Estimation of the disorder degree of the classical static noise using three entangled qubits as quantum probes

The paper investigates the estimation of the disorder degree of the classical static noise using three entangled qubits as quantum probes together with the tools of local quantum estimation theory. Three probing schemes namely common environment (CE), independent environments (IEs) and mixed environments (MEs) are investigated and the optimal initial state preparation of the probes taken as a partially depolarized GHZ state. The results show that: (i) the IEs probing scheme allows one to achieve better estimation precision compared to both MEs and CE schemes respectively; (ii) the higher is the initial amount of entanglement of the probes, the larger is the estimation precision, independently of the scheme considered; (iii) both small and large values of the disorder parameter are uniformly estimable at the optimal interaction time; (iv) entangled qubits probes quickly encode information about the disorder parameter than single-qubit probe; (v) there is an improvement in the estimation of the disorder parameter when entangled probes interacting either in IEs or MEs are used instead of a single probe, demonstrating that a single probe is not sufficient to optimally estimate the disorder parameter of the static noise. On the other hand, we have also investigated the relationship between the residual amount of entanglement present in the probes at the optimal interaction time and the estimation precision of the disorder parameter. We show that the higher the residual amount of entanglement at the optimal interaction time, the smaller the estimation precision.     

Tripod型双电磁诱导透明原子系统中压缩光的传输

采用海森堡-朗之万方法理论研究了Tripod型双电磁诱导透明原子系统中压缩态探针场的传输特性.研究结果表明:通过双透明窗口压缩光可实现双通道传输,且每个通道可以被独立操控;当两束耦合场的频率失谐相等时,输出探针场的压缩度可以得到更好的保持.此外,输出探针场的压缩度可以通过耦合场的拉比频率、原子的光学厚度和基态退相干率以及探测频率来操控.该研究结果为进一步优化多通道量子存储提供依据.     

Quantum Coherence in Non-Markovian Quantum Channels

We numerically simulate quantum coherence in a system of two qubits interacting with a reservoir for non-Markovian channels. The explicit form of the master equation is taken in terms of density-operator elements and is solved according to the initial conditions. In particular, we consider the effect of an Ohmic reservoir (OR) with Lorentz–Drude regularization (LDR) on the extent of coherence during dynamics. We describe the dynamical behavior of the coherence for low, intermediate, and high-temperature reservoirs. We explain the effect of the ratio of the cutoff frequency (CF) to the quantum system frequency and the effect of temperature on the quantum coherence. We show that a decreasing ratio enhances coherence, while an increasing temperature decreases it.

     

Non-Markovian dynamics of two non-coupled qubits interacting with two separate reservoirs with different spectral densities

The dynamics of two non-coupled qubits independently interacting with their reservoirs is solved by the time convolutionless projection operator method. We study two-qubit quantum correlation dynamics for two different types of spectral densities, which are a Lorentzian distribution and an Ohmic spectral density with a Lorentzian-Drude cutoff function. For two qubits initially prepared in the initial Bell state, quantum discord can keep longer time and reach larger values in nonMarkovian reservoirs for the first spectral distribution or by reducing the cutoff frequency for the second case. For the initial Bell-like state, the dynamic behaviors of quantum discord and entanglement are compared. The results show that a long time of quantum correlation can be obtained by adjusting some parameters in experiment and further confirm that the discord can capture quantum correlation in addition to entanglement.     

Dissipative multistate systems in the scaling limit

The dynamics of dissipative multistate systems is studied using integral equations that are derived within the framework of the path integral-formulation of quantum mechanics. As an illustrative example we study a 5-state system coupled to a harmonic reservoir with an Ohmic spectral density with a high cutoff frequency. The dominant exchange mechanism determining the dynamics as well as the range of validity of different approximations to the influence functional are investigated. Besides depending on temperature and the strength of the system-bath coupling, both the exchange mechanism and the accuracy of different approximations to the influence functional depend on the state in which the system is initially prepared.     

Information transmission with Einstein-Podolsky-Rosen pairs and imperfect Bell-state measurement

We propose a feasible information transmission protocol using EPR (Einstein-Podolsky-Rosen) pairs. The transmitter only need distinguish partially Bell states and publicize the corresponding classical information, and the recipient performs single-qubit measurements to obtain secret information. It can save a lot of classical communication cost than the general ones using quantum teleportation. Its feasibility in noise and loss channel is also taken into account.     

Fast initial qubit dephasing and the influence of substrate dimensions on error correction rates

Keeping single-qubit quantum coherence above some threshold value not far below unity is a prerequisite for fault-tolerant quantum error correction (QEC). We study the initial dephasing of solid-state qubits in the independent-boson model, which describes well recent experiments on quantum dot (QD) excitons both in bulk and in substrates of reduced geometry such as nanotubes. Using explicit expressions for the exact coherence dynamics, a minimal QEC rate is identified in terms of error threshold, temperature, and qubit-environment coupling strength. This allows us to systematically study the benefit of a current trend towards substrates with reduced dimensions.     

New results on ultrafast coherent excitation of molecular vibrations in liquids

Coherent Raman probe scattering experiments are performed to study dynamical processes of polyatomic liquids at 300 K. For single homogeneous transitions the dephasing timeT ₂ is readily obtained from time resolved investigations. Spectral studies show an interesting time dependent shift in scattered frequency. After the excitaiton the vibrating molecules are shown to relax freely with their resonance frequency. Multiple, equally spaced transitions exhibit a beating phenomenon which provides the dephasing time and the frequency interval between neighboring vibrational states. Inhomogeneously broadened systems do not allow a ready determination of the dephasing time by the present probing technique. Previous experiments on the subject have to be reconsidered.     

Role of environment and confinement in quantum dissipative dynamics: A pedestrian approach

In this paper, the long time and short time behavior of a free quantum Brownian particle and of a quantum harmonic oscillator coupled to a quantum heat bath is investigated for different spectral density functions. Not only are the usual Ohmic, Drude or radiation heat bath schemes employed to derive explicit expressions for the generalized susceptibility χ(t)$\chi \left(t\right)$ and the position correlation function σ(t)$\sigma \left(t\right)$ but also the most intriguing frequency dependent heat bath spectra are considered. Emphasis is laid on the dependence on the low-frequency behavior but in some cases also the high-frequency cutoff becomes relevant. The present investigation also sheds light on the role of boundary in long time and short time behavior of quantum dissipative systems.     

量子点腔非谐振耦合系统纯退相干的特性及应用

采用非相干泵浦、受激辐射和纯退相干的量子主方程研究了量子点腔耦合系统,得出腔与量子点发射光谱解析解.理论分析显示,在非谐振耦合系统中纯退相干能使腔发射谱产生明显的移位效应,从而可以解释“非谐振耦合腔有效发射”效应.为了进一步研究纯退相干在量子点腔耦合系统上的应用,引入了系统有效耦合率和单光子源效率,并通过比较有效耦合率与腔耗散定义出好腔与坏腔机制.选取两组依据实验数据作为参量,在共振与失谐时研究了纯退相干对系统有效耦合率和单光子源效率的影响.结果表明:纯退相干可提高失谐系统有效耦合率与单光子源效率,从而可能使坏腔转变为好腔|两组参量中有较大耦合效率一组在一定范围内满足好腔机制,其单光子源效率明显优于另一组.在非谐振耦合系统比较了好腔机制与坏腔机制的激光,好腔机制是实现单量子点激光的必要条件|由于非谐振耦合系统Fano因子无最大值出现,从而该系统可能无激光阈值.     

Environmental parameter estimation with the two-level atom probes

A novel scheme is proposed to estimate three environmental parameters, the detuning, the temperature and the squeezing strength with one-qubit or two-qubit probes. Quantum Fisher information and the fidelity of the atom probes are calculated. When the detuning between the frequency of cavity field and the atomic transition frequency is estimated, the dynamics of quantum Fisher information shows oscillatory and rising behaviors. To estimate the temperature of the thermal reservoir, the one-qubit probe with the superposition initial state is more favorable than the two-qubit probe with the entangled initial state. When the squeezing strength of the squeezed vacuum reservoir is estimated, we find that the estimation precision is significantly improved by utilizing the two-qubit probe with the maximal entangled initial state. Our work provides a potential application in the open quantum system and quantum information processing.     

Induced probe-beam modulation and resonatorless optical bistability in semiconductor (CdSxSe1−x)-doped glass

Induced probe-beam modulation in semiconductor-doped glass was observed. The dephasing between pump and probe modulation is dependent on the pump power, the modulation frequency and the selenium concentration. At room temperature, with a proper choice of these parameters, the dephasing can change from 0° (laser-induced clearing) to –180° (laser-induced darkening). A slow switching behaviour and optical bistability without external feedback are also reported.     

Quantum mechanical model for two-state jump Markovian process

A quantum mechanical model is given which is equivalent to the stochastic dephasing subject to the two-state jump Markovian process. The stochastic variable corresponds to a Hermitian operator of a spin-1/2 system which is embedded in a thermal reservoir, where the time-evolution of the spin-1/2 system is described by the quantum master equation of the Lindblad form.     

On the origin of time-resolved pump–probe differential signal from excitons in quantum wells

We investigate the origin and nature of femtosecond time-resolved (TR) pump–probe differential (PPD) signal from the exciton resonance in semiconductor quantum wells (QWs) in the coherent regime. The excitonic PPD signal is expressed as a sum of interferences of the excitonic nonlinear polarization with the incident probe field and the linear polarization induced by the probe. A nonzero excitonic TR-PPD signal from QWs can occur for time t larger than the pulse width (t_p) when the exciton dephasing time is longer than t_p, and this signal can originate only from the second interference term for both the negative and positive pump–probe delays, the contribution from the first interference term being zero for t>t_p. This is in contrast with the case of PPD signal in the spectral domain where the first interference term plays a major role. We discuss the effects of many body interactions on the TR-PPD signal. It is shown that excitation-induced dephasing can cause a significant increase in the rise time of the TR-PPD signal.     

Reprint of : Single-electron coherence: Finite temperature versus pure dephasing

We analyze a coherent injection of single electrons on top of the Fermi sea in two situations, at finite-temperature and in the presence of pure dephasing. Both finite-temperature and pure dephasing change the property of the injected quantum states from pure to mixed. However, we show that the temperature-induced mixedness does not alter the coherence properties of these single-electron states. In particular two such mixed states exhibit perfect antibunching while colliding at an electronic wave splitter. This is in striking difference with the dephasing-induced mixedness which suppresses antibunching. On the contrary, a single-particle shot noise is suppressed at finite temperatures but is not affected by pure dephasing. This work therefore extends the investigation of the coherence properties of single-electron states to the case of mixed states and clarifies the difference between different types of mixedness.     

Geometric quantum discord and non-Markovianity of structured reservoirs

The reservoir memory effects can lead to information backflow and recurrence of the previously lost quantum correlations. We establish connections between the direction of information flow and variation of the geometric quantum discords (GQDs) measured respectively by the trace distance, the Hellinger distance, and the Bures distance for two qubits subjecting to the bosonic structured reservoirs, and unveil their dependence on a factor whose derivative signifies the (non-)Markovianity of the dynamics. By considering the reservoirs with Lorentzian and Ohmic-like spectra, we further demonstrated that the non-Markovianity induced by the backflow of information from the reservoirs to the system enhances the GQDs in most of the parameter regions. This highlights the potential of non-Markovianity as a resource for protecting the GQDs.     

Effects of single-qubit quantum noise on entanglement purification

We study the stability under quantum noise effects of the quantum privacy amplification protocol for the purification of entanglement in quantum cryptography. We assume that the E91 protocol is used by two communicating parties (Alice and Bob) and that the eavesdropper Eve uses the isotropic Bužek-Hillery quantum copying machine to extract information. Entanglement purification is then operated by Alice and Bob by means of the quantum privacy amplification protocol and we present a systematic numerical study of the impact of all possible single-qubit noise channels on this protocol. We find that both the qualitative behavior of the fidelity of the purified state as a function of the number of purification steps and the maximum level of noise that can be tolerated by the protocol strongly depend on the specific noise channel. These results provide valuable information for experimental implementations of the quantum privacy amplification protocol.     

Single-electron coherence: Finite temperature versus pure dephasing

We analyze a coherent injection of single electrons on top of the Fermi sea in two situations, at finite-temperature and in the presence of pure dephasing. Both finite-temperature and pure dephasing change the property of the injected quantum states from pure to mixed. However, we show that the temperature-induced mixedness does not alter the coherence properties of these single-electron states. In particular two such mixed states exhibit perfect antibunching while colliding at an electronic wave splitter. This is in striking difference with the dephasing-induced mixedness which suppresses antibunching. On the contrary, a single-particle shot noise is suppressed at finite temperatures but is not affected by pure dephasing. This work therefore extends the investigation of the coherence properties of single-electron states to the case of mixed states and clarifies the difference between different types of mixedness.     

Entanglement and non-Markovianity of a spin-S system in a dephasing environment

We study the entanglement （measured by negativity） evolution and the non-Markovianity for the dynamical process of a spin-S system embedded in dephasing environments. The exact analytical solution is presented, which shows that the decoherence function governs the evolutions of coherence, entanglement, and the non-Markovianity of the correspond- ing dynamical processes. For Ohmic and sub-Ohmic reservoirs, the negativity decreases monotonically in time and the corresponding dynamics is Markovian. While for super-Ohmic reservoirs with non-monotonic decoherence function, the negativity appears as the phenomenon of revival and the corresponding dynamics is non-Markovian. The relation between non-Markovianity and the system dimension is studied.     

实现D-J量子算法的物理方案

利用腔QED技术,我们在本文提出了两个物理方案用来实现最简单版本的Deutsch-Jozsa(D-J)量子算法.第一个方案是比较理想的方案,这个方案可以推广到多个量子比特输入的Deutsch-Jozsa量子算法.我们只需要通过实现控制-非门和一系列单个量子比特操作,就可以简单的实现该方案.我们在这个方案中详细地讨论了基于腔QED技术最简单版本的Deutsch-Jozsa量子算法的实现过程.另一个方案是不需要控制-非门的更简单的方案,但是这个方案仅仅适用于实现这种最简单版本的Deutsch-Jozsa量子算法,这个方案只需要实现单个量子比特操作即可.显然,该方案比第一个方案更简化.我们的这两个方案可能是实现量子计算机的一个重要环节.