库仑势对抛物量子点量子比特消相干的影响

采用Pekar类型的变分方法,在电子与体纵光学声子强耦合的条件下,计算得出了抛物量子点中电子的基态能量和第一激发态能量及其相应的本征波函数.量子点中这样的二能级体系可以作为一个量子比特.由于声子的自发辐射,造成量子比特的消相干,讨论了消相干时间与库仑结合参数,耦合强度,受限长度,色散系数的变化关系. 关键词： 量子点 量子比特 量子信息 消相干     

The decoherence of the triangular and Coulomb bound potential quantum dot qubit

We study the eigenenergies and eigenfunctions of the ground and first-excited states of an electron which is strongly coupled to an LO-phonon in a quantum dot with a triangular bound potential and Coulomb bound potential by using the Pekar variational method. This system may be used as a two-level qubit. Phonon spontaneous emission causes the decoherence of the qubit. Numerical calculations are performed on the decoherence rate as a function of the polar angle, the Coulomb binding parameter, the coupling strength, the confinement length of the quantum dot and the dispersion coefficient.     

量子环中量子比特的消相干时间

在量子环中电子与体纵光学声子强耦合的情况下,通过求解能量本征方程,得出了电子的基态和第一激发态的本征能量及其波函数,进而以电子-声子体系的基态与第一激发态构造一个量子比特.结果讨论了消相干时间与耦合强度,色散系数以及量子环内径、外径的变化关系.     

Controlled decoherence of floating flux qubits

In Born-Markov approximation,this paper calculates the energy relaxation time T 1 and the decoherence time T 2 of a floating flux qubit by solving the set of Bloch-Redfield equations.It shows that there are two main factors influencing the floating flux qubits:coupling capacitor in the circuit and the environment resistor.It also discusses how to improve the quantum coherence time of a qubit.Through shunt connecting/series connecting inductive elements,an inductive environment resistor is obtained and further the reactance component of the environment resistor is improved,which is beneficial to the enhancement of decoherence time of floating flux qubits.     

The initial decoherence of the mesoscopic Josephson junction flux qubit

For a mesoscopic radio frequency superconducting quantum interference device (rfSQUID), at a degeneracy point, the system reduces to a quantum two-state system which can be used as a flux qubit. When the noise environment is equivalent to a harmonic oscillators bath, by virtue of an operator-norm measure for the short time decoherence, this paper investigates the initial decoherence of the flux qubit operating in the ohmic noise environment and illustrates its property by means of the numerical evaluation.     

Tuning rf-SQUID flux qubit system's potential with magnetic flux bias

At an extremely low temperature of 20 mK, we measured loop current in a tunable rf superconducting quantum interference device (SQUID) with a dc-SQUID. By adjusting the magnetic flux applied to the rf-SQUID loop (Φ_f) and the small dc-SQUID (Φ_f^cjj), respectively, the potential shape of the system can be fully controlled in situ. Variations of transition step and overlap size in switching current with the barrier flux bias are analyzed, from which we can obtain some relevant device parameters and built up a model to explain the experimental phenomenon.     

Criticality-based quantum metrology in the presence of decoherence

Because quantum critical systems are very sensitive to the variation of parameters around the quantum phase transition (QPT), quantum criticality has been presented as an efficient resource for metrology. In this paper, we address the issue whether the divergent feature of the inverted variance is realizable in the presence of noise when approaching the QPT. Taking the quantum Rabi model (QRM) as an example, we obtain the analytical result for the inverted variance with single-photon relaxation. We show that the inverted variance may be convergent in time due to the noise. Since the precision of the metrology is very sensitive to the noise, as a remedy, we propose squeezing the initial state to improve the precision under decoherence. In addition, we also investigate the criticality-based metrology under the influence of the two-photon relaxation. Strikingly, although the maximum inverted variance still manifests a power-law dependence on the energy gap, the exponent is positive and depends on the dimensionless coupling strength. This observation implies that the criticality may not enhance but weaken the precision in the presence of two-photon relaxation, due to the non-linearity introduced by the two-photon relaxation.     

Steering quantum nonlocalities of quantum dot system suffering from decoherence

The important applications of quantum dot system are to implement logic operations and achieve universal quantum computing based on different quantum nonlocalities. Here, we characterize the quantum steering, Bell nonlocality, and nonlocal advantage of quantum coherence (NAQC) of quantum dot system suffering nonunital and unital channels. The results reveal that quantum steering, Bell nonlocality, and NAQC can display the traits of dissipation, enhancement, and freezing. One can achieve the detections of quantum steering, Bell nonlocality, and NAQC of quantum dot system in different situations. Among these quantum nonlocalities, NAQC is the most fragile, and it is most easily influenced by different system parameters. Furthermore, considering quantum dot system coupling with amplitude damping channel and phase damping channel, these quantum nonlocalities degenerate with the enlargement of the channel parameters $t$ and $\varGamma$. Remarkably, measurement reversal can effectively control and enhance quantum steering, Bell nonlocality, and NAQC of quantum dot system suffering from decoherence, especially in the scenarios of the amplitude damping channel and strong operation strength.     

Nonlocal advantage of quantum coherence and entanglement of two spins under intrinsic decoherence

We investigate the nonlocal advantage of quantum coherence(NAQC) and entanglement for two spins coupled via the Heisenberg interaction and under the intrinsic decoherence. Solutions of this decoherence model for the initial spin-1/2 and spin-1 maximally entangled states are obtained, based on which we calculate the NAQC and entanglement. In the weak region of magnetic field, the NAQC behaves as a damped oscillation with the time evolves, while the entanglement decays exponentially(behaves as a damped oscillation) for the spin-1/2(spin-1) case. Moreover, the decay of both the NAQC and entanglement can be suppressed significantly by tuning the magnetic field and anisotropy of the spin interaction to some decoherence-rate-determined optimal values.     

Measurement-induced disturbance in Heisenberg XY spin model with Dzialoshinskii–Moriya interaction under intrinsic decoherence

Quantum correlations measured by measurement-induced disturbance （MID） in a two-qubit Heisenberg XY spin model with Dzialoshinskii-Moriya （DM） interaction under intrinsic decoherence are investigated. MID is studied un-der various circumstances and the influences of the external dependencies on the final quantum state which has stable MID are discussed. Two kinds of initial quantum states are considered as well as different conclusions. MID appears to decay periodically during the processing of intrinsic decoherence; both DM interaction and intrinsic decoherence have a negative impact on the correlations. The MID of the stable state depends on several factors, except the parameter of the intrinsic decoherence. Moreover, we find a special initial state that is able to maintain the maximum quantum correlations during the processing of intrinsic decoherence.     

Optimal quantum parameter estimation of two-qutrit Heisenberg XY chain under decoherence

Adopting the Milburn decoherence model, we investigate the performance of quantum Fisher information of the twoqutrit isotropic Heisenberg XY chain under decoherence. We find that the quantum Fisher information with respect to the decoherence rate and the magnetic field decreases exponentially in the long-time limit, which significantly reduces the precision of optimal quantum estimation. We also show that with the increase of the decoherence rate or the magnetic field,the QFIs go down considerably. Furthermore, we find that the precision of optimal quantum estimation can be enhanced by the entanglement in the input state.     

Distributed quantum computation with superconducting qubit via LC circuit using dressed states

A scheme is proposed where two superconducting qubits driven by a classical field interacting separately with two distant LC circuits connected by another LC circuit through mutual inductance,are used for implementing quantum gates.By using dressed states,quantum state transfer and quantum entangling gate can be implemented.With the help of the time-dependent electromagnetic field,any two dressed qubits can be selectively coupled to the data bus (the last LC circuit),then quantum state can be transferred from one dressed qubit to another and multi-mode entangled state can also be formed.As a result,the promising perspectives for quantum information processing of mesoscopic superconducting qubits are obtained and the distributed and scalable quantum computation can be implemented in this scheme.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity. We also find that the stationary quantum discord can be increased by applying a classical driving field. Furthermore, we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence. Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity.We also find that the stationary quantum discord can be increased by applying a classical driving field.Furthermore,we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence.Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

The decoherence of the parabolic linear bound potential quantum dot qubit

On the condition of electric-LO phonon strong coupling in parabolic quantum dot, we obtain the eigenenergies of the ground state and the first-excited state, the eigenfuctions of the ground state and the first-excited state by using variational method of Pekar type. This system in quantum dot may be employed as a two-level quantum system—qubit. The phonon spontaneous emission causes the decoherence of the qubit. The relations between the decoherence time with the coupling strength, the confinement length, the coefficient dispersion are discussed.     

在不同初态下Dzyaloshinskii-Moriya相互作用及内禀退相干对海森伯系统的量子纠缠的影响

研究了存在内禀退相干时,对于不同的系统初态,具有DM相互作用和各向异性的三粒子XXZ海森伯模型的对纠缠动力学特性.得出了一些结论:系统的对纠缠度与各向异性参数?无关,但内禀退相干对系统的纠缠有明显的抑制作用;在内禀退相干存在时,若系统初态为纠缠态,选择合适的DM相互作用的参数,系统的对纠缠有一个非零的稳定值;系统初态为分离态时,系统的对纠缠会随时间震荡衰减,并且每次震荡会出现纠缠突然死亡现象,系统的对纠缠最终达到解纠缠状态.因此,选择合适的系统初态和DM相互作用参数可以有效地控制系统的对纠缠.     

囚禁离子与调制热库相互作用时的退相干特征

解析地分析了在囚禁离子中由热库导致量子退相干的控制问题,提出了一个基于热库调制的抑制退相干的一般方法.热库的调制可利用外加的光场与离子相互作用来实现.应用这个方法,适当地选择热库调制参数,可以有效消除量子退相干.计算表明:在高的调制频率下,用这个机制来抑制退相干的效果与宇称激励机制相似.宇称激励机制不仅要求外场是±π脉冲而且要求脉冲既快又强,而热库调制机制仅要求外场频率足够高,因此,热库调制机制优于宇称激励机制.     

Dzyaloshinskii-Moriya相互作用和內禀退相干对三粒子XXZ海森堡系统的量子纠缠的影响

根据Milburn理论, 研究了考虑内禀退相干情况下, 三粒子XXZ海森堡系统在Dzyaloshinskii-Moriya(DM)相互作用和各向异性参数的影响下的量子纠缠演化特性。 分析了不同DM相互作用和內禀退相干因子等参数对量子对纠缠度的影响。 研究表明： 系统的对纠缠度与各向异性参数Δ无关, 但DM相互作用和內禀退相干都对系统的对纠缠都有明显的影响。 当无DM相互作用只存在內禀退相干时, 系统的三对对纠缠度各不相同; 引入DM相互作用后, 系统的对纠缠度的稳定值将改变; 选择合适的DM相互作用参数时, 系统的对纠缠度随时间做震荡, 随着时间的延长, 震荡减弱至一个非零的稳定值; 并且系统的对纠缠随內禀退相干增大, 震荡幅度变小, 震荡时间变短。 因此, 在內禀退相干存在时, 合适的DM相互作用参数可以有效的控制对纠缠。     

Dzyaloshinskii-Moriya相互作用和內禀退相干对三粒子XXZ海森堡系统的量子纠缠的影响

根据Milburn理论,研究了考虑内禀退相干情况下,三粒子XXZ海森堡系统在DzyaloshinskiiMoriya(DM)相互作用和各向异性参数的影响下的量子纠缠演化特性.分析了不同DM相互作用和內禀退相干因子等参数对量子对纠缠度的影响.研究表明:系统的对纠缠度与各向异性参数Δ无关,但DM相互作用和內禀退相干都对系统的对纠缠都有明显的影响.当无DM相互作用只存在內禀退相干时,系统的三对对纠缠度各不相同;引入DM相互作用后,系统的对纠缠度的稳定值将改变;选择合适的DM相互作用参数时,系统的对纠缠度随时间做震荡,随着时间的延长,震荡减弱至一个非零的稳定值;并且系统的对纠缠随內禀退相干增大,震荡幅度变小,震荡时间变短.因此,在內禀退相干存在时,合适的DM相互作用参数可以有效的控制对纠缠.