Quantum recurrences in driven power-law potentials

The recurrence phenomena of an initially well-localized wave packet are studied in periodically driven power-law potentials. For our general study we divide the potentials in two kinds, namely tightly binding and loosely binding potentials. In the presence of an external periodically modulating force, these potentials may exhibit classical and quantum chaos. We show that in the dynamics of a quantum wave packet in the modulated power law potentials quantum recurrences occur at various time scales. We develop general analytical relations for these times and discuss their parametric dependence.     

Quantum effect in one-body dissipation

The influence of quantum effects on the one-body dissipation has been studied in a Monte-Carlo simulation method. The dissappearance of quantum effects with increasing amplitude of the oscillation is demonstrated.     

Quantum localization in one-body dissipation

We show, using the quantum Fermi Accelerator model, that the energy dissipated may saturate in a time of the order of the characteristic period of the mean field oscillation. This localization time increases if the periodicity of the driving field is broken by the addition of a second driving frequency in an irrational ratio with the first. However, energy localization is still present, contrary to standard expectations.     

Quantum ohmic dissipation: Transition from coherent to incoherent dynamics

We investigate the real-time dynamics of a particle in a double well coupled to phonons with ohmic dissipation. We refine previous results about the T=0 symmetry breaking: the critical behaviour of the order parameter is displayed and the symmetry is shown to be only fully broken for infinite coupling.     

Quantum dissipation in vacuum neutrino oscillation

The pattern of neutrino oscillations changes when one considers these particles evolving as an open quantum system. In this article we derive seven possibilities that change the two-generation neutrino survival probability due to quantum dissipation and decoherence. We find entirely original probabilities assuming that these effects can be parametrized by the addition of only one phenomenological constant keeping complete positivity. We observe that a relaxation effect of the system shows an unusual mechanism of flavor conversion, the appearance of CP-violation effects for Majorana neutrinos, besides an appreciable change in the behavior of high energy neutrinos.     

Quantum control of two-qubit entanglement dissipation

We investigate quantum control of the dissipation of entanglement under environmental decoherence. We show by means of a simple two-qubit model that standard control methods – coherent or openloop control – will not in general prevent entanglement loss induced by a Markovian environment. However, we propose a control method utilizing a Wiseman–Milburn feedback/measurement control scheme which will effectively negate environmental entanglement dissipation.     

Quantum and classical dissipation of charged particles

A Hamiltonian approach is presented to study the two dimensional motion of damped electric charges in time dependent electromagnetic fields. The classical and the corresponding quantum mechanical problems are solved for particular cases using canonical transformations applied to Hamiltonians for a particle with variable mass. Green’s function is constructed and, from it, the motion of a Gaussian wave packet is studied in detail.     

光量子存储

光纤通信改变了人们感知信息的方式,人们可以充分利用网络获取或者交换信息。在量子领域,光纤内传输的单光子就是一种几乎理想的量子信息载体,它具有自由度多(包括极化、空间、相位和振幅等)、易探测、传输速度快、距离长、损耗低等优点。科学家们正致力于发展量子信息网络中的各类技术以便实现远距离加密传输、分布式量子计算以及量子感知等。为了实现这些远大目标,人们需要克服很多核心技术困难,包括紧凑的纠缠光子源、高效的单光子探测器和可操控频率、波形以及时空自由度的单光子传输设备等。此外,量子存储也是需要突破的核心技术之一,它可作为量子网络中的节点,暂时存储光子量子态,以便进行下一步的量子信息处理。     

Quantum random access memory

A random access memory (RAM) uses n bits to randomly address N=2(n) distinct memory cells. A quantum random access memory (QRAM) uses n qubits to address any quantum superposition of N memory cells. We present an architecture that exponentially reduces the requirements for a memory call: O(logN) switches need be thrown instead of the N used in conventional (classical or quantum) RAM designs. This yields a more robust QRAM algorithm, as it in general requires entanglement among exponentially less gates, and leads to an exponential decrease in the power needed for addressing. A quantum optical implementation is presented.     

Quantum memory for squeezed light

We produce a 600-ns pulse of 1.86-dB squeezed vacuum at 795 nm in an optical parametric amplifier and store it in a rubidium vapor cell for 1 mus using electromagnetically induced transparency. The recovered pulse, analyzed using time-domain homodyne tomography, exhibits up to 0.21+/-0.04 dB of squeezing. We identify the factors leading to the degradation of squeezing and investigate the phase evolution of the atomic coherence during the storage interval.     

Quantum chaos, thermalization and dissipation in nuclear systems

Nuclei have complex energy-level sequence with statistical properties in agreement with canonical random matrix theory. This agreement appears when the one-particle one-hole states are mixed completely with two-particle two-hole states. In the transition, there is a new universality which we present here, bringing about a relation between dynamics and statistics. We summarize also the role of chaos in thermalization and dissipation in isolated systems like nuclei. The methods used to bring forth this understanding emerge from random matrix theory, semiclassical physics, and the theory of dynamical systems.     

量子存储(Quantum memory)

量子存储技术在量子信息理论的各个领域都有着十分重要的意义.例如在BB84密钥分配方案中,引入量子存储器的"先公布基后测量"的策略就可以使它的效率提高一倍从而接近100%.     

Quantum memory with optically trapped atoms

We report the experimental demonstration of quantum memory for collective atomic states in a far-detuned optical dipole trap. Generation of the collective atomic state is heralded by the detection of a Raman scattered photon and accompanied by storage in the ensemble of atoms. The optical dipole trap provides confinement for the atoms during the quantum storage while retaining the atomic coherence. We probe the quantum storage by cross correlation of the photon pair arising from the Raman scattering and the retrieval of the atomic state stored in the memory. Nonclassical correlations are observed for storage times up to 60 mus.     

Quantum memory for images with feedback

A spatially multimode scheme of the quantum memory for light is presented that can be considered to be a thin hologram, whose long-lived spin subsystem of a multiatomic ensemble stores image peculiarities, such as the multimode superposition and entangled quantum states, which is unattainable for ordinary holography. In the recording process, the measurement of polarization parameters of the transmitted light wave and the feedback are used for the action on an ensemble of fixed atoms (obtained, for example, by means of the laser cooling). The information capacity of a quantum hologram with a feedback will considerably exceed that of a spatially singlemode quantum memory.     

Quantum ohmic dissipation: Particle on a one-dimensional periodic lattice

We investigate the dynamics of a particle on a periodic lattice, coupled to phonons with ohmic dissipation. At T = 0 a symmetry breaking appears, which corresponds to a transition between a localized and a delocalized regime. For T > 0 we recover a diffusive motion for which the diffusion coefficient is computed.     

Quantum computing using dissipation to remain in a decoherence-free subspace

We propose a new approach to the implementation of quantum gates in which decoherence during the gate operations is strongly reduced. This is achieved by making use of an environment induced quantum Zeno effect that confines the dynamics effectively to a decoherence-free subspace.