Enhanced cross-phase modulation based on a double electromagnetically induced transparency in a four-level tripod atomic system

We report experimental observations on the simultaneous electromagnetically induced transparency (EIT) effects for probe and trigger fields (double EIT) as well as the enhanced cross-phase modulation (XPM) between the two fields in a four-level tripod EIT system of the D1 line of 87Rb atoms. The XPM coefficients (larger than 2 x 10(-5) cm2/W) and the accompanying transmissions (higher than 60%) are measured at a slight detuning of the probe field from the exact EIT-resonance condition. The system and enhanced cross-Kerr nonlinearities presented here can be applied to quantum information processes.     

Generation and storage of double slow light pulses in a solid

We experimentally study the generation and storage of double slow light pulses in a Pr³⁺:Y₂SiO₅ crystal. Under electromagnetically induced transparency, a single signal pulse is stored in the spin coherence of the crystal. By simultaneously switching on two control fields to recall the stored information, the spin coherence is converted into two slow light pulses with distinct frequencies. Furthermore, the storage and controlled retrieval of double slow light pulses are obtained by manipulating the control fields. This study of double slow light pulses may have practical applications in information processing and all-optical networks. vspace2mm     

Electromagnetically induced transparency‐based slow and stored light in warm atoms

This paper reviews recent efforts to realize a high‐efficiency memory for optical pulses using slow and stored light based on electromagnetically induced transparency (EIT) in ensembles of warm atoms in vapor cells. After a brief summary of basic continuous‐wave and dynamic EIT properties, studies using weak classical signal pulses in optically dense coherent media are discussed, including optimization strategies for stored light efficiency and pulse‐shape control, and modification of EIT and slow/stored light spectral properties due to atomic motion. Quantum memory demonstrations using both single photons and pulses of squeezed light are then reviewed. Finally a brief comparison with other approaches is presented.     

固体材料中原子相干效应的研究进展

原子相干效应是光与物质相互作用的结果,它导致了一系列重要的物理现象。目前原子相干的实验研究工作主要在原子气体中开展,而与之相比固体材料中的相关实验研究具有更实际的应用前景。本文系统介绍了近年来固体材料中原子相干效应的研究进展,主要涉及电磁感应光透明、光速减慢与相干存储、存储光信息的可控制擦除、基于光存储的全光路由、双光脉冲的速度减慢和可逆存储和基于原子相干的增强四波混频等基本内容,最后还讨论了其在相关领域的应用。     

Controllable shape-matched propagation of two signal beams in a double-Λ atomic ensemble

We study a four-level double-Λ atomic ensemble interacting with two time-dependent signal fields and two stationary control fields. Though, in each Λ channel, a pair of signal and control fields couple resonantly with the two lower levels of atoms, the occurrences of electromagnetically induced transparency (EIT) is affected by the coherence of the four fields. In the discussion of atomic susceptibilities, we show that the quantum coherence between the two lower levels can be either formed or released according to the phase matching of the four fields. We analyze the propagation equation of the two signal fields, and find two characteristic solutions: the stationary transmission wave and the transient decay wave. The former corresponds to a correlated EIT effect in which two signal pulses are shape-matched. The latter is an opposite effect to the correlated EIT in which two pulses quench simultaneously, thus named as the correlated two-signal absorption (CTSA). We propose the CTSA condition in correspondence with the EIT condition. The numerical simulation shows that the double-Λ configuration is capable of manipulating synchronous optical signals and thus provides multiplicity and versatility in quantum information process.     

Controllable beating signal using stored light pulse

We experimentally study the controllable generation of a beating signal using stored light pulses based on electromagnetically induced transparency(EIT) in a solid medium. The beating signal relies on an asymmetric procedure of light storage and retrieval. After storing the probe pulse into the spin coherence under the EIT condition, two-color control fields with opposite detunings instead of the initial control field are used to scatter the stored spin coherence. The controllable beating signal is generated due to alternative constructive and destructive interferences in the retrieved signal intensities. The beating of the two-color control fields is mapped into the beating of weak probe fields by using atomic spin coherence. This beating signal will be important in precise atomic spectroscopy and fast quantum limited measurements.     

Enhanced frequency conversion of nonadiabatic pulses in a double Λ system driven by two pumps with and without carrier beams

We show that nonadiabatic, resonant amplitude- and phase-modulated pulses can be frequency converted with greater efficiency than adiabatic resonant pulses in a double Λ system, interacting with two strong cw beams on one side of the system, and a weak pulsed probe on the other. Indeed, in this double EIT (electromagnetically induced transparency) configuration, conversion efficiencies close to unity, similar to those achieved using highly detuned pulses, can be obtained using highly nonadiabatic resonant pulses. The distance at which the maximum conversion occurs is shorter than in a coherently-prepared Λ system. This counteracts the increased absorption that occurs in the double EIT configuration, so that both produce similar conversion efficiencies. The absorption experienced by matched nonadiabatic pulses in the double EIT system, at all propagation distances, can be overcome by superimposing the nonadiabatic pulses as amplitude modulations on carrier fields. Thus we demonstrate the formation of adiabatons in the double EIT system, and of diabatons in both the coherently-prepared Λ system and the double EIT system. Both the diabatons and adiabatons satisfy pulse-matching conditions. In addition, the asymptotic amplitude of the complementary amplitude modulations is proportional to the ratio of the pump to probe carrier Rabi frequencies, and is the same in each of the configurations.     

Image information transfer via electromagnetically induced transparency-based slow light

In this work, we experimentally demonstrate an image information transfer between two channels by using slow light based on electromagnetically induced transparency(EIT) in a solid. The probe optical image is slowed due to steep dispersion induced by EIT. By applying an additional control field to an EIT-driven medium, the slowed image is transferred into two information channels. Image intensities between two information channels can be controlled by adjusting the intensities of the control fields. The similarity of output images is further analyzed. This image information transfer allows for manipulating images in a controlled fashion, and will be important in further information processing.     

Enhanced Cross-Phase Modulation via Phase Control in a Quantum dot Nanostructure

A four-level quantum dot (QD) nanostructure interacting with four fields (two weak near-infrared (NIR) pulses and two control fields) forms the well-known double-cascade configuration. We investigate the cross-phase modulation (XPM) between the two NIR pulses. The results show, in such a closed-loop scheme, that the XPM can be greatly enhanced, while the linear absorption and two-photon absorption (gain) can be efficiently depressed by tuning the relative phase among the applied fields. This protocol may have potential applications in NIR all-optical switch design and quantum information processing with the solid-state materials.     

Where is the energy of slow light stored?

We analyze the energy storage process of light propagating with slow group velocity in a sample where electromagnetically induced transparency (EIT) is created by a strong coupling field. We compare the formation of slow light in EIT and in self-induced transparency (SIT). For SIT, soliton-like propagation of light with essentially reduced group velocity takes place because of the temporary storage of an appreciable part of the pulse energy in the atoms. For EIT, no energy of the probe is stored in the atoms. This energy is transformed to the coupling field and leaves the sample with phase velocity c without absorption. Slow light is formed by a low frequency coherence induced at the input by the probe and coupling fields in a two-quantum excitation process. This coherence propagates as a “spin wave” with small group velocity, and at a large distance from the input, the coherence rules the process of the energy transformation from the coupling field to the probe, reproducing exactly the temporal profile of the probe at the input.     

双光子失谐对慢光和光存储影响的实验研究

利用电磁感应透明(EIT)效应在87Rb热原子气室中进行了慢光和光存储的实验研究,在单光子红失谐650 MHz处测量了双光子失谐对光脉冲延迟和光存储的影响.结果表明:在双光子失谐0—0.5 MHz范围内存在显著的光脉冲延迟和光存储恢复信号,其慢光波形与理论计算结果基本相符;而恢复光脉冲信号随着双光子失谐的变化出现形变,这是由于多个EIT子系统之间的干涉引起的.这一研究结果为连续变量光场在热原子系综中的存储提供了实验参考.     

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.     

Steady-state linear optical properties and Kerr nonlinear optical response of a four-level quantum dot with phonon-assisted transition

The linear optical properties and Kerr nonlinear optical response in a four-level loop configuration Ga As/Al Ga As semiconductor quantum dot are analytically studied with the phonon-assisted transition(PAT). It is shown that the changes among a single electromagnetically induced transparency(EIT) window, a double EIT window and the amplification of the probe field in the absorption curves can be controlled by varying the strength of PAT κ. Meanwhile, double switching from the anomalous dispersion regime to the normal dispersion regime can likely be achieved by increasing the Rabi energy of the external optical control field. Furthermore, we demonstrate that the group velocity of the probe field can be practically regulated by varying the PAT and the intensity of the optical control field. In the nonlinear case, it is shown that the large SPM and XPM can be achieved as linear absorption vanishes simultaneously, and the PAT can suppress both third-order self-Kerr and the cross-Kerr nonlinear effect of the QD. Our study is much more practical than its atomic counterpart due to its flexible design and the controllable interference strength, and may provide some new possibilities for technological applications.     

Light storage via slow-light four-wave mixing

We experimentally demonstrate a light storage via slow-light four-wave mixing in a solid-state medium with a four-level double lambda scheme. Using slow light based on electromagnetically induced transparency, we obtain a slowed four-wave mixing signal pulse together with the slowed probe pulse. During the propagation of light pulses, the storage and retrieval of both the slowed four-wave mixing pulse and the slowed probe pulse are studied by manipulating the intensities of the control fields.     

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.     

Slow light with cavity electromagnetically induced transparency

We report an experimental observation of slow light propagation in cold Rb atoms exhibiting cavity electromagnetically induced transparency (EIT). The steep slope of the atomic dispersion manifested by EIT reduces the light group velocity. The cavity filtering and feedback further contribute to the slowdown and delay of the light pulse propagation. A combination of the cavity and the EIT atomic system significantly improves the performance of the slow light propagation. A propagation time delay of approximately 200 ns was observed in the cavity and Rb EIT system, which is approximately 70 times greater than the time delay calculated for the light pulse propagation through the same Rb EIT system without the cavity.     

回音壁微腔光力系统的相干控制与完全相干透射

本文研究了两侧同时输入的回音壁模谐振双微腔光力系统中电磁诱导透明的相干调控.通过改变双微腔两侧探测场的强度比值及相位差,可以有效控制电磁诱导透明窗口的宽度和深度,对探测场的吸收和色散等性质实施显著的影响,并且能够在特殊频率处产生关于探测场的完全相干透射现象.     

Low-light-level cross-phase-modulation based on stored light pulses

We experimentally demonstrate a low-light-level cross-phase-modulation (XPM) scheme based on the light-storage technique in laser-cooled 87Rb atoms. The proposed scheme can achieve a similar phase shift and has the same figure of merit as one using static electromagnetically induced transparency under the constant coupling field. Nevertheless, the phase shift and the energy loss of a probe pulse induced by a signal pulse are neither influenced by the coupling intensity nor by the atomic optical density in the light-storage XPM scheme. This scheme enhances the flexibility of the experiment and makes possible conditional phase shifts on the order of pi with single photons.     

Copropagating superluminal and slow light manifested by electromagnetically assisted nonlinear optical processes

We report an experimental observation of nonlinear optical gain and loss assisted by electromagnetically induced transparency (EIT) and simultaneous superluminal and subluminal light propagations. Two circular components of a linearly polarized light initiate third-order nonlinear processes in which one circular component is attenuated while the other component is amplified in a three-level EIT system. Near the atomic resonance, the attenuated circular component experiences steep normal dispersion and propagates with a slow group velocity, while the amplified circular component experiences steep anomalous dispersion and propagates with a superluminal group velocity.     

Control of Group Velocity via Spontaneous Generated Coherence and Kerr Nonlinearity

A four-level N-type atomic medium is considered to study the effect of spontaneous generated coherence(SGC) and Kerr nonlinearity on light pulse propagation. A light pulse is propagating inside the medium where each atom follows four-level N-type atom-field configuration of rubidium(85Rb) atom. The atom-field interaction leads to electromagnetically induced transparency(EIT) process. The atom-field interaction is accompanied by normal dispersion and in the presence of SGC and Kerr nonlinearity the dispersion property of the proposed atomic medium is modified,which leads to enhancement of positive group index of the medium. The enhancement of positive group index then leads to slow group velocity inside the medium. A more slow group velocity is also investigated by incorporated the collective effect of SGC and Kerr nonlinearity. The control of group velocity inside a four-level N-type atomic medium via collective effect of SGC and Kerr nonlinearity is the major part of this work.