光学腔增强光与冷原子耦合及自旋波读出效率的实验研究

在~(87)Rb冷原子系综中,通过自发拉曼散射过程产生了光子与原子自旋波的关联对。利用光学腔增强光与原子的相互作用,使光子和原子自旋波关联对的产生率增加了3.8倍。同时,我们也研究了光学腔对原子自旋波读出效率的影响。结果表明,腔使自旋波的读出效率提高了1.5倍。本文的研究结果为下一步产生高恢复效率的光与原子量子纠缠提供了实验基础。     

冷原子系综中光纤腔增强且高保真度的光学存储

利用原子系综中的Duan-Lukin-Cirac-Zoller (DLCZ)过程可产生光与原子记忆(自旋波)量子纠缠,该纠缠可作为量子中继的重要元件.随着量子信息研究的深入发展,人们对量子信息存储其灵活多样性、可控性等方面提出更高的要求.本文在冷原子系综中演示了一种基于DLCZ过程的光纤腔增强且高保真度的光学存储方案,即将87Rb原子系综放于设计的光纤腔中,通过光纤腔增强“写出”和“读出”光子与原子系综的耦合实现自旋波量子信息的有效恢复,同时具有较高的保真度.观察到有腔且锁定的情况下斯托克斯光子产生概率比无腔时增加4.6倍,原子自旋波读出效率增加1.6倍,实验实现22%的读出效率并具有92%的量子态保真度,该读出效率对应一个40%的本质读出效率.这种高度可恢复、高量子态保真度的原子-光子纠缠源,可为未来长距离量子通信及广域大规模量子网络构建的实现提供另一种有效的途径.     

读出效率对光与原子纠缠产生的影响

在光与原子纠缠态产生中,自旋波读出效率是影响纠缠质量的一个重要因素.本文在实验和理论上研究了读出效率与纠缠质量(Bell参量)的关系.实验上利用~(87)Rb冷原子系综中的自发Raman散射过程产生了光与原子量子纠缠.通过改变读光功率或OD (光学厚度),实现了读出效率的变化.在此基础上,研究了光与原子纠缠质量(Bell参量)随读出效率变化的关系.该实验将为高保真度的光与原子纠缠产生提供帮助.     

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     

电磁诱导光透明过程中两原子的自旋极化特性

本文研究了电磁诱导光透明过程中两原子的自旋极化矢量.对于数态探测场,在光信息转移过程中,两原子自旋极化矢量始终固定在z方向,大小变化非常明显,两原子一般处于混合态;对于相干态探测场,自旋极化矢量大小变化很微弱,两量子比特基本处于纯态,这有利于改善系综内原子之间的相干性,但自旋极化矢量的方向在x-z平面内.     

电磁诱导光透明过程中两原子的自旋极化特性

本文研究了电磁诱导光透明过程中两原子的自旋极化矢量。对于数态探测场,在光信息转移过程中,两原子自旋极化矢量始终固定在z方向,大小变化非常明显,两原子一般处于混合态;对于相干态探测场,自旋极化矢量大小变化很微弱,两量子比特基本处于纯态,这有利于改善系综内原子之间的相干性,但自旋极化矢量的方向在x-z平面内。     

光学腔增强Duan-Lukin-Cirac-Zoller量子记忆读出效率的研究

量子中继是长距离纠缠分发的关键组成部分,而基于原子系综存储的读出效率是量子中继能否实用化的一个重要指标.本文利用冷原子系综中的自发拉曼散射过程产生Duan-Lukin-Cirac-Zoller量子记忆,在原子系综周围搭建环形腔,增强光与原子相互作用,从而提高读出效率,然而,腔内原子的能级分裂使量子记忆的读出效率降低.本文研究了读出效率随读光相对于原子共振线失谐量的变化关系.结果显示:当读光的失谐量为80 MHz时,本质读出效率为45%,这时腔对读出效率的增强倍数为1.68倍.     

自旋波波长对量子存储寿命的影响研究

利用电磁感应透明效应的动力学过程我们在87Rb冷原子系综中实验实现光学信号的存储和释放.通过改变信号光和耦合光的角度实现了自旋波波长的变化.在此基础上我们研究了存储寿命与自旋波波长的关系,结果表明,光存储寿命随自旋波波长(角度)的增加(减小)而增加,当自旋波波长大于2.2 mm(信号光和耦合光的角度小于0.02度)时,存储寿命可达到2.4 ms.     

冷85Rb原子D2线电磁诱导透明的实验研究

对85Rb原子D2线三能级系统的电磁诱导透明进行了详细的实验研究.得到了不同耦合强度下探测光的吸收线型,并对气室中热原子与磁光阱中冷原子的电磁诱导透明光谱进行了比较.     

光泵浦效应对电磁诱导透明介质极化特性的影响

本文主要研究光泵浦效应对电磁诱导透明介质极化特性的影响。理论分析了耦合场分别与探针场同向和反向传播时,介质对探针场的色散和吸收特性。同时理论分析表明,相对二能级原子来说,由于耦合光的泵浦作用,在多普勒吸收基础上,原子对探针光的吸收增强,并且这种泵浦作用与耦合场传播方向无关。实验上验证了上述理论研究中原子介质对探针场的吸收特性。进一步地,实验研究了耦合光功率、频率失谐及粒子数密度等参量对泵浦率的影响,特别是对吸收宽度和深度的影响。     

Enhancement of electromagnetically induced transparency in room temperature alkali metal vapor

Electromagnetically induced transparency (EIT) has led to several quantum optics effects such as lasing without inversion or squeezed light generation. More recently quantum memories based on EIT have been experimentally implemented in different systems such as alkali metal atoms. In this system the excited state of the optical transition splits into several sublevels due to the hyperfine interaction. However, most of the theoretical models used to describe the experimental results are based on a Λ-system with only one excited state. In this article, we present a theoretical model for the Λ-type interaction of two light, fields and an atomic system with multiple excited state. In particular we show that if the control and probe fields are orthogonally circularly polarized the EIT effect in an alkali-metal vapor can almost disappears. We also identify the reasons of this reduction and propose a method to recover the transparency via velocity selective optical pumping.     

Quantum memory based on $\mathsf{\Lambda}$-atoms ensemble with two-photon resonance EIT

We study the -atoms ensemble based quantum memory for the quantum information carried by a probe light field. Two atomic Rabi transitions of the ensemble are coupled to the quantum probe field and classical control field respectively with a same detuning. Our analysis shows that the dark states and dark-state polaritons can still exist for the present case of two-photon resonance EIT. Starting from these dark states we can construct a complete class of eigen-states of the total system. A explicit form of the adiabatic condition is also given in order to achieve the memory and retrieve of quantum information.Received: 8 June 2004, Published online: 10 August 2004PACS: 03.67.-a Quantum information - 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 03.65.Fd Algebraic methods     

Objectively discerning Autler-Townes splitting from electromagnetically induced transparency

Autler-Townes splitting (ATS) and electromagnetically induced transparency (EIT) both yield transparency in an absorption profile, but only EIT yields strong transparency for a weak pump field due to Fano interference. Empirically discriminating EIT from ATS is important but so far has been subjective. We introduce an objective method, based on Akaike's information criterion, to test ATS vs EIT from experimental data for three-level atomic systems and determine which pertains. We apply our method to a recently reported induced-transparency experiment in superconducting-circuit quantum electrodynamics.     

Manipulating light pulses via dynamically controlled photonic band gap

When a resonance associated with electromagnetically induced transparency in an atomic ensemble is modulated by an off-resonant standing light wave, a band of frequencies can appear for which light propagation is forbidden. We show that dynamic control of such a band gap can be used to coherently convert a propagating light pulse into a stationary excitation with nonvanishing photonic component. This can be accomplished with high efficiency and negligible noise even at the level of few-photon quantum fields thereby facilitating possible applications in quantum nonlinear optics and quantum information.     

Low-frequency vacuum squeezing via polarization self-rotation in Rb vapor

We observed squeezed vacuum light at 795 nm in (87)Rb vapor via resonant polarization self-rotation and report noise sidebands suppression of approximately 1 dB below shot-noise level spanning from 30 kHz to 1.2 MHz frequencies. To our knowledge, this is the first demonstration of submegahertz quadrature vacuum squeezing in atomic systems. The spectral range of observed squeezing matches well typical bandwidths of electromagnetically induced transparency (EIT) resonances, making this simple technique for generation of optical fields with nonclassical statistics at atomic transitions wavelengths attractive for EIT-based quantum information protocols applications.     

Frequency‐sensitive switching control effect induced by two‐photon resonance in an EIT‐based layered medium

The optical response of an atomic vapor can be coherently manipulated by tunable quantum interference occurring in atomic transition processes. A periodic layered medium whose unit cells consist of a dielectric and an EIT (electromagnetically induced transparency) atomic vapor is designed for light propagation manipulation. Such an EIT‐based periodic layered medium exhibits a flexible frequency‐sensitive optical response, where a very small change in probe frequency can lead to a drastic variation of reflectance and transmittance. As the destructive quantum interference relevant to two‐photon resonance arises in EIT atoms interacting with both control and probe fields, the controllable optical processes that depend sensitively on the external control field will take place in this EIT‐based periodic layered medium. Such a frequency‐sensitive and field‐controlled optical behavior of reflection and transmission in the EIT photonic crystal can be applicable to designs of new devices such as photonic switches, photonic logic gates and photonic transistors, where one laser field can be controlled by the other one, and would have potential applications in the areas of integrated optical circuits and other related techniques (e.g., all‐optical instrumentations).     

Linewidth of electromagnetically induced transparency under motional averaging in a coated vapor cell

The linewidth of electromagnetically induced transparency(EIT) in a coated Rb vapor cell was studied under a magnetic field gradient.The nonlinear broadening of the EIT linewidth with the magnetic field gradient was observed.It was found that the motional averaging of the field gradient was more pronounced at higher laser intensities and larger beam sizes.In the same regime,there was a small linewidth decrease with the increasing magnetic field gradient.We have established a Monte-Carlo model,which gave results in good qualitative agreement with our experiment.Physics pictures for the above phenomena were also suggested.These results provide an understanding of the EIT linewidth behavior under motional averaging,and should be useful for applications in quantum optics and metrology based on coated vapor cells.     

Coherent control of stationary light pulses

We present a detailed analysis of the recently demonstrated technique to generate quasi-stationary pulses of light [M. Bajcsy, A.S. Zibrov, M.D. Lukin, Nature (London) 426 (2003) 638] based on electromagnetically induced transparency. We show that the use of counter-propagating control fields to retrieve a light pulse, previously stored in a collective atomic Raman excitation, leads to quasi-stationary light field that undergoes a slow diffusive spread. The underlying physics of this process is identified as pulse matching of probe and control fields. We then show that spatially modulated control-field amplitudes allow us to coherently manipulate and compress the spatial shape of the stationary light pulse. These techniques can provide valuable tools for quantum nonlinear optics and quantum information processing.     

Linewidth of electromagnetically induced transparency under motional averaging in coated vapor cell

The linewidth of electromagnetically induced transparency (EIT) in a coated Rb vapor cell was studied under a magnetic field gradient. The nonlinear broadening of the EIT linewidth with the magnetic field gradient was observed. It was found that the motional averaging of the field gradient was more pronounced at higher laser intensities and larger beam sizes. In the same regime, there was a small linewidth decrease with the increasing magnetic field gradient. We have established a Monte-Carlo model, which gave results in good qualitative agreement with our experiment. Physics pictures for the above phenomena were also suggested. These results provide an understanding of the EIT linewidth behavior under the motional averaging, and should be useful for applications in quantum optics and metrology based on coated vapor cells.     

Quantum study of information delay in electromagnetically induced transparency

Using electromagnetically induced transparency (EIT), it is possible to delay and store light in atomic ensembles. Theoretical modeling and recent experiments have suggested that the EIT storage mechanism can be used as a memory for quantum information. We present experiments that quantify the noise performance of an EIT system for conjugate amplitude and phase quadratures. It is shown that our EIT system adds excess noise to the delayed light that has not hitherto been predicted by published theoretical modeling. In analogy with other continuous-variable quantum information systems, the performance of our EIT system is characterized in terms of conditional variance and signal transfer.