Jun Jing

Non-Markovian relaxation of a three-level system: quantum trajectory approach.

The non-Markovian dynamics of a three-level quantum system coupled to a bosonic environment is a difficult problem due to the lack of an exact dynamic equation such as a master equation. We present for the first time an exact quantum trajectory approach to a dissipative three-level model. We have established a convolutionless stochastic Schrödinger equation called the time-local quantum state diffusion (QSD) equation without any approximations, in particular, without Markov approximation. Our exact time-local QSD equation opens a new avenue for exploring quantum dynamics for a higher dimensional quantum system coupled to a non-Markovian environment.

Control of decoherence with no control

A common philosophy in control theory is the control of disorder by order. Control of decoherence is no exception; strategies aimed at suppressing quantum decoherence adopt this point of view. Here we predict an anomalous phenomenon in open quantum systems-control of disorder by (even more) disorder. It is shown that suppression of decoherence can be achieved using the most disordered white noise field, specifically a white Poissonian noise field. This phenomenon seems to be another anomaly in quantum mechanics and may offer a new strategy in quantum control practices.

Many-body quantum trajectories of non-Markovian open systems

We have derived for the first time a set of non-Markovian quantum trajectory equations for manybody and multi-state quantum open systems. The exact time-local (time-convolutionless) quantum trajectory equations can be used to simulate quantum dynamics of coupled open systems interacting with a bosonic environment at zero temperature. Our general results are explained and illustrated with several atomic and optical open systems.

Fundamental Speed Limits to the Generation of Quantumness

Quantum physics dictates fundamental speed limits during time evolution. We present a quantum speed limit governing the generation of nonclassicality and the mutual incompatibility of two states connected by time evolution. This result is used to characterize the timescale required to generate a given amount of quantumness under an arbitrary physical process. The bound is found to be tight under pure dephasing dynamics. More generally, our analysis reveals the dependence on the initial and final states and non-Markovian effects.

Nonperturbative leakage elimination operators and control of a three-level system.

Dynamical decoupling operations have been shown to reduce errors in quantum information processing. Leakage from an encoded subspace to the rest of the system space is a particularly serious problem for which leakage elimination operators (LEOs) were introduced. Here we provide an analysis of nonideal pulses, rather than the well-understood idealization or bang-bang controls. Under realistic conditions, we show that these controls will provide the same protection from errors as idealized controls. Our work indicates that the effectiveness of LEOs depends on the integral of the pulse sequence in the time domain, which has been missing because of the idealization of pulse sequences. Our results are applied to a three-level system for the nitrogen-vacancy centers under an external magnetic field and are illustrated by the fidelity dynamics of LEO sequences, ranging from regular rectangular pulses, random pulses, and even disordered (noisy) pulses.

Breakdown of the rotating-wave approximation in the description of entanglement of spin-anticorrelated states

It is well established that an entanglement encoded in the Bell states of a two-qubit system with correlated spins exhibits completely different evolution properties from that encoded in states with the anticorrelated spins. A complete and abrupt loss of the entanglement, called the entanglement sudden death, can be found to occur for the spin-correlated states, but the entanglement evolves without any discontinuity or decays asymptotically for the spin-anticorrelated states. We consider the evolution of an initial entanglement encoded in the spin-anticorrelated states and demonstrate that the asymptotic behavior predicted before occurs only in the weak-coupling limit or equivalently when the rotating-wave approximation (RWA) is made on the interaction Hamiltonian of the qubits with the field. If we do not restrict ourselves to the RWA, we find that the entanglement undergoes a discontinuity, the sudden-death phenomenon. We illustrate this behavior by employing an efficient scheme for entanglement evolution between two cold-trapped atoms located inside a single-mode cavity. Although only a single excitation is initially present in the system, we find that the two-photon excited state, which plays the key role for the discontinuity in the behavior of the entanglement, gains a population over a short time of the evolution. When the RWA is made on the interaction, the two-photon excited state remains unpopulated for all times and the discontinuity is absent. We attribute this phenomenon to the principle of complementarity between the evolution time and energy, and the presence of the counter-rotating terms in the interaction Hamiltonian.

One Component Dynamical Equation and Noise Induced Adiabaticity

Department of Physics, Ocean University of China, Qingdao 266100, China(Dated: July 22, 2013)The adiabatic theorem addresses the dynamics of a target instantaneous eigenstate of a time-dependent Hamiltonian. We use a Feshbach P-Q partitioning technique to derive a closed one-component integro-differential equation. The resultant equation properly traces the footprint of thetarget eigenstate. The physical significance of the derived dynamical equation is illustrated by bothgeneral analysis and concrete examples. Surprisingly, we find an anomalous phenomenon showingthat a dephasing white noise can enhance and even induce adiabaticity. This new phenomenon maynaturally occur in many physical systems. We also show that white noises can also shorten the totalduration of dynamic processes such as adiabatic quantum computing.

Transient unidirectional energy flow and diode-like phenomenon induced by non-Markovian environments.

Relying on an exact time evolution scheme, we identify a novel transient energy transfer phenomenon in an exactly-solvable quantum microscopic model consisting of a three-level system coupled to two non-Markovian zero-temperature bosonic baths through two separable quantum channels. The dynamics of this model can be solved exactly using the quantum-state-diffusion equation formalism, demonstrating finite intervals of unidirectional energy flow across the system, typically, from the non-Markovian environment towards the more Markovian bath. Furthermore, when introducing a spatial asymmetry into the system, an analogue of the rectification effect is realized. In the long time limit, the dynamics arrives at a stationary state and the effects recede. Understanding temporal characteristics of directional energy flow will aid in designing microscopic energy transfer devices.

Nonperturbative quantum dynamical decoupling

Jun Jing, Lian-Ao Wu , Marcelo S. Sarandy and J. Gonzalo Muga Department of Physics, Shanghai University, Shanghai 200444, China Ikerbasque, Basque Foundation for Science, 48011 Bilbao and Department of Theoretical Physics and History of Science, The Basque Country University (UPV/EHU), PO Box 644, 48080 Bilbao, Spain Instituto de F́ısica, Universidade Federal Fluminense, Av. Gal. Milton Tavares de Souza s/n, Gragoatá, 24210-346, Niterói, RJ, Brazil 4 Department of Physical Chemistry, The Basque Country University (UPV/EHU), PO Box 644, 48080 Bilbao, Spain (Dated: May 7, 2014)

Entanglement dephasing dynamics driven by a bath of spins

We have studied the entanglement dynamics for a two-qubit system coupled to a spin environment of different configurations by a z–x-type interaction. Quantum dynamics of the models considered in this paper is solved analytically. Moreover, we show that simple and concise results may be obtained when certain approximations are properly made. Our purpose is to find out how the entanglement of a central spin system is affected by the pre-designed factors of the system and its environment, such as their initial states and the coupling constants between the system and its environment. Clearly, how the system is coupled to its environment will inevitably change the feature of entanglement evolution of the central spin system. Our major findings include the following: (i) the entanglement of the system of interest is sensitive to the number of spins in the environment, (ii) the initial states of the environment can profoundly affect the dynamics of the entanglement of the central spin system and (iii) the entangled environment can speed up the decay and revival of the entanglement of the central spin system. Our results exhibit some interesting features that have not been found for a bosonic environment.