Generation of any superposition of coherent states along a straight line via resonant atom-cavity interaction

This paper proposes a scheme for generating arbitrary superpositions of several coherent states along a straight line for a cavity mode. In the scheme, several atoms are sent through a cavity initially in a strong coherent state. The superposition of several coherent states with desired coefficients may be generated if each atom is detected in the excited state after it exits the cavity. The scheme is based on resonant atom--cavity interaction and no classical field is required during and after the atom--cavity interaction. Thus, the scheme is very simple and the interaction time is very short, which is important in view of decoherence.     

Cavity-Induced Enhancement of Squeezing in Resonance Fluorescence of a V-Type Three-Level Atom

The quadrature squeezing spectra in the resonance fluorescence of a V-type three-level atom driven by a coherent field and coupled to a single-mode cavity is investigated. For weak excitation, the fluorescence field exhibit squeezing in the out-of-phase quadrature. The coupling between the atom and the cavity mode can greatly enhance the squeezing centred at the laser frequency. More importantly, for strong excitation, under the effect of the cavity-atom coupling, the in-phase quadrature of fluorescence can exhibit two-mode squeezing at the two inner sideband frequencies. By working in the dressed-state representation and hiring secular approximation, we give an analytical explanation for the effect. The result shows, under appropriate conditions, the squeezing can be greatly enhanced by appropriately tuning the cavity resonant frequency.     

远程控制光场的量子统计性质

考虑双模纠缠相干光场,将其中一束光场注入腔中与一个二能级原子发生共振相互作用,总系统在腔量子电动力学演化过程中,对原子作选择性的测量,通过操纵参加相互作用的时间以及选择适当的光场参量可控制未参加相互作用光场的量子统计性质,在一定条件下可产生反聚束、压缩态等非经典光场,并且通过适当的控制和调整这些参量可以改变未参加相互作用光场的反聚束和压缩特性的强弱。这样,通过利用腔与原子之间简单的共振相互作用和相干光场之间的纠缠关联,实现了远程的、无直接量子测量的控制或改变相干光场的非经典性质这一目的。     

Quantum state engineering by superpositions of coherent states along a straight line with a single atomic state measurement

A new scheme is proposed for preparation of a type of nonclassical state in cavity QED. In the scheme, an atom either flying through or trapped within a cavity, is controlled by the classical Stark effect; this makes it interact alternately with a (resonant) classical field and with the (dispersive) cavity field. The cavity field, which allows an arbitrary displacement operation during the process, after the detection on the atom, finally collapses to the specific superpositions of coherent states, with their weighting factors controllable. The scheme is also applied for preparation of superpositions of motional coherent states for a trapped ion. The scheme is in contrast to all the previous ones, and thus provides a new perspective for quantum state engineering.     

三能级原子与奇偶纠缠相干光作用的光场压缩

采用求解薛定谔方程和数值计算方法,研究了V型三能级原子与双模奇偶纠缠相干光场相互作用过程中的光场压缩效应,讨论了压缩效应与双模奇偶纠缠相干光场的纠缠程度、系统失谐量、双模光场的平均光子数和原子基态概率幅的依赖关系。结果表明：光场压缩效应与双模奇偶纠缠相干光场的纠缠程度、失谐量、平均光子数和原子初态相关联：双模纠缠相干光场处于非纠缠状态时的光场压缩量比光场处于纠缠状态时要大;原子处在单纯的基态或激发态时光场都有明显的压缩现象出现;而原子初态中基态和激发态的概率幅较接近时无光场压缩现象;无论光场是否处于纠缠态,只有两模平均光子数接近时,光场才会出现压缩效应。     

Preparation of Highly Squeezed States and Multi—component Entangled Coherent States via the Raman Interaction

A method is presented for generating highly squeezed states of a cavity field via the atom-cavity field interaction of the Raman type.In the scheme a sequence of three-level Λ-type atoms interacts with a cavity field,displaced by a classical source,in a Raman manner.Then the atomic states are measured.By this way the cavity field may collapse onto a superposition of several coherent states,which exhibits strong squeezing.The scheme can also be used to prepare superpositions of many two-mode coherent states for two cavity fields.The coherent states in each mode are on a straight line.This is the first way for preparing multi-component entangled coherent states of this type in cavity QED.     

强场与级联型三能级原子相互作用中原子的偶极压缩效应

采用数值计算方法研究了强相干初态光场与级联型三能级原子相互作用中原子的偶极压缩效应结果表明:级联型三能级原子偶极算符的两个正交分量均呈现出周期性的压缩现象,原子存在偶极压缩,且压缩的最大深度比弱场作用情况下要深得多;随着n值的不断增大,偶极算符两个正交分量的振荡频率随之增大,场与原子的非线性耦合越来越强,且振荡重复地出现复苏缩现象.     

Strong-Driving-Assisted Probabilistic State Preparation in Cavity QED

An alternative scheme is proposed for preparing the superpositions of coherent states with controllable weighting factors along a straight line for a cavity field. The scheme is based on the interaction of a single-mode cavity field with a resonant two-level atom driven by a strong classical field. It is in contrast to the previous methods used in cavity QED of injecting a coherent state into a cavity via a microwave source. In the scheme, the interaction between the cavity mode and atoms is fully resonant, thus the required interaction time is greatly shortened. Moreover, the present scheme requires smaller numbers of operations. In view of decoherence, a reduction of interaction time and numbers of operations for the state preparation is very important for experimental implementation of quantum state engineering.     

Strong-Driving-Assisted Probabilistic State Preparation in Cavity QED

An alternative scheme is proposed for preparing the superpositions of coherent states with controllable weighting factors along a straight line for a cavity field. The scheme is based on the interaction of a single-mode cavity field with a resonant two-level atom driven by a strong classical field. It is in contrast to the previous methods used in cavity QED of injecting a coherent state into a cavity via a microwave source. In the scheme, the interaction between the cavity mode and atoms is fully resonant, thus the required interaction time is greatly shortened. Moreover, the present scheme requires smaller numbers of operations. In view of decoherence, a reduction of interaction time and numbers of operations for the state preparation is very important for experimental implementation of quantum state engineering.     

腔内混合态原子系统中辐射场的压缩效应

本文应用熵最大原理，假定腔内二能级原子起初就没有关于系统所处态的任何信息，研究了腔内原子与腔场相互作用的压缩行为，得出了原子初始处于任意统计混合态下，当初始激发足够强时，辐射场将产生极其接近于纯态情况在相同激发时的强压缩现象的重要结果，同时还讨论了产生压缩的混态范围。     

Generation and Preparation of the Sustained Optimal Entropy Squeezing State of a Motional Atom Inside Vacuum Cavity

Considering two two-level atoms initially in Bell state, we send one atom into a vacuum cavity while leaving the other outside, and consider the motion of atom inside the cavity. Using quantum information entropy squeezing theory, the time evolution of the entropy squeezing factor of atom inside the cavity is discussed for two cases, i.e., before and after performing rotation operations and measuring atom outside, the influences of the field mode structure and atomic motions on the atomic entropy squeezing are evaluated. It is shown that atom inside the cavity has no entropy squeezing phenomenon before operating atom outside the cavity. However, the optimal entropy squeezing phenomenon of period T = 2π/p emerges and constant entropy squeezing phenomenon can occur by adjusting rotation operation to R(π/4), and setting the field mode structure parameter 0 < p < 50. In particular, if choosing p > 50, a sustained optimal entropy squeezing state (SOESS) can be generated. We also present the schematic circuit diagram of preparation of SOESS. Our proposal provides a possible way for the initial decoherent state recovering into sustained maximal coherent superposition state of single atom in the quantum noise environment.     

Generation of quantum switch and nonlocal fields with an external field in a high quality dispersive cavity

We present a scheme to prepare an optical “quantum switch”, a superposition of “open” and “closed” states. The scheme is based on the interaction of an Λ-type three-level atom with a single-mode of quantized cavity field and an external classical driving field, in the regime where the atom and fields are highly detuned. We show how this interaction can be used to generate coherent states of the cavity field in contrast to the usual method used in microwave cavity QED of injecting a coherent state into a cavity. A combination of switches could be used to prepare a quantum superposition of coherent field states located simultaneously in two cavities. Compared with previous proposals, our scheme is simplified due to economizes the Ramsey zone and the time required for the state generation is short.     

利用V型原子与单模光场的非共振相互作用制备四光子相干态

提出了一种利用简并V型三能级原子与振幅很大的单模相干光场的远离共振相互作用,制备四光子相干态的有效新方案.同时证明了在相同的条件下,按照相同的方法不能制备出四成分以上的相干态的叠加态, 即当在腔中注入的第三个原子的速度与第二个原子的速度相等时,腔场将保持这种四光子相干态不变.     

腔损对单原子微激射器中产生稳定四阶与振幅平方压缩态的影响

研究了利用相干陷落方法在单原子微激射器中产生具有四阶压缩和振幅平方压缩性质的稳态腔场.结果表明,在目前微激射器实验的腔损条件下,如果注入腔中的原子初始处于相干态,只要原子束的通量适当大,当原子在腔中的飞行时间满足陷落条件时,腔场可进入相干陷落态,此时腔场呈现稳定的四阶压缩和振幅平方压缩性质 关键词：     

Entropy squeezing of the field interacting with a nearly degenerate V-type three-level atom

The position- and momentum-entopic squeezing properties of the optical field in the system of a nearly degenerate three-level atom interacting with a single-mode field are investigated. Calculation results indicate that when the field is initially in the vacuum state, it may lead to squeezing of the position entropy or the momentum entropy of the field if the atom is prepared properly. The effects of initial atomic state and the splitting of the excited levels of the atom on field entropies are discussed in this case. When the initial field is in a coherent state, we find that position-entropy squeezing of the field is present even if the atom is prepared in the ground state. By comparing the variance squeezing and entropy squeezing of the field we confirm that entropy is more sensitive than variance in measuring quantum fluctuations.     

双光子过程耦合腔系统中光场的量子特性

本文研究了双光子过程原子与耦合腔相互作用系统中腔场的压缩效应和反聚束效应.考虑系统总激发数等于2的情况,利用数值计算方法讨论了腔场间的耦合强度变化和原子与腔场间耦合强度变化对反聚束效应的影响.研究结果表明:腔场不呈现出压缩效应;腔场的反聚束效应与原子与腔场的耦合系数之间,以及与腔场间的耦合系数之间都存在着非线性关系,     

对双模反关联光场非经典特性的远程控制

考虑初始处于纠缠态的两个V型三能级原子之一注入腔场中,并且与初始处于SU(2)相干态的双模场发生共振相互作用的情况。采用数值计算方法研究了双模关联场的非经典特性。讨论了对腔外原子选择性测量、双模总光子数和配分参量对双模关联场的差压缩、柯西-施瓦茨(Cauchy-Schwarz)不等式、模间相干性以及光子亚泊松(sub-Poisson)分布的影响。研究结果表明:对腔外原子的选择性测量或增加两模的总光子数,a模的差压缩和光子sub-Poisson分布都明显地增强,但两模之间的反相关明显地减弱。     

强场作用下二能级原子双光子过程中原子的压缩效应

本文采用时间演化算符方法和数值计算研究了强场作用下二能级原子双光子过程中原子的压缩效应.结果表明:原子出现周期性的压缩,且压缩的最大深度比弱场情况下要深得多;随着n.值的不断增大,场与原子的非线性耦合越来越强,振荡重复地出现复苏收缩现象.     

Generation of entangled coherent states for two cavity modes via resonant interaction with a V-type three-level atom

This paper proposes a scheme for the generation of entangled coherent states for two cavity modes. In the scheme a V-type three-level atom is sent through a two-mode cavity filled with a coherent field. After the atom cavity interaction and detection of the atomic state the cavity modes may evolve to a superposition of two-mode coherent states. As the scheme is based on resonant atom-cavity interaction, the required interaction time is short, which is important in view of the decoherence. Moreover, additional classical pulses are unnecessary before and after the atom-cavlty interaction.     

三能级原子系统中原子偶极压缩和光场压缩间的关联

通过考察三能级原子与单模和双模场相互作用系统中原子偶极压缩和光场压缩随时间的演化规律, 研究了原子偶极压缩与光场压缩之间的关系, 讨论了原子偶极压缩与光场压缩之间相互转换的特征, 并给出了初始处于偶极压缩状态的原子辐射压缩光的条件。