Algebraic Formulation of Quantum Decoherence

An algebraic formalism for quantum decoherence in systems with continuous evolution spectrum is introduced. A certain subalgebra, dense in the characteristic algebra of the system, is defined in such a way that Riemann–Lebesgue theorem can be used to explain decoherence in a well defined final pointer basis.     

Dynamics of Optically Driven Exciton and Quantum Decoherence

By using the normal ordering method, we study the state evolution of an optically driven excitons in aquantum well immersed in a leaky cavity, which was introduced by Yu-Xi Liu et al. [Phys. Rev. A63 (2001) 033816]. Theinfluence of the externallaser field on the quantum decoherence of a mesoscopically superposed state of the excitons isinvestigated. Our result shows that, the classical field can compensate the energy dissipation of the excitons. Althoughthe decoherence rate of the excitonic Schrodinger cat state does not depend on the external field, the phase of thedecoherence factor can be well controlled by adjusting the amplitude of the external field as well as the detuning betweenthe field and the transition frequency of the atom.     

Return Probability of Quantum and Correlated Random Walks

The analysis of the return probability is one of the most essential and fundamental topics in the study of classical random walks. In this paper, we study the return probability of quantum and correlated random walks in the one-dimensional integer lattice by the path counting method. We show that the return probability of both quantum and correlated random walks can be expressed in terms of the Legendre polynomial. Moreover, the generating function of the return probability can be written in terms of elliptic integrals of the first and second kinds for the quantum walk.     

Quantum Monty Hall Problem under Decoherence

We study the effect of decoherence on quantum Monty Hall problem under theinfluence of amplitude damping, depolarizing, and dephasing channels. It isshown that under the effect of decoherence, there is a Nash equilibrium ofthe game in case of depolarizing channel for Alice's quantum strategy.Whereas in case of dephasing noise, the game is not influenced by thequantum channel. For amplitude damping channel, Bob's payoffs are foundsymmetrical about a decoherence of 50% and the maximum occurs at this value of decoherence for his classical strategy. However, it is worth-mentioning that in case of depolarizing channel, Bob's classical strategy remains always dominant against any choice of Alice's strategy.     

Quantum Monty Hall Problem under Decoherence

We study the effect of decoherence on quantum Monty Hall problem under the influence of amplitude damping, depolarizing, and dephasing channels. It is shown that under the effect of decoherence, there is a Nash equilibrium of the game in case of depolarizing channel for Alice＇s quantum strategy. Whereas in case of dephasing noise, the game is not influenced by the quantum channel. For amplitude damping channel, Bob＇s payoffs are found symmetrical about a decoherence of 50% and the maximum occurs at this value of decoherence for his classical strategy. However, it is worth-mentioning that in case of depolarizing channel, Bob＇s classical strategy remains always dominant against any choice of Alice＇s strategy.     

On Exciton Decoherence in Quantum Dots

The effects resulting due to dressing of an exciton with phonons are analyzed as the source of unavoidable decoherence of orbital degrees of freedom in quantum dots. The dressing with longitudinal optical phonons results in energetic shift of order of a few meV even of the ground state of exciton in a state-of-the-art InAs/GaAs dot and the mediating role of longitudinal acoustical phonons is essential in this process. The characteristic time needed for dressing of the exciton with optical phonons is of a picosecond order. That time can be regarded as the lower limit for decoherence for optically driven quantum gates employing self-assembled quantum dot structures.     

消干效应和量子力学新解释的意义

简单介绍了消干 (decoherence)效应以及量子力学的一种新解释———由Griffiths,Gell Mann和Omn埁ｓ等独立提出的“消干历史解释”的由来和要点 ,评述了这一进展在量子理论发展中的意义 .     

Dynamical Suppression of Decoherence in Two-Qubit Quantum Memory

In this paper, we have detailedly studied the dynamical suppression of the phase damping for the two-qubit quantum memory of Ising model by the quantum “bang-bang“ technique. We find the sequence of periodic radiofrequency pulses repetitively to flip the state of the two-qubit system and quantitatively find that these pulses can be used to effectively suppress the phase damping decoherence of the quantum memory and freeze the system state into its initial state. The general sequence of periodic radio-frequency pulses to suppress the phase damping of multi-qubit of Ising model is also given.     

Dynamical Suppression of Decoherence in Two-Qubit Quantum Memory

In this paper, we have detailedly studied the dynamical suppression of the phase damping for the two-qubit quantum memory of Ising model by the quantum “bang-bang” technique. We find the sequence of periodic radio-frequency pulses repetitively to flip the state of the two-qubit system and quantitatively find that these pulses can be used to effectively suppress the phase damping decoherence of the quantum memory and freeze the system state into its initial state. The general sequence of periodic radio-frequency pulses to suppress the phase damping of multi-qubit of Ising model is also given.     

Conditions for Collective Decoherence

An efficient decoherence-reducing strategy exists if two qubits can be made decohered collectively. This paper serves to derive the explicit conditions for coJJective decoherence from a practical decoherence model. We describe two kinds of collective decoherence. In each case, a corresponding decoherence-reducing strategy is proposed.     

Normal Ordering Solution to Quantum Dissipation and Its Induced Decoherence

We implement the normal ordering technique to study the quantum dissipation of a single mode harmonic oscillator system. The dynamic evolution of the system is investigated for a reasonable initial state by solving the Schrödinger equation directly through the normal ordering technique. The decoherence process of the system for the cases T=0 K and T≠0 K is investigated as an application.     

Quantum Correlations in a Two-Qubit Heisenberg XX Model under Intrinsic Decoherence

Taking into account the intrinsic decoherence,we have investigated quantum correlations in a two-qubit Heisenberg XX model when a nonuniform magnetic field is included.We compare entanglement measured by entanglement of formation,quantum discord and measurement-induced measurement(MID)and illustrate their diferent characteristics.Quantum discord and MID show the same features and always exist even though there is no entanglement in the long time limit.In the time evolution,quantum discord could be generated or enhanced to the stable value,while MID just decreases to the stable value.     

Effect of Decoherence Induced by a Spin Environment on Quantum Channel Capacity

We investigate the effect of decoherence from a spin environment on the quantum channel capacity.Our results imply that the time evolution of the quantum channel capacity depends on the number of freedom degrees of the environment,the tunneling element,the initial state of the environment,and the system-environment coupling strength.From the analysis,we find that the strong tunneling elements and the weak coupling strength can enhance the quantum channel capacity while the environment with a large number of freedom degrees and the strong coupling strength will shrink it.     

Decoherence of a Double Quantum Dot Charge Qubit Approached with Redfield Equation

In this paper, we investigate the decoherence time of a double quantum dot （DQD） charge qubit in three kinds of baths through solving dynamics of the qubit. The dynamics of the qubit is investigated with Redfield master equation. It is shown that the decoherence time of the qubit in Ohmic bath has the same order of magnitude as the experiments reported. When the environment is modeled with the supra-Ohmic bath the decoherence time of the qubit is shorter than the experimental result. And when modeled with the sub-Ohmic bath the decoherence time of the qubit is longer than the experimental result.     

Quantum Correlations in a Two-Qubit Heisenberg XX Model under Intrinsic Decoherence

Taking into account the intrinsic decoherence, we have investigated quantum correlations in a two-qubit Heisenberg XX model when a nonuniform magnetic field is included. We compare entanglement measured by entanglement of formation, quantum discord and measurement-induced measurement (MID) and illustrate their different characteristics. Quantum discord and MID show the same features and always exist even though there is no entanglement in the long time limit. In the time evolution, quantum discord could be generated or enhanced to the stable value, while MID just decreases to the stable value.     

Shannon Entropy and Diffusion Coefficient in Parity-Time Symmetric Quantum Walks

Non-Hermitian topological edge states have many intriguing properties, however, to date, they have mainly been discussed in terms of bulk–boundary correspondence. Here, we propose using a bulk property of diffusion coefficients for probing the topological states and exploring their dynamics. The diffusion coefficient was found to show unique features with the topological phase transitions driven by parity–time (PT)-symmetric non-Hermitian discrete-time quantum walks as well as by Hermitian ones, despite the fact that artificial boundaries are not constructed by an inhomogeneous quantum walk. For a Hermitian system, a turning point and abrupt change appears in the diffusion coefficient when the system is approaching the topological phase transition, while it remains stable in the trivial topological state. For a non-Hermitian system, except for the feature associated with the topological transition, the diffusion coefficient in the PT-symmetric-broken phase demonstrates an abrupt change with a peak structure. In addition, the Shannon entropy of the quantum walk is found to exhibit a direct correlation with the diffusion coefficient. The numerical results presented herein may open up a new avenue for studying the topological state in non-Hermitian quantum walk systems.     

Transport Efficiency of Continuous-Time Quantum Walks on Graphs

Continuous-time quantum walk describes the propagation of a quantum particle (or an excitation) evolving continuously in time on a graph. As such, it provides a natural framework for modeling transport processes, e.g., in light-harvesting systems. In particular, the transport properties strongly depend on the initial state and specific features of the graph under investigation. In this paper, we address the role of graph topology, and investigate the transport properties of graphs with different regularity, symmetry, and connectivity. We neglect disorder and decoherence, and assume a single trap vertex that is accountable for the loss processes. In particular, for each graph, we analytically determine the subspace of states having maximum transport efficiency. Our results provide a set of benchmarks for environment-assisted quantum transport, and suggest that connectivity is a poor indicator for transport efficiency. Indeed, we observe some specific correlations between transport efficiency and connectivity for certain graphs, but, in general, they are uncorrelated.