertial frames are discussed.

Harnessing non-Markovian quantum memory by environmental coupling

Controlling the non-Markovian dynamics of open quantum systems is essential in quantum information technology since it plays a crucial role in preserving quantum memory. Albeit in many realistic scenarios the quantum system can simultaneously interact with composite environments, this condition remains little understood, particularly regarding the effect of the coupling between environmental parts. We analyze the non-Markovian behavior of a qubit interacting at the same time with two coupled single-mode cavities which in turn dissipate into memoryless or memory-keeping reservoirs. We show that increasing the control parameter, that is the two-mode coupling, allows for triggering and enhancing a non-Markovian dynamics for the qubit starting from a Markovian one in absence of coupling. Surprisingly, if the qubit dynamics is non-Markovian for zero control parameter, increasing the latter enables multiple transitions from non-Markovian to Markovian regimes. These results hold independently on the nature of the reservoirs. This work highlights that suitably engineering the coupling between parts of a compound environment can efficiently harness the quantum memory, stored in a qubit, based on non-Markovianity.

Entanglement measure and dynamics of multiqubit systems: non-Markovian versus Markovian and generalized monogamy relations

In this paper, we first present a simple measure for multiqubit entanglement based on the strategy of bipartite cuts and the measure of negativity. Then, we establish generalized monogamy inequalities and associated partition-dependent residual entanglement (PRE) accounting for arbitrary partitions of a multiqubit system. By virtue of the defined quantities, we investigate the entanglement dynamics of a system of N qubits, either in the Greenberger–Horne–Zeilinger (GHZ)-type state or in the W state, interacting with N independent reservoirs in both Markovian and non-Markovian regimes. We observe entanglement revivals of qubits at instantaneous points of disappearance or after a finite interval of abrupt vanishing due to the memory effect of non-Markovian reservoirs. We also follow the whole entanglement evolution in terms of the PRE to demonstrate the process of transition between the bipartite entanglement of all possible bipartitions and the multipartite entanglement. In particular, we show that the change in time of entanglement formats differs qualitatively for the GHZ-type and W states.

On conditions for atomic entanglement sudden death in cavity QED

In this paper, we study the entanglement dynamics of atoms coupled to cavity fields. We consider conditions characterized not only by the purity of the atomic initial state but also by the cavity photon number. By studying three different models within the framework of cavity QED, we show that the so-called atomic entanglement sudden death always occurs if initially the atomic state is sufficiently impure and/or the cavity photon number is nonzero.

Classical-driving-assisted quantum speed-up

We propose a method of accelerating the speed of evolution of an open system by an external classical driving field for a qubit in a zero-temperature structured reservoir. It is shown that, with a judicious choice of the driving strength of the applied classical field, a speed-up evolution of an open system can be achieved in both the weak system-environment couplings and the strong system-environment couplings. By considering the relationship between non-Makovianity of environment and the classical field, we can drive the open system from the Markovian to the non-Markovian regime by manipulating the driving strength of classical field. That is the intrinsic physical reason that the classical field may induce the speed-up process. In addition, the roles of this classical field on the variation of quantum evolution speed in the whole decoherence process is discussed.

Economical and feasible controlled teleportation of an arbitrary unknown N-qubit entangled state

We propose a new quantum protocol to teleport an arbitrary unknown N-qubit entangled state from a sender to a fixed receiver under the control of M (M < N) controllers. In comparison with other existing protocols, ours is more economical and more feasible. The quantum resource required is just M Greenberger–Horne–Zeilinger trios plus (N − M) Einstein–Podolsky–Rosen pairs. The techniques required are only N Bell measurements by the sender, a von Neumann measurement by a controller and N single-qubit transformations by the receiver. The rule for the receiver to reconstruct the desired state is derived explicitly in the most general case.

Entanglement dynamics for a six-qubit model in cavity QED

We study entanglement dynamics for a six-qubit model in cavity QED, where three two-level atoms A, B and C are initially prepared in a Greenberger–Horne–Zeilinger (GHZ)-like state and locally coupled with the independent cavities a, b and c, respectively. We consider entanglement evolution via negativity for the tripartite subsystems of atoms and the corresponding cavities. By choosing two types of GHZ-like states as atomic initial states, we shall show that the entanglement of atoms may or may not exhibit entanglement sudden death (ESD) depending on the type of atomic initial state.