Quantum speed limit time and entanglement in a non-Markovian evolution of spin qubits of coupled quantum dots

Quantum speed limit time and entanglement in a system composed of coupled quantum dots are investigated.The excess electron spin in each quantum dot constitutes the physical system(qubit).Also the spin interaction is modeled through the Heisenberg model and the spins are imposed by an external magnetic field.Taking into account the spin relaxation as a non-Markovian process,the quantum speed limit and entanglement evolution are discussed.Our findings reveal that increasing the magnetic field leads to the faster quantum evolution.In addition,the temperature increment causes the longer quantum speed limit time as well as the entanglement degradation.

Quantum speed limit time of a two-level atom under different quantum feedback control

We investigate the quantum speed limit time （QSLT） of a two-level atom under quantum-jump-based feedback control or homodyne-based feedback control. Our results show that the two different feedback control schemes have different influences on the evolutionary speed. By adjusting the feedback parameters, the quantum-jump-based feedback control can induce speedup of the atomic evolution from an excited state, but the homodyne-based feedback control cannot change the evolutionary speed. Additionally, the QSLT for the whole dynamical process is explored. Under the quantum-jump-based feedback control, the QSLT displays oscillatory behaviors, which implies multiple speed-up and speed-down processes during the evolution. While, the homodyne-based feedback control can accelerate the speed-up process and improve the uniform speed in the uniform evolution process.

Quantum speed limit time of a non-Hermitian two-level system

We investigated the quantum speed limit time of a non-Hermitian two-level system for which gain and loss of energy or amplitude are present. Our results show that, with respect to two distinguishable states of the non-Hermitian system, the evolutionary time does not have a nonzero lower bound. The quantum evolution of the system can be effectively accelerated by adjusting the non-Hermitian parameter, as well as the quantum speed limit time can be arbitrarily small even be zero.

Influences of spin-orbit interaction on quantum speed limit and entanglement of spin qubits in coupled quantum dots

Quantum speed limit and entanglement of a two-spin Heisenberg XYZ system in an inhomogeneous external magnetic field are investigated.The physical system studied is the excess electron spin in two adjacent quantum dots.The influences of magnetic field inhomogeneity as well as spin-orbit coupling are studied.Moreover,the spin interaction with surrounding magnetic environment is investigated as a non-Markovian process.The spin-orbit interaction provides two important features:the formation of entanglement when two qubits are initially in a separated state and the degradation and rebirth of the entanglement.

Quantum speed limits for Bell-diagonal states

The lower bounds of the evolution time between two distinguishable states of a system, defined as quantum speed limit time, can characterize the maximal speed of quantum computers and communication channels. We study the quantum speed limit time between the composite quantum states and their target states in the presence of nondissipative decoherence.For the initial states with maximally mixed marginals, we obtain the exact expressions of the quantum speed limit time which mainly depend on the parameters of the initial states and the decoherence channels. Furthermore, by calculating the quantum speed limit time for the time-dependent states started from a class of initial states, we discover that the quantum speed limit time gradually decreases in time, and the decay rate of the quantum speed limit time would show a sudden change at a certain critical time. Interestingly, at the same critical time, the composite system dynamics would exhibit a sudden transition from classical decoherence to quantum decoherence.

Controlling Speedup Induced by a Hierarchical Environment

Quantum speedup of an open quantum system can be induced by the non-Markovian effect of the environment. Although an environment with a higher degree of non-Markovianity may seem like it should cause a faster speed of quantum evolution, this seemingly intuitive thinking may not always be correct. To clarify this point, we give a mechanism for controlling speedup of a single qubit that is coupled to a hierarchical photonic-crystal (PC) environment, which contains a defect single-mode cavity and a semi-infinite one-dimensional (1D) waveguide. Via studying the dynamics of the qubit, we reveal that with a judicious choice of the qubit-cavity coupling strength and the memory time of the waveguide environment, a speed-up evolution can be achieved. In particular, we found that the quantum speedup is not entirely attributed to the non-Markovianity, but to the increase of the total amount of flow information. That is the intrinsic physical reason that the hierarchical environment may induce the speed-up process. Our results may open new perspectives for detecting quantum speedup in realistic environments.

纠缠相干光场对量子态最大演化速率的操控

考虑双模纠缠相干光场,将其中一束光场注入腔中与一个二能级原子发生共振相互作用,根据腔量子电动力学理论推导出原子系统的演化态.针对原子系统初始状态到目标演化态的动力学过程,利用量子速率极限时间概念来表征原子系统量子态的最大演化速率.通过调节双模纠缠相干光场的相干参数来操控原子系统量子态动力学过程中所能达到的最大演化速率.结果发现:与原子发生相互作用光场的相干参数在一定条件下可以对系统量子态的最大演化速率产生明显的影响;并且当该光场参数不能很好地来操控量子态的最大演化速率时,双模纠缠相干光场间的量子关联可以实现未参加相互作用光场参数对原子系统量子态最大演化速率的远程调控.

量子速率极限时间

系统从初始态演化到目标态所需要的最短时间被定义为量子速率极限时间.此概念可以被用来估量由给定量子态到期望目标态的最大演化速率.本文主要回顾了从封闭系统量子速率极限时间过渡到开放系统量子速率极限时间的发展历程,并讨论了非马尔科夫环境对开放系统量子速率极限时间的影响.希望能对今后量子速率极限时间的发展有所帮助.

系统动力学演化中量子加速的研究

在一个二能级原子与单模腔场相互作用的系统中,基于归一化条件获得原子的密度矩阵及原子处于激发态的布居数,给出了系统演化的量子加速公式。采用纵向加速和横向加速来描述系统动力学演化的过程,利用数值计算的方法讨论了原子与腔场的耦合强度和演化时间对系统演化加速的影响。研究结果表明腔与原子的耦合能够加速量子态的纵向演化;随着演化时间的增加,量子态的横向加速增强,并伴随着周期性振荡。