Long time evolution of a spin interacting with a spin bath in arbitrary magnetic field

We introduce a completely different method to calculate the evolution of a spin interacting with a sufficient large spin bath,especially suitable for treating the central spin model in a quantum dot(QD).With only an approximation on the envelope of central spin,the symmetry can be exploited to reduce a huge Hilbert space which cannot be calculated with computers to many small ones which can be solved exactly.This method can be used to calculate spin-bath evolution for a spin bath containing many(say,1000)spins,without a perturbative limit such as strong magnetic field condition,and works for long-time regime with sufficient accuracy.As the spin-bath evolution can be calculated for a wide range of time and magnetic field,an optimal dynamic of spin flip-flop can be found,and more sophisticated approaches to achieve extremely high polarization of nuclear spins in a QD could be developed.     

Impurity spin dynamics and quantum coherence in mesoscopic rings

We investigate the effects of impurity spin dynamics on quantum coherence in mesoscopic metallic rings. Spin relaxation induces temporal current fluctuations which are closely related to persistent currents in such systems. The persistent current is suppressed in the presence of magnetic impurities. We discuss spin-polarisation effects, spin-glass ordering, and the Kondo effect which are shown to restore the current even in the presence of magnetic scattering.     

Enhancing the coherence of a spin qubit by operating it as a feedback loop that controls its nuclear spin bath

In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time T?* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.     

Quantum coherence and correlation dynamics of two-qubit system in spin bath environment

The quantum entanglement,discord,and coherence dynamics of two spins in the model of a spin coupled to a spin bath through an intermediate spin are studied.The effects of the important physical parameters including the coupling strength of two spins,the interaction strength between the intermediate spin and the spin bath,the number of bath spins and the temperature of the system on quantum coherence and correlation dynamics are discussed in different cases.The frozen quantum discord can be observed whereas coherence does not when the initial state is the Bell-diagonal state.At finite temperature,we find that coherence is more robust than quantum discord,which is better than entanglement,in terms of resisting the influence of environment.Therefore,quantum coherence is more tenacious than quantum correlation as an important resource.     

Zero-dimensional spin accumulation and spin dynamics in a mesoscopic metal island

We have measured electron spin accumulation at 4.2 K and at room temperature in an aluminum island with all dimensions (400 nm x 400 nm x 30 nm) smaller than the spin relaxation length. For the first time, we obtain uniform spin accumulation in a four-terminal lateral device with a magnitude exceeding the Ohmic resistance in the island. By controlling the magnetization directions of the four magnetic electrodes that contact the island, we have performed a detailed study of the spin accumulation. Spin precession measurements confirm the uniformity of our system and provide an accurate method to extract the spin relaxation time.     

Geometric phase of a qubit interacting with a squeezed-thermal bath

We study the geometric phase of an open two-level quantum system under the influence of a squeezed, thermal environment for both non-dissipative as well as dissipative system-environment interactions. In the non-dissipative case, squeezing is found to have a similar influence as temperature, of suppressing geometric phase, while in the dissipative case, squeezing tends to counteract the suppressive influence of temperature in certain regimes. Thus, an interesting feature that emerges from our work is the contrast in the interplay between squeezing and thermal effects in non-dissipative and dissipative interactions. This can be useful for the practical implementation of geometric quantum information processing. By interpreting the open quantum effects as noisy channels, we make the connection between geometric phase and quantum noise processes familiar from quantum information theory.     

Entanglement of a two-particle Gaussian state interacting with a heat bath

The effect of a thermal reservoir is investigated on a bipartite Gaussian state. We derive a pre-Lindblad master equation in the non-rotating wave approximation for the system. We then solve the master equation for a bipartite harmonic oscillator Hamiltonian with entangled initial state. We show that for strong damping the loss of entanglement is the same as for freely evolving particles. However, if the damping is small, the entanglement is shown to oscillate and eventually tend to a constant non-zero value.     

Persistent spin current in a mesoscopic hybrid ring with spin-orbit coupling

We investigate the equilibrium property of a mesoscopic ring with a spin-orbit interaction. It is well known that, for a normal mesoscopic ring threaded by a magnetic flux, the electron acquires a Berry phase that induces the persistent (charge) current. Similarly, the spin of an electron acquires a spin Berry phase traversing a ring with a spin-orbit interaction. It is this spin Berry phase that induces a persistent spin current. To demonstrate its existence, we calculate the persistent spin current without an accompanying charge current in the normal region in a hybrid mesoscopic ring. We point out that this persistent spin current describes the real spin motion and can be observed experimentally.     

存在自旋轨道耦合的介观小环中的持续自旋流

文章作者研究了存在自旋轨道耦合的介观小环的平衡态性质.此前人们已经知道,在有磁通穿过的介观小环中,绕环运动的电子会产生一附加的Berry相位而导致持续电流;同样地,在仅有自旋轨道耦合的体系中,电子绕环运动也应当会产生附加的自旋Berry相位,进而驱动持续自旋流.文章作者通过对一个有正常区和自旋轨道耦合区的复合小环的计算,结果表明,无电流伴随的纯持续自旋流的确存在.文章作者指出,这持续自旋流描述真实的自旋运动,并且它能被实验观测.     

Isothermal resistivity for interacting charged particles in contact with a bath

To calculate the stationary resistivity, the role of the coupling to a bath is pointed out. In the case of two kinds of interacting charged particles, the coupling to a bath is taken into account by relaxation terms in the equation of motion of the density operator so that the correct balance equations for the impuls transfer are obtained. Higher orders of a perturbation expansion of the resistivity with respect to the coupling constant are given. For special cases the Matthiesen rule is derived, and deviations from the Matthiesen rule are discussed.     

A functional renormalization group approach to non-equilibrium properties of mesoscopic interacting quantum systems

We present an extension of the concepts of the functional renormalization group approach to quantum many-body problems in non-equilibrium situations. The approach is completely general and allows calculations for both stationary and time-dependent situations. As a specific example we study the stationary state transport through a quantum dot with local Coulomb correlations. We discuss the influence of finite bias voltage and temperature on the current and conductance.     

Possible evidence of a spontaneous spin polarization in mesoscopic two-dimensional electron systems

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems.     

Entanglement evolution of a two-qubit system interacting with a quantum spin environment

We investigate the entanglement dynamics and decoherence of a two-qubit system under a quantum spin environment at finite temperature in the thermodynamics limit. For the case under study, we find different initial states will result in different entanglement evolution, what deserves mentioning here is that the state |Ψ=cosα|01+sinα|10 is most robust than other states when π/2<α<π, since the entanglement remains unchanged or increased under the spin environment. In addition, we also find the anisotropy parameter Δ can suppress the destruction of decoherence induced by the environment, and the undesirable entanglement sudden death arising from the process of entanglement evolution can be efficiently controlled by the inhomogeneous magnetic field ζ.     

Rabi oscillations revival induced by time reversal: a test of mesoscopic quantum coherence

Using an echo technique proposed by Morigi et al., we have time-reversed the atom-field interaction in a cavity quantum electrodynamics experiment. The collapse of the atomic Rabi oscillation in a coherent field is reversed, resulting in an induced revival signal. The amplitude of this "echo" is sensitive to nonunitary decoherence processes. Its observation demonstrates the existence of a mesoscopic quantum superposition of field states in the cavity between the collapse and the revival times.     

Coherent incoherent transition in the spin boson model with a super-ohmic bath

The temperature effect on tunnelling splitting in the spin--boson model with a super-ohmic bath is studied by the small polaron theory. The coherent--incoherent transition temperature is calculated and its dependence on dissipation strength and bare tunnelling splitting is analysed. In additional to the traditional transition point described in textbooks, a new kind of transition is found in the low dissipation region, showing different temperature dependence in the transition. The relation to the corresponding transition in the polaron--phonon system is also discussed.