加光子相干场作用下原子线性熵的演化特性

研究了在加光子相干场作用下,处于混态的二能级原子与加光子相干场相互作用系统中原子线性熵的演化规律.结果表明:当相干态平均光子数和加光子数较小时,原子线性熵的振动周期依赖于原子处于基态的概率;当加光子相干态的平均光子数较大时,原子线性熵呈现崩塌与回复现象,其回复周期随加光子数的增加而增加,与原子的初态无关.     

制备腔场纠缠压缩相干态的新方案

提出了一种利用V型三能级原子与大振幅相干态腔场的Raman相互作用来制备相干压缩纠缠态的方案．在这个方案中,一个V型三能级原子连续通过多个处于压缩相干态的腔,通过Raman相互作用,整个系统处于纠缠态．而后,对原子进行探测,则腔场组成的系统坍缩到多模压缩相干纠缠态．     

一种相干叠加态与二能级原子相互作用系统中场熵及反转粒子数的演化特性

研究了压缩相干态和真空态构成的相干叠加态光场与二能级原子相互作用系统中场熵的演化特性及粒子数反转效应,并与相干态和真空态构成的相干叠加态光场的场熵特性及粒子数反转效应进行了比较.研究结果表明,两种叠加态光场的场熵都随时间呈准周期变化,反转粒子数随着时间呈周期性的崩塌-复原变化.与相干态和真空态构成的相干叠加态相比,压缩相干态和真空态构成的相干叠加态与二能级原子的纠缠更强,反转粒子数随着时间变化所呈现崩塌-复原效应的周期变得更短.     

单模辐射场作用过程中二能级原子的Wigner-Yanase偏态信息

研究了与单模辐射场作用过程中二能级原子的Wigner-Yanase信息.原子与真空场作用时,总的Wigner-Yanase偏态信息呈现出周期性的丢失与恢复现象;原子与相干光场作用时,随着时间的演化,总的Wigner-Yanase偏态信息会完全丢失掉,但合适的原子初态及强的相干场,对偏态信息的丢失有延缓作用.     

与ν↑型三能级原子非共振相互作用的SU（1，1）场中光子的聚束与反聚束效应

研究了双模SU(1，1)相干态场与ν↑型三能级原子非共振相互作用系统中场模失谐量、Kerr介质以及原子初态对光场的二阶相干度的影响。结果表明：Kerr介质对光场二阶相干度不产生明显的影响；场模失谐量减弱原子初态对模场1的影响，增大原子初态对模场2的影响；原子初始状态对光场两模的二阶相干度都有影响，但其对模场2的影响较其对模场1的影响更强。     

与Λ型三能级原子非共振相互作用的SU(1,1)场中光子的聚束与反聚束效应

研究了双模SU(1,1)相干态场与Λ型三能级原子非共振相互作用系统中场模失谐量、Kerr介质以及原子初态对光场的二阶相干度的影响.结果表明:Kerr介质对光场二阶相干度不产生明显的影响;场模失谐量减弱原子初态对模场1的影响,增大原子初态对模场2的影响;原子初始状态对光场两模的二阶相干度都有影响,但其对模场2的影响较其对模场1的影响更强.     

驱动单个原子制备纠缠相干态

提出一种利用经典场驱动单个原子与一个多模腔场相互作用制备腔场的多模纠缠相干态的方案.交替调整原子的跃迁频率,使原子与经典场及多模腔场交替作用,通过对原子的选态测量使多模腔场塌缩为多模纠缠相干态.     

利用 型三能级原子与腔场的非共振相互作用制备原子纠缠态

提出一种利用 型三能级原子与相干态腔场的非共振相互作用制备原子纠缠态的方案。研究表明，在简单的条件下，可获得多种形式的原子纠缠态。     

耗散腔中两二能级原子的量子退相干

在能量耗散腔中,原子用泡利算符描述,光场用相干态描述,运用密度矩阵理论,得到了两二能级原子密度矩阵元的演化规律及退相干因子.分析与单模辐射场作用过程中原子态的量子相干性.结果表明: 原子态的量子相干性的演化特性取决于原子-场耦合常数、光场的平均光子数和衰变系数.     

利用Raman型的Jaynes-Cummings模型传送原子态

提出了一种利用V型三能级原子与相干态腔的Raman相互作用来传送原子态的方案。     

Microwave quantum logic gates for trapped ions

Control over physical systems at the quantum level is important in fields as diverse as metrology, information processing, simulation and chemistry. For trapped atomic ions, the quantized motional and internal degrees of freedom can be coherently manipulated with laser light. Similar control is difficult to achieve with radio-frequency or microwave radiation: the essential coupling between internal degrees of freedom and motion requires significant field changes over the extent of the atoms' motion, but such changes are negligible at these frequencies for freely propagating fields. An exception is in the near field of microwave currents in structures smaller than the free-space wavelength, where stronger gradients can be generated. Here we first manipulate coherently (on timescales of 20 nanoseconds) the internal quantum states of ions held in a microfabricated trap. The controlling magnetic fields are generated by microwave currents in electrodes that are integrated into the trap structure. We also generate entanglement between the internal degrees of freedom of two atoms with a gate operation suitable for general quantum computation; the entangled state has a fidelity of 0.76(3), where the uncertainty denotes standard error of the mean. Our approach, which involves integrating the quantum control mechanism into the trapping device in a scalable manner, could be applied to quantum information processing, simulation and spectroscopy.     

Sub-poissonian loading of single atoms in a microscopic dipole trap.

The ability to manipulate individual atoms, ions or photons allows controlled engineering of the quantum state of small sets of trapped particles; this is necessary to encode and process information at the quantum level. Recent achievements in this direction have used either trapped ions or trapped photons in cavity quantum-electrodynamical systems. A third possibility that has been studied theoretically is to use trapped neutral atoms. Such schemes would benefit greatly from the ability to trap and address individual atoms with high spatial resolution. Here we demonstrate a method for loading and detecting individual atoms in an optical dipole trap of submicrometre size. Because of the extremely small trapping volume, only one atom can be loaded at a time, so that the statistics of the number of atoms in the trap, N, are strongly sub-poissonian (DeltaN2 approximately 0.5N). We present a simple model for describing the observed behaviour, and we discuss the possibilities for trapping and addressing several atoms in separate traps, for applications in quantum information processing.     

Cavity QED with a Bose-Einstein condensate

Cavity quantum electrodynamics (cavity QED) describes the coherent interaction between matter and an electromagnetic field confined within a resonator structure, and is providing a useful platform for developing concepts in quantum information processing. By using high-quality resonators, a strong coupling regime can be reached experimentally in which atoms coherently exchange a photon with a single light-field mode many times before dissipation sets in. This has led to fundamental studies with both microwave and optical resonators. To meet the challenges posed by quantum state engineering and quantum information processing, recent experiments have focused on laser cooling and trapping of atoms inside an optical cavity. However, the tremendous degree of control over atomic gases achieved with Bose-Einstein condensation has so far not been used for cavity QED. Here we achieve the strong coupling of a Bose-Einstein condensate to the quantized field of an ultrahigh-finesse optical cavity and present a measurement of its eigenenergy spectrum. This is a conceptually new regime of cavity QED, in which all atoms occupy a single mode of a matter-wave field and couple identically to the light field, sharing a single excitation. This opens possibilities ranging from quantum communication to a wealth of new phenomena that can be expected in the many-body physics of quantum gases with cavity-mediated interactions.     

Entanglement of single-atom quantum bits at a distance

Quantum information science involves the storage, manipulation and communication of information encoded in quantum systems, where the phenomena of superposition and entanglement can provide enhancements over what is possible classically. Large-scale quantum information processors require stable and addressable quantum memories, usually in the form of fixed quantum bits (qubits), and a means of transferring and entangling the quantum information between memories that may be separated by macroscopic or even geographic distances. Atomic systems are excellent quantum memories, because appropriate internal electronic states can coherently store qubits over very long timescales. Photons, on the other hand, are the natural platform for the distribution of quantum information between remote qubits, given their ability to traverse large distances with little perturbation. Recently, there has been considerable progress in coupling small samples of atomic gases through photonic channels, including the entanglement between light and atoms and the observation of entanglement signatures between remotely located atomic ensembles. In contrast to atomic ensembles, single-atom quantum memories allow the implementation of conditional quantum gates through photonic channels, a key requirement for quantum computing. Along these lines, individual atoms have been coupled to photons in cavities, and trapped atoms have been linked to emitted photons in free space. Here we demonstrate the entanglement of two fixed single-atom quantum memories separated by one metre. Two remotely located trapped atomic ions each emit a single photon, and the interference and detection of these photons signals the entanglement of the atomic qubits. We characterize the entangled pair by directly measuring qubit correlations with near-perfect detection efficiency. Although this entanglement method is probabilistic, it is still in principle useful for subsequent quantum operations and scalable quantum information applications.     

异模双光子与二能级原子相互作用系统的纠缠演化

本文从研究初态为一般纠缠态的两个异模光子与初态为一般叠加态的二能级原子的相互作用着手,对得到的三体纠缠态的纠缠度进行了分析,并进一步讨论了系统初态为某些特殊状态时的纠缠演化。最终都得出了简明的演化公式,并且在某些特殊状态时发现系统将演化成三体GHZ态。     

简并二能级系统中的正负色散(英文)

通过解光学布洛赫方程,考察了简并二能级系统的吸收色散性质。采用一束平行线偏振光(π)泵浦和一束横向线偏振光(σ±)探测,在有磁场的情况下简并被破坏,改变耦合光强时发现了失谐零点正负色散之间的转化,这样我们就得到了慢光速与超光速之间的转变。在其它泵探光场偏振情况下同样有正负色散变化,它普遍存在于简并能级系统中。同时我们还发现了吸收谱中的电磁诱导透明、电磁诱导吸收、Mollow吸收谱型等现象。     

Resonant quantum transitions in trapped antihydrogen atoms

The hydrogen atom is one of the most important and influential model systems in modern physics. Attempts to understand its spectrum are inextricably linked to the early history and development of quantum mechanics. The hydrogen atom's stature lies in its simplicity and in the accuracy with which its spectrum can be measured and compared to theory. Today its spectrum remains a valuable tool for determining the values of fundamental constants and for challenging the limits of modern physics, including the validity of quantum electrodynamics and--by comparison with measurements on its antimatter counterpart, antihydrogen--the validity of CPT (charge conjugation, parity and time reversal) symmetry. Here we report spectroscopy of a pure antimatter atom, demonstrating resonant quantum transitions in antihydrogen. We have manipulated the internal spin state of antihydrogen atoms so as to induce magnetic resonance transitions between hyperfine levels of the positronic ground state. We used resonant microwave radiation to flip the spin of the positron in antihydrogen atoms that were magnetically trapped in the ALPHA apparatus. The spin flip causes trapped anti-atoms to be ejected from the trap. We look for evidence of resonant interaction by comparing the survival rate of trapped atoms irradiated with microwaves on-resonance to that of atoms subjected to microwaves that are off-resonance. In one variant of the experiment, we detect 23 atoms that survive in 110 trapping attempts with microwaves off-resonance (0.21 per attempt), and only two atoms that survive in 103 attempts with microwaves on-resonance (0.02 per attempt). We also describe the direct detection of the annihilation of antihydrogen atoms ejected by the microwaves.     

Quantum jumps of light recording the birth and death of a photon in a cavity

A microscopic quantum system under continuous observation exhibits at random times sudden jumps between its states. The detection of this quantum feature requires a quantum non-demolition (QND) measurement repeated many times during the system's evolution. Whereas quantum jumps of trapped massive particles (electrons, ions or molecules) have been observed, this has proved more challenging for light quanta. Standard photodetectors absorb light and are thus unable to detect the same photon twice. It is therefore necessary to use a transparent counter that can 'see' photons without destroying them. Moreover, the light needs to be stored for durations much longer than the QND detection time. Here we report an experiment in which we fulfil these challenging conditions and observe quantum jumps in the photon number. Microwave photons are stored in a superconducting cavity for times up to half a second, and are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms, highly correlated in the same state, are interrupted by sudden state switchings. These telegraphic signals record the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons should open new perspectives for the exploration of the quantum-to-classical boundary.     

Coherent control of optical information with matter wave dynamics

In recent years, significant progress has been achieved in manipulating matter with light, and light with matter. Resonant laser fields interacting with cold, dense atom clouds provide a particularly rich system. Such light fields interact strongly with the internal electrons of the atoms, and couple directly to external atomic motion through recoil momenta imparted when photons are absorbed and emitted. Ultraslow light propagation in Bose-Einstein condensates represents an extreme example of resonant light manipulation using cold atoms. Here we demonstrate that a slow light pulse can be stopped and stored in one Bose-Einstein condensate and subsequently revived from a totally different condensate, 160 mum away; information is transferred through conversion of the optical pulse into a travelling matter wave. In the presence of an optical coupling field, a probe laser pulse is first injected into one of the condensates where it is spatially compressed to a length much shorter than the coherent extent of the condensate. The coupling field is then turned off, leaving the atoms in the first condensate in quantum superposition states that comprise a stationary component and a recoiling component in a different internal state. The amplitude and phase of the spatially localized light pulse are imprinted on the recoiling part of the wavefunction, which moves towards the second condensate. When this 'messenger' atom pulse is embedded in the second condensate, the system is re-illuminated with the coupling laser. The probe light is driven back on and the messenger pulse is coherently added to the matter field of the second condensate by way of slow-light-mediated atomic matter-wave amplification. The revived light pulse records the relative amplitude and phase between the recoiling atomic imprint and the revival condensate. Our results provide a dramatic demonstration of coherent optical information processing with matter wave dynamics. Such quantum control may find application in quantum information processing and wavefunction sculpting.     

Single artificial-atom lasing

Solid-state superconducting circuits are versatile systems in which quantum states can be engineered and controlled. Recent progress in this area has opened up exciting possibilities for exploring fundamental physics as well as applications in quantum information technology; in a series of experiments it was shown that such circuits can be exploited to generate quantum optical phenomena, by designing superconducting elements as artificial atoms that are coupled coherently to the photon field of a resonator. Here we demonstrate a lasing effect with a single artificial atom--a Josephson-junction charge qubit--embedded in a superconducting resonator. We make use of one of the properties of solid-state artificial atoms, namely that they are strongly and controllably coupled to the resonator modes. The device is essentially different from existing lasers and masers; one and the same artificial atom excited by current injection produces many photons.