Quantum leakage of collective excitations of atomic ensemble induced by spatial motion

We generalize the conception of quantum leakage for the atomic collective excitation states. By making use of the atomic coherence state approach, we study the influence of the atomic spatial motion on the symmetric collective states of 2-level atomic ensemble due to inhomogeneous coupling. In the macroscopic limit, we analyze the quantum decoherence of the collective atomic state by calculating the quantum leakage for a very large ensemble at a finite temperature. Our investigations show that the fidelity of the atomic system will not be good in the case of atom numberN→∞. Therefore, quantum leakage is an inevitable problem in using the atomic ensemble as a quantum information memory. The detailed calculations shed theoretical light on quantum processing using atomic ensemble collective qubit.     

Coherent-state qubits: entanglement and decoherence

Arbitrary superpositions of any two optical coherent states are investigated as realizations of qubits for quantum information processing. Decoherence of these coherent-state qubits is described in detail, and visualized using a suitable adaptive Bloch-sphere. The entanglement that can be created by a beam splitter from these states is quantified, and its decoherence behavior is analyzed.Received: 13 May 2004, Published online: 10 August 2004PACS: 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements - 03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bells inequalities, GHZ states, etc.) - 03.67.-a Quantum information     

Quantum fluctuations in holographic teleportation of optical images

We investigate in detail the quantum fluctuations in the quantum holographic teleportation protocol that we recently proposed [11]. This protocol implements a continuous variable teleportation scheme that enables the transfer of the quantum state of spatially multimode electromagnetic fields, preserving their quantum correlations in space-time, and can be used to perform teleportation of 2D optical images. We derive a characteristic functional, which provides any arbitrary spatio-temporal correlation function of the teleported field, and calculate the fidelity of the teleportation scheme for multimode Gaussian input states. We show that for multimode light fields one has to distinguish between a global and a reduced fidelity. While the global fidelity tends to vanish for teleportation of fields with many degrees of freedom, the reduced fidelity can be made close to unity by choosing properly the number of essential degrees of freedom and the spatial bandwidth of the EPR beams used in the teleportation scheme.Received: 16 March 2004, Published online: 11 May 2004PACS: 03.67.-a Quantum information - 03.65.Bz Foundations, theory of measurement, miscellaneous theories (including Aharonov-Bohm effect, Bell inequalities, Berrys phase) - 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements     

基于电磁诱导透明机制的压缩光场量子存储

光场的量子存储不仅是构建量子计算机的重要基础,而且是实现量子中继和远距离量子通信的核心部分.由于存在不可避免的光学损耗,光学参量放大器产生的压缩真空态光场将变为压缩热态光场,不再是最小不确定态.因此,压缩热态光场的量子存储是实现量子互联网的关键.在原子系综中利用电磁诱导透明机制能够实现量子态在光场正交分量和原子自旋波之间的相互映射,即受控量子存储.本文根据量子存储的保真度边界,研究了实现压缩热态光场量子存储的条件.量子存储的保真度边界是通过经典手段能够达到的最大保真度,当保真度大于该边界时,就实现了量子存储.通过数值计算分析了不同情况下压缩热态光场的量子存储保真度边界,以及存储保真度随存储效率的变化关系,得到了实现量子存储的条件,为连续变量量子存储实验设计提供了直接参考.     

在五能级三重∧型原子系综中利用暗态极化子理论制备W态(英文)

在由三个经典控制场驱动的五能级三重(?)型原子系综与三个多模量子光场相互作用的系统中,得到了该系统的极化子.利用求得的极化子结果研究了光量子态存储到原子激发态,或从原子系综中释放出光量子信息.在释放过程中,通过绝热调节控制场的Rabi频率,能得到纠缠光子态.尤其是在一定条件下,能利用该系统能制备一类W态,这类态在量子信息处理中有潜在的应用.     

Doppler-free spectroscopy in driven three-level systems

We demonstrate two techniques for studying the features of three-level systems driven by two lasers (called control and probe), when the transitions are Doppler broadened as in room-temperature vapor. For -type systems, the probe laser is split to produce a counter-propagating pump beam that saturates the transition for the zero-velocity atoms. Probe transmission then shows Doppler-free peaks which can even have sub-natural linewidth. For V-type systems, the transmission of the control beam is detected as the probe laser is scanned. The signal shows Doppler-free peaks when the probe laser is resonant with transitions for the zero-velocity group. Both techniques greatly simplify the study of three-level systems since theoretical predictions can be directly compared without complications from Doppler broadening and the presence of multiple hyperfine levels in the spectrum.Received: 10 September 2003, Published online: 6 January 2004PACS: 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 42.50.-p Quantum optics     

Quantum memory for photons in case of many close lying exciton resonances in solids

The possibility of storage of quantum information with photons is studied in the case of resonant transitions via many close lying exciton levels in a solid with impurity -atoms. The upper levels of the impurity atom form resonant Fano states, similar to the autoionization atomic states, due to the configuration interaction with the continuum of the exciton band. In this case slowing of light pulses is shown to be realistic, in the presence of the control field, down to the group velocity much lower than that in vacuum. The possibility of storage and reconstruction of a quantum pulse is studied in the case of the instantaneous switching on/off of the control field. It is shown that the signal quantum pulse cannot be stored undistorted for differing values of Fano parameters and for non-zero two-photon detuning and decay rate between the lower levels (decoherence). However, for small difference of the Fano parameters and for small values of the two-photon detuning and the decoherence there is no distortion in the case where the length of the pulse is much longer than the linear absorption (amplification) length, so the shape and quantum state of the light pulse can be restored.     

Adiabatic Passage of Collective Excitations in Atomic Ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Rmann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.     

Adiabatic Passage of Collective Excitations in Atomic Ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Ramann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.     

电磁诱导光透明过程中的Wigner-Yanse偏振信息

利用约化密度矩阵及信息的定义,研究了电磁诱导光透明机理下, 控制场变化过程中探测场与原子系综的Wigner-Yanse偏振信息, 结果表明:探测场信息转移过程中,原子的信息量不仅依赖于光子的数目及光子的状态, 还依赖于系综内原子的数目;调节控制场,使探测场不能通过介质时,探测场完成信息转移, 原子系综内单个原子信息量获得最大值,但探测场的信息量并没有完全地转移到原子系综. 关键词： 电磁诱导光透明 暗态极化子 Wigner-Yanse偏振信息     

Spinor slow-light and dirac particles with variable mass

We consider the interaction of two weak probe fields of light with an atomic ensemble coherently driven by two pairs of standing wave laser fields in a tripod-type linkage scheme. The system is shown to exhibit a Dirac-like spectrum for light-matter quasiparticles with multiple dark states, termed spinor slow-light polaritons. They posses an "effective speed of light" given by the group velocity of slow light, and can be made massive by inducing a small two-photon detuning. Control of the two-photon detuning can be used to locally vary the mass including a sign flip. Particularly, this allows the implementation of the random-mass Dirac model for which localized zero-energy (midgap) states exist with unusual long-range correlations.     

Quantum memory process with a four-level atomic ensemble

We examine in detail the quantum memory technique for photons in a double Λ atomic ensemble in this work. The novel application of the present technique to create two different quantum probe fields as well as entangled states of them is proposed. A larger zero-degeneracy class besides dark-state subspace is investigated and the adiabatic condition is confirmed in the present model. We extend the single-mode quantum memory technique to the case with multi-mode probe fields, and reveal the exact pulse matching phenomenon between two quantized pulses in the present system.     

Quantum well states in thin (110)-oriented Au films and k-space symmetry

We present electron diffraction and electron photoemission results for thin Au films grown on Nb(100) in a hexagonal close-packed stacking sequence, which is unusual for Au. Strong d-band [48] quantum size effects occur in photo-electron spectroscopy from 5-26 monolayer thick ( ) oriented Au films whose confined direction ([11.0]) is not perpendicular to any face of the bulk Brillouin zone. Also in this case, the energetics of the quantum well states can be explained by a discretisation of the bulk band structure corresponding to the quantum well. However, the bulk states corresponding to the quantum well states do not lie in the confined direction of the first bulk Brillouin zone, contrary to what is required by the quantum well. This can be remedied by the construction of a layer symmetry adapted Brillouin zone, which is consistent with the symmetry of the quantum well but different from the bulk one. We subsequently determine, for the first time, the periodicity of the quantum well states from their measured ). This periodicity proves to be consistent with the newly introduced Brillouin zone.Received: 1 December 2003, Published online: 15 March 2004PACS: 73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems - 82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.) - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections - 71.20.Gj Other metals and alloys     

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.     

Continuous weak measurement and nonlinear dynamics in a cold spin ensemble

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a nonperturbing measurement on the much longer time scale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.     

Optical Switch Based on Dressed Intracavity Dark States

We present a scheme to realize two‐direction optical switch by a single‐mode optical cavity containing some four‐level atoms. The high switching efficiency can be obtained through low photon loss and large third‐order nonlinear susceptibility of this N‐type atomic system in cavity. Without the microwave source, it can be reduced to a Λ‐type atomic system where a coupling laser is used to realize single intracavity electromagnetically induced transparency (EIT). Namely, the probe field can be transmitted almost totally at resonance. Thus a two‐direction optical switch is operated and the state for forward (backward) direction is set as “open” (“closed”). When microwave source is introduced, dressed splitting of intracavity dark state happens. The probe field is reflected almost completely at resonance and the state of the optical switch at forward and backward directions (transmitted and reflected channels) is shifted as “closed” and “open”, respectively. Moreover, this scheme is much advantageous to realize splitting of intracavity dark state because weak microwave field () induces the coupling between intracavity dark state and one sublevel of ground state. While a strong pump laser () which couples the intracavity dark state with an excited level is applied to realize this splitting in ref. [Phys. Rev. A 85 013814 (2012)].     

冷原子系综中两正交光场偏振模高效率存储的实验研究

高效率光量子信息存储是可扩展光量子信息处理的一个重要工具.本文对一个冷原子系综中两正交光场偏振模的高效率存储进行了实验研究.通过在雪茄型冷原子系统上施加一个中等强度的磁场,消除了原子Zeeman子能级的简并性,从而使磁敏感自旋波从电磁感应透明系统中被移出,由此完成了两正交光场偏振模高效率、长寿命的量子存储.实验测量了两偏振模存储效率与存储时间以及实验重复频率的关系,结果表明,随着重复频率的增加,存储效率逐渐降低,在10 Hz时,测量得到两偏振模存储效率为30%,同时存储寿命达到3 ms.测量结果为偏振纠缠在冷原子系统中的存储提供了重要的实验基础.     

Generalized two-mode harmonic oscillator model: squeezed number state solutions and nonadiabatic Berry’s phase

A generalized two-mode harmonic oscillator model is investigated within the framework of its general dynamical algebra so(3,2). Two types of eigenstates, formulated as extended su(1,1), su(2) squeezed number states are found respectively. The nonadiabatic Berrys phase for this system with the cranked time-dependent Hamiltonian is also given.Received: 16 January 2004, Published online: 10 August 2004PACS: 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements - 03.65.Fd Algebraic methods - 03.65.Vf Phases: geometric; dynamic or topological     

Quantum light memory using quantum dot spins in a microdisk cavity via Raman process

A solid state quantum circuit where an ensemble of self-assembled quantum dots in a microdisk cavity served as long-lived quantum light memory, is investigated. It is shown that via laser coupling Raman process, the coherent transfer between the light field (qubits) and the ensemble spin states of the quantum dots can be efficient and fast. The coherence properties of the system are analyzed, which enables us to obtain a long coherence time.