Coherence properties and quantum state transportation in an optical conveyor belt

We have prepared and detected quantum coherences of trapped cesium atoms with long dephasing times. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The limiting dephasing effects are experimentally identified, and we present an analytical model of the reversible and irreversible dephasing mechanisms. Our experimental methods are applicable at the single-atom level. Coherent quantum bit operations along with quantum state transport open the route towards a "quantum shift register" of individual neutral atoms.     

蓝移阱中单个铯原子基态磁不敏感态的相干操控

单个中性原子的超精细微波跃迁能级的相干性是基于中性原子量子计算、量子信息处理和量子模拟的基础.我们在实验上利用微波双光子拉曼过程实现了蓝移阱中铯原子基态超精细态|6S1/2,F=3,mF=-1〉和|6S1/2,F=4,mF=1〉间的相干操控,并研究了其相对能级频移随磁场的变化,获得了"魔术"磁场的大小为1.4(2)Gauss(1 Gauss=10-4 T).结果表明,利用魔术磁场可大幅改善超精细态|6S1/2,F=3,mF=-1?和|6S1/2,F=4,mF=1〉之间的相干性,测量到的相干时间可达1.0(1)s.     

长距离长期稳定的量子密钥分发系统

介绍了最近完成的长距离长期稳定的量子密钥分发系统.该系统利用往返光路补偿光纤偏振 抖动和相位漂移的原理,采用结电容平衡魔T网络耦合的单光子探测技术,在506km单模 光纤中实现了长时间（大于12h）稳定的量子密钥分发实验.单脉冲平均光子数为007, 误码率为4%,其中单光子探测器的探测效率大于5%,单脉冲暗计数低于29×10^{-6. 关键词： 量子保密通信 量子密钥分发 单光子探测}     

2 GHz clock quantum key distribution over 260 km of standard telecom fiber

We report a demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length. The differential phase shift QKD protocol was chosen and implemented with a 2 GHz system clock rate. By careful optimization of the 1 bit delayed Faraday-Michelson interferometer and the use of the superconducting single photon detector (SSPD), we achieved a quantum bit error rate below 2% when the fiber length was no more than 205 km, and of 3.45% for a 260 km fiber with 52.9 dB loss. We also improved the quantum efficiency of SSPD to obtain a high key rate for 50 km length.     

One-electron parametric oscillator

The parametric oscillation of a trapped electron is studied and used to measure enhanced spontaneous emission. Hysteresis in this motion provides a one bit memory to store information about excitations made with the electron in the dark.Dedicated to H. Walther on the occasion of his 60th birthday     

Differential light-shift cancellation in a magnetic-field-insensitive transition of 87rb

The precise measurement of transition frequencies of trapped atomic samples is susceptible to inaccuracy arising from the inhomogeneous differential shift of the relevant energy levels in the presence of the trapping fields. We demonstrate near-complete cancellation of the differential ac Stark shift ("light shift") of a two-photon magnetic-field-insensitive microwave hyperfine (clock) transition in ^{87}Rb atoms trapped in an optical lattice. Up to 95(2)% of the differential light shift is cancelled while maintaining magnetic-field insensitivity. This technique should have applications in quantum information and frequency metrology.     

Coupling a single atomic quantum bit to a high finesse optical cavity

The quadrupole S(1/2)-D(5/2) optical transition of a single trapped Ca+ ion, well suited for encoding a quantum bit of information, is coherently coupled to the standing wave field of a high finesse cavity. The coupling is verified by observing the ion's response to both spatial and temporal variations of the intracavity field. We also achieve deterministic coupling of the cavity mode to the ion's vibrational state by selectively exciting vibrational state-changing transitions and by controlling the position of the ion in the standing wave field with nanometer precision.     

Operational quantification of continuous-variable correlations

We quantify correlations (quantum and/or classical) between two continuous-variable modes as the maximal number of correlated bits extracted via local quadrature measurements. On Gaussian states, such "bit quadrature correlations" majorize entanglement, reducing to an entanglement monotone for pure states. For non-Gaussian states, such as photonic Bell states, photon-subtracted states, and mixtures of Gaussian states, the bit correlations are shown to be a monotonic function of the negativity. This quantification yields a feasible, operational way to measure non-Gaussian entanglement in current experiments by means of direct homodyne detection, without a complete state tomography.     

A New Approach to Measurement of Heating Rate of Trapped Ions

We propose a new scheme to estimate the heating rate of trapped ions in thermal states. By applying a controlled-U gate between the internal and the motional states of one of the trapped ions, we could obtain the mean phonon number from the population of the internal state of the ion. The imperfection due to fluctuations of the relevant parameters in real experiments is considered and we anaiyze the experimental feasibility of our scheme with sophisticated ion trap techniques.     

偏振光学系统中相位延迟机理及其应用

为了实现偏振编码的自由空间量子密钥分发实验,研制了偏振保持的光机系统,并对该系统所采用的相位延迟传输机理及应用进行了研究,建立了偏振误码率在允许范围内的量子链路.首先,采用矩阵光学理论对偏振光的方位角、相位延迟与消光比的关系进行了介绍.接着,通过矩阵光学理论及实验验证了偏振光学系统的相位延迟线性叠加原理.然后,在相位延迟线性叠加原理的基础上,设计了一套偏振保持光学系统,并通过理论分析及实验验证了此系统具备良好的偏振保持效果.最后,将偏振保持光学系统的设计机理应用于量子通信光机系统的设计之中,并取得了良好的设计效果.实验结果表明:相位之间的相互抵消可以有效地进行偏振保持设计,最终设计的量子通信光机系统的偏振消光比优于500∶1.满足了自由空间量子通信实验中对偏振误码率的要求.     

一种基于确定性量子密钥分发误码判据的相位调制器半波电压的精确测定方法

基于相位编码的量子密钥分发系统需要对信息加载的相位调制器的半波电压进行精确的测定以减小量子密钥的误码率,相位调制器半波电压的测量精度直接影响到了量子密钥分发系统的最终误码.本文提出了一种基于确定性量子密钥分发误码率判据的相位调制器半波电压的精确测定方法,所采用相位调制器的半波电压的测量精度达到了2mV,实验结果表明这种方法可以用于量子密钥分发实际应用系统中实时获得不同条件下的行波相位调制器的半波电压以最大程度地减小由于相位信息不准确加载而带来的系统误码.     

Trapped-ion quantum simulator: experimental application to nonlinear interferometers

We show how an experimentally realized set of operations on a single trapped ion is sufficient to simulate a wide class of Hamiltonians of a spin-1/2 particle in an external potential. This system is also able to simulate other physical dynamics. As a demonstration, we simulate the action of two nth order nonlinear optical beam splitters comprising an interferometer sensitive to phase shift in one of the interferometer beam paths. The sensitivity in determining these phase shifts increases linearly with n, and the simulation demonstrates that the use of nonlinear beam splitters (n=2,3) enhances this sensitivity compared to the standard quantum limit imposed by a linear beam splitter (n=1).     

Exciting collective oscillations in a trapped 1D gas

We report on the realization of a trapped one-dimensional Bose gas and its characterization by means of measuring its lowest lying collective excitations. The quantum degenerate Bose gas is prepared in a 2D optical lattice, and we find the ratio of the frequencies of the lowest compressional (breathing) mode and the dipole mode to be (omega(B)/omega(D))(2) approximately 3.1, in accordance with the Lieb-Liniger and mean-field theory. For a thermal gas we measure (omega(B)/omega(D))(2) approximately 4. By heating the quantum degenerate gas, we have studied the transition between the two regimes. For the lowest number of particles attainable in the experiment the kinetic energy of the system is similar to the interaction energy, and we enter the strongly interacting regime.     

Measurement of the coherence time of the ground-state Zeeman states in <Superscript>40</Superscript>Ca<Superscript>+</Superscript>

The transition between the two Zeeman sublevels of ²S_1/2 in a single trapped ⁴⁰Ca⁺ ion is directly excited by a radio-frequency magnetic field. A coherence time of 29.4±2.5 ms is measured by Ramsey interferometry. The ratio of the maximum Rabi frequency of the Zeeman transition (∼250 kHz) to the inverse of the coherence time is 7.3×10³, which is considered to be a figure of merit for Zeeman states when they are used as a quantum bit. Several applications of the Zeeman qubit to quantum information processing are also discussed.     

量子阱激光器混沌相位控制同步以及编码研究

提出外部光注入多量子阱激光器激光混沌相位控制同步，及其混沌相位相移键控外调制ON/ON全光保密通信系统，数值模拟了在相位控制器控制下的激光混沌同步.通过连续ON/ON键控调制相位控制器调制通过相位控制器的激光相位相移，实现激光混沌相位相移键控连续调制，设置接收系统相位控制器，控制通过接收相位控制器的激光相位相移，以实现激光混沌解调.分别数值模拟了该系统调制速率01和05Gbit/s混沌相位相移ON/ON键控通信，进行了混沌ON/ON键控调制速率分析，以及系统参数失配和系统抗噪声扰动的数值分析. 关键词： 混沌 同步 通信 多量子阱激光器     

Optical pumping of trapped neutral molecules by blackbody radiation

Optical pumping by blackbody radiation is a feature shared by all polar molecules and fundamentally limits the time that these molecules can be kept in a single quantum state in a trap. To demonstrate and quantify this, we have monitored the optical pumping of electrostatically trapped OH and OD radicals by room-temperature blackbody radiation. Transfer of these molecules to rotationally excited states by blackbody radiation at 295 K limits the 1/e trapping time for OH and OD in the X(2)Pi(3/2), v" =0, J"=3/2(f) state to 2.8 and 7.1 s, respectively.     

Comment on “A special attack on the multiparty quantum secret sharing of secure direct communication using single photons”

In this comment, we show that the special attack [S.-J. Qin, F. Gao, Q.-Y. Wen, F.-C. Zhu, Opt. Commun. 281 (2008) 5472.], which claims to be able to obtain all the transmitted secret message bit values of the protocol of the multiparty quantum secret sharing of secure direct communication using single photons with random phase shift operations, fails. Furthermore, a class of similar attacks are also shown to fail to extract the secrete message.     

Universal Ratio of Coulomb Interaction to Geometric Quantization in (In,Ga)As/GaAs Quantum Dots

We study the photoluminescence of self-assembled (In,Ga)As/GaAs quantum dot ensembles with varying confinement potential height. The low energy shift of the s-shell emission with increasing excitation power gives a measure of the Coulomb interaction in these structures as it results from carrier–carrier interactions between the optically injected exciton complexes. When dividing this shift by the dot level splitting, determined by the geometric confinement, we obtain a universal function of the number of involved excitons that is independent of the confinement potential height. This shows an identical scaling of Coulomb interaction and geometric quantization with varying confinement.     

Quantum Computation Toward Quantum Gravity

The aim of this paper is to enlighten the emerging relevance of Quantum Information Theory in the field of Quantum Gravity. As it was suggested by J. A. Wheeler, information theory must play a relevant role in understanding the foundations of Quantum Mechanics (the "It from bit" proposal). Here we suggest that quantum information must play a relevant role in Quantum Gravity (the "It from qubit" proposal). The conjecture is that Quantum Gravity, the theory which will reconcile Quantum Mechanics with General Relativity, can be formulated in terms of quantum bits of information (qubits) stored in space at the Planck scale. This conjecture is based on the following arguments: a) The holographic principle, b) The loop quantum gravity approach and spin networks, c) Quantum geometry and black hole entropy. From the above arguments, as they stand in the literature, it follows that the edges of spin networks pierce the black hole horizon and excite curvature degrees of freedom on the surface. These excitations are micro-states of Chern-Simons theory and account of the black hole entropy which turns out to be a quarter of the area of the horizon, (in units of Planck area), in accordance with the holographic principle. Moreover, the states which dominate the counting correspond to punctures of spin j = 1/2 and one can in fact visualize each micro-state as a bit of information. The obvious generalization of this result is to consider open spin networks with edges labeled by the spin –1/ 2 representation of SU(2) in a superposed state of spin "on" and spin "down." The micro-state corresponding to such a puncture will be a pixel of area which is "on" and "off" at the same time, and it will encode a qubit of information. This picture, when applied to quantum cosmology, describes an early inflationary universe which is a discrete version of the de Sitter universe.