Hybrid quantum processors: molecular ensembles as quantum memory for solid state circuits

We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as long-lived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman processes. We show that, for convenient trap-surface distances of a few microm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.     

Teleportation of an atomic ensemble quantum state

We propose a protocol to achieve high fidelity quantum state teleportation of a macroscopic atomic ensemble using a pair of quantum-correlated atomic ensembles. We show how to prepare this pair of ensembles using quasiperfect quantum state transfer processes between light and atoms. Our protocol relies on optical joint measurements of the atomic ensemble states and magnetic feedback reconstruction.     

基于光学-微波同步的低噪声微波产生方法

低噪声微波在冷原子光钟、光子雷达、大科学装置远程同步等领域具有重要的应用价值.本文介绍了一种基于光学-微波相位探测技术的低噪声微波产生方案,利用光纤环路光学-微波鉴相器,将超稳激光的频率稳定度相干传递至介质振荡器.实验采用梳齿相位参考至超稳激光的窄线宽掺铒光纤飞秒光学频率梳,结合光纤环路光学-微波鉴相器和精密锁相装置,将7 GHz介质振荡器同步至光频梳重复频率的高次谐波,同步后的光脉冲序列与微波信号的剩余相位噪声为–100 d Bc/Hz@1 Hz,定时抖动为8.6 fs [1 Hz—1.5 MHz];通过搭建两套低噪声微波产生系统,测得7 GHz微波的剩余相位噪声为–90 d Bc/Hz@1 Hz,对应的频率稳定度为4.8×10^–15@1 s.该研究结果对基于光学相干分频的低噪声微波产生提供了一种新思路.     

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.     

Dynamics of Spectral Composition of Fluorescence of Cold Atoms in External Electric and Magnetic Fields

A consistent quantum-mechanical approach is used to calculate a fluorescence signal from a cold atomic ensemble excited by pulsed radiation and exposed to a DC electric or magnetic field. The short time Fourier transform is used to analyze the spectral composition of this signal and to investigate its changes over time after the end of excitation. It is demonstrated that external fields substantially change both the shape of the spectrum and its dynamics. The discovered effects are explained as being the result of action of these fields on the spectrum of states of the two-atomic quasi-molecular clusters that randomly form in disordered ensembles.     

Holographic quantum computing

We propose to use a single mesoscopic ensemble of trapped polar molecules for quantum computing. A "holographic quantum register" with hundreds of qubits is encoded in collective excitations with definite spatial phase variations. Each phase pattern is uniquely addressed by optical Raman processes with classical optical fields, while one- and two-qubit gates and qubit readout are accomplished by transferring the qubit states to a stripline microwave cavity field and a Cooper pair box where controllable two-level unitary dynamics and detection is governed by classical microwave fields.     

Four-wave mixing of optical and microwave fields

We demonstrate degenerate four-wave mixing involving both optical and microwave fields. This four-wave mixing process, with fields that differ in frequency by 5 orders of magnitude, results from stimulated Raman scattering of the optical field from an atomic ground-state Zeeman coherence in warm rubidium vapor, which is induced and maintained by the microwave field.     

Efficient quantification of non-gaussian spin distributions

We study theoretically and experimentally the quantification of non-gaussian distributions via nondestructive measurements. Using the theory of cumulants, their unbiased estimators, and the uncertainties of these estimators, we describe a quantification which is simultaneously efficient, unbiased by measurement noise, and suitable for hypothesis tests, e.g., to detect nonclassical states. The theory is applied to cold 87Rb spin ensembles prepared in non-gaussian states by optical pumping and measured by nondestructive Faraday rotation probing. We find an optimal use of measurement resources under realistic conditions, e.g., in atomic ensemble quantum memories.     

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.     

Scheme for the preparation of macroscopicW-type state of atomic ensembles in cavity QED coulped with optical fibers

We propose a potentially practical scheme to generate macroscopic W-type state of N atomic ensembles in cavity QED system consisting of N atomic ensembles trapped in N single-mode cavities connected by(N 1)optical fibers.We show that the N-qubit W-type state of atomic ensembles can be realized with high success probabilities if the coulping strength of the cavity-fiber is much stronger than that of cavity-atom.We also show that both the growth of atomic number in each ensemble and the increase of the number of atomic ensembles can diminish the detrimental influence from dissipative processes.This idea provides a scalable way to an atomic-ensemble-based quantum network,which is plausible with current available technology.     

Optical bistability via amplitude and phase control of a microwave field

We investigate the steady-state optical bistability behavior in a three-level Λ-type atomic system closed by a microwave field under the condition that the applied fields are in resonance with corresponding atomic transitions. It is shown that the bistable hysteresis cycles can be controlled by both the amplitude and the phase of the microwave field.     

Controlled deflection of cold atomic clouds and of Bose-Einstein condensates

We present a detailed, realistic proposal and analysis of the implementation of a cold atom deflector using time-dependent far off-resonance optical guides. An analytical model and numerical simulations are used to illustrate its characteristics when applied to both non-degenerate atomic ensembles and to Bose-Einstein condensates. Using for all relevant parameters values that are achieved with present technology, we show that it is possible to deflect almost entirely an ensemble of ⁸⁷Rb atoms falling in the gravity field. We discuss the limits of this proposal, and illustrate its robustness against non-adiabatic transitions.     

A quantum computer in the scheme of an atomic quantum transistor with logical encoding of qubits

A scheme of a multiqubit quantum computer on atomic ensembles using a quantum transistor implementing two qubit gates is proposed. We demonstrate how multiatomic ensembles permit one to work with a large number of qubits that are represented in a logical encoding in which each qubit is recorded on a superposition of single-particle states of two atomic ensembles. The access to qubits is implemented by appropriate phasing of quantum states of each of atomic ensembles. An atomic quantum transistor is proposed for use when executing two qubit operations. The quantum transistor effect appears when an excitation quantum is exchanged between two multiatomic ensembles located in two closely positioned QED cavities connected with each other by a gate atom. The dynamics of quantum transfer between atomic ensembles can be different depending on one of two states of the gate atom. Using the possibilities of control for of state of the gate atom, we show the possibility of quantum control for the state of atomic ensembles and, based on this, implementation of basic single and two qubit gates. Possible implementation schemes for a quantum computer on an atomic quantum transistor and their advantages in practical implementation are discussed.     

Synchronized independent narrow-band single photons and efficient generation of photonic entanglement

We create independent, synchronized single-photon sources with built-in quantum memory based on two remote cold atomic ensembles. The synchronized single photons are used to demonstrate efficient generation of entanglement. The resulting entangled photon pairs violate a Bell's inequality by 5 standard deviations. Our synchronized single photons with their long coherence time of 25 ns and the efficient creation of entanglement serve as an ideal building block for scalable linear optical quantum information processing.     

用错位相位光栅产生的可调光学双阱

提出了用相位型错位光栅产生光学双阱的新方案.用平面光波(或TEM₀₀模式高斯光波)照射、正透镜聚焦,在透镜焦平面上产生的适用于冷原子或冷分子囚禁的多对可调光学双阱.计算和推导了双阱的光强分布、强度梯度以及光阱的几何参数与光学系统参数间的解析关系,研究了双阱到单阱三种不同的演化过程.同时还计算了光学双阱囚禁冷原子的光学偶极势和光子散射速率.研究发现,该方案不仅简单可行、操作方便,而且在原子物理、原子光学、分子光学和量子光学领域中有着广阔的应用前景. 关键词： 原子光学 相位光栅 光学双阱 冷原子囚禁     

Collective atomic spin squeezing and control

Using a quantum theory for an ensemble of two- or three-level atoms driven by electromagnetic fields in an optical cavity, we show that the various spins associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand self-spin squeezing when the input fields are coherent ones and the atomic ensemble exhibits a large non-linearity; on the other hand squeezing transfer when one of the incoming fields is squeezed. Received 14 August 2001 and Received in final form 7 November 2001     

冷铷原子样品中高效静态光信号的生成与调制

为了提高光信息处理元件的性能,实现高效率光信号的静止与存储,本文建立了由双向耦合场耦合的冷铷（87Rb）原子的四能级双Λ型能级机制,并对此机制在生成静态光信号时要求的高效性、持续时间,控制耦合场所需的操作条件以及对信号场强度的相位调制等进行了研究。首先,选取四能级双Λ型87Rb冷原子精细能级,用一对反向传播的行波激光场对能级进行近共振耦合,并沿耦合场方向输入一个正向弱光信号进行探测。接着,通过适当的绝热开启、关闭耦合场,存储光信号和生成高保真度静态光信号。然后,通过选取87Rb原子的不同精细能级结构,得出实现静态光所需满足的必要条件。最后,采用相位调制法对光信号进行处理。结果表明：生成的静态光信号具有高效性,过程持续时间约为80μs;相位调制法可以周期调节静态光脉冲的强度。在此机制下生成的静态光信号满足高效性、易于全光调节和长时效性等要求。     

Optical quantum simulation of Abelian gauge field using cold atomic ensembles coupled with arrays of optical cavities

A potentially practical scheme is proposed to realize optical quantum simulation of artificial Abelian gauge field in a scalable architecture consisting of cold atomic ensembles with optical cavities.In the present model,the collective excitations of cold atomic ensembles can be converted to the bosonic modes within the low-excitation limit,where the structure of two-dimension(2D)square plaquette enables the polaritons to move like a charged particle subjected to an external magnetic field.We find that the energy spectrum of this hybrid system exhibits a shape of Hofstadter buttery.Our work provides a different perspective to the quantum simulation of condensed matter and many-body physics in the context of cavity quantum electrodynamics.The experimental feasibility are justified using the existing techniques.     

多个量子节点确定性纠缠的建立

量子纠缠是一种重要的量子资源,在多个空间分离的量子存储器间建立确定性的量子纠缠,然后在用户控制的时刻将所存储的量子纠缠转移到量子信道中进行信息的分发和传送,这对于实现量子信息网络是至关重要的.本文介绍了用光学参量放大器制备与铷原子D1吸收线对应的非经典光场,而且在三个空间分离的原子系综中确定性量子纠缠的产生、存储和转移.利用电磁感应透明光和原子相互作用的原理,将制备的多组分光场纠缠态模式映射到三个远距离的原子系综以建立原子自旋波之间的纠缠.然后,存储在原子系综中的纠缠态通过三个量子通道,纠缠态的量子噪声被转移到三束空间分离的正交纠缠光场.三束释放的光场间纠缠的存在验证了该系统具有保持多组分纠缠的能力.这个方案实现了三个量子节点间的纠缠,并且可以直接扩展到具有更多节点的量子网络,为未来实现大型量子网络通信奠定了基础.     

相干诱导冷原子微腔中的动态光存储

将行波耦合激光、驻波光栅激光和静磁场作用于一个超冷原子系综,获得了由两个Bragg反射区和一个电磁感应透明区构成的动态可控光学微腔。对耦合激光、光栅激光和静磁场进行时间调制,将一个弱探测激光送入这一相干诱导光学微腔,使其形成周期振荡,然后再根据需要在一定时间延迟之后将其导出,将这一伴随着较弱能量损耗的探测脉冲受限传播过程视为一个有效的动态光存储机制。对提出的信息存储机制进行了数值模拟,讨论了它的优点和实用价值,提出了它的发展前景。