An atom and a photon

The coherent control of single-photon emitters as, e.g., single ions or atoms, is a crucial element for mapping quantum information between light and matter. The possibility of generating entanglement between a photon and the emitter system provides an interface between matter-based quantum memories and photonic quantum communication channels, which is the essential resource for quantum repeaters and other future quantum information applications. To generate entangled atom-photon states, in our experiment, we store a single ⁸⁷Rb atom in an optical dipole trap. The single-atom/single-photon character is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom allowed us to determine the mean kinetic energy of the atom corresponding to 105 μK. We describe a single-atom state analysis method which allowed us to characterize the entanglement between the atom and a single photon emitted in the spontaneous decay. We obtain an entanglement fidelity of 89% that clearly shows the high degree of entanglement in our system and potential for further applications in quantum communication.     

Deterministic and storable single-photon source based on a quantum memory

A single-photon source is realized with a cold atomic ensemble (87Rb atoms). A single excitation, written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a single-photon source is well suited for future large-scale realization of quantum communication and linear optical quantum computation.     

Simultaneous magneto-optical trapping of a boson-fermion mixture of metastable helium atoms

We simultaneously confine fermionic metastable 3He atoms and bosonic metastable 4He atoms in a magneto-optical trap. The trapped clouds, containing up to 1.5 x 10(8) atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He and 4He atoms are indicated.     

Nonlinear phenomenology from quantum mechanics: soliton in a lattice

We study a soliton in an optical lattice holding bosonic atoms quantum mechanically using both an exact numerical solution and quantum Monte Carlo simulations. The computation of the state is combined with an explicit account of the measurements of the numbers of the atoms at the lattice sites. In particular, importance sampling in the quantum Monte Carlo method arguably produces faithful simulations of individual experiments. Even though the quantum state is invariant under lattice translations, an experiment may show a noisy version of the localized classical soliton.     

On the resistance of relativistic quantum cryptography in open space at finite resources

The security of keys for the basic nonrelativistic BB84 protocol has been examined for more than 15 years. A simple proof of security for the case of a single-photon source of quantum states and finite sequences has been only recently obtained using entropy uncertainty relations. However, the existing sources of states are not strictly single-photon. Since sources are not single-photon and losses in a quantum channel??open space??are not a priori known and vary, nonrelativistic quantum cryptographic systems in open space cannot guarantee the unconditional security of keys. Recently proposed relativistic quantum cryptography removes fundamental constraints associated with non-single-photon sources and losses in open space. The resistance of a fundamentally new family of protocols for relativistic quantum key distribution through open space has been analyzed for the real situation with finite lengths of transmitted sequences of quantum states. This system is stable with real sources of non-single-photon states (weakened laser radiation) and arbitrary losses in open space.     

Ultracold Li + Li2 collisions: bosonic and fermionic cases

We have carried out quantum dynamical calculations of vibrational quenching in Li + Li(2) collisions for both bosonic (7)Li and fermionic (6)Li. These are the first ever such calculations involving fermionic atoms. We find that for the low initial vibrational states considered here (v < or = 3), the quenching rates are not suppressed for fermionic atoms. This contrasts with the situation found experimentally for molecules formed via Feshbach resonances in very high vibrational states.     

Two-Mode Maximally Entangled State Generated by the Microtoroid Cavity-Atom System

Schemes are proposed for producing maximally entangled states for two modes via a single atoms resonating with an ultra-high Q microtoroidal cavity. This system would lead to the generation of the single-photon entangled state or single-atom single-photon entangled state. The maximal entanglements are highly relative to the distances between the atom and the microtoroidal cavity as well as interaction times. All of these states have high fidelities.     

Degenerate Bose-Fermi mixture of metastable atoms

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10(6) atoms of each isotope at temperatures below 1 microK, and achieve a fermionic degeneracy parameter of T/TF = 0.45. Because of their high internal energy, the detection of individual metastable atoms with subnanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.     

Efficient and robust initialization of a qubit register with fermionic atoms

We show that fermionic atoms have crucial advantages over bosonic atoms in terms of loading in optical lattices for use as a possible quantum computation device. After analyzing the change in the level structure of a nonuniform confining potential as a periodic potential is superimposed to it, we show how this structure combined with the Pauli principle and fermion degeneracy can be exploited to create unit occupancy of the lattice sites with very high efficiency.     

Seeing Wave-Particle Superposition with Cavity Input-Output Process

We present an experimental protocol to implement quantum delay-choice experiment in the context of cavity input-output process. In our protocol, the single-atom is employed as ancillary qubit to test the wave-particle feature of a single photon. With the cavity input-output process, we show that the controlled phase shift gate between single-atom and single-photon can be naturally used to generate the controlled Hadamard gate, which thus allows us to construct the quantum circuit for realizing the quantum delay-choice experiment. We also demonstrate the photonic wavelike and particlelike states can be simultaneously observed in our platform. Our protocol may open a new prospect using cavity quantum electrodynamics system to study some counterintuitive fundamental phenomenons in quantum mechanics.     

Single-photon sources

The article surveys the state of the art in the design, development and application of devices for deterministically generating single photons on demand. Both the defined function and requisite form of such ‘single-photon’ sources are explained in detail. Their attributes and characteristics, in particular the photon-counting statistics of the light that they generate, are presented in conjunction with the experimental apparatus (most notably the Hanbury-Brown and Twiss interferometer) for measuring them. Promising applications of single-photon sources within quantum key distribution, quantum information processing, as well as metrology and fundamental tests of quantum mechanics, are described. The utility and relative advantages of single-photon sources vis-á-vis more conventional sources of light are explained in terms of application-specific requirements and the respective abilities of different sources to fulfil them. The article collects, classifies and sorts the most significant work towards realizing practical single-photon sources to date. Though emanating from a diverse set of technological disciplines, with different research and application objectives in mind, the relative advantages and drawbacks of each approach are assessed, to give the reader a broad yet coherent and critical review of a rapidly developing research front.     

Ion crystal transducer for strong coupling between single ions and single photons

A new approach for the realization of a quantum interface between single photons and single ions in an ion crystal is proposed and analyzed. In our approach the coupling between a single photon and a single ion is enhanced via the collective degrees of freedom of the ion crystal. Applications including single-photon generation, a memory for a quantum repeater, and a deterministic photon-photon, photon-phonon, or photon-ion entangler are discussed.     

Atomic four-wave mixing: fermions versus bosons

We compare four-wave mixing in quantum degenerate gases of bosonic and fermionic atoms. We find that matter-wave gratings formed from either bosonic or fermionic atoms can in principle exhibit nearly identical Bragg scattering and four-wave mixing properties. This implies that effects such as coherent matter-wave amplification and superradiance can occur in degenerate Fermi gases. This effect is due to constructive many-particle quantum interferences, which in the boson case are interpreted as "Bose enhancement."     

Localization of bosonic atoms by fermionic impurities in a three-dimensional optical lattice

We observe a localized phase of ultracold bosonic quantum gases in a 3-dimensional optical lattice induced by a small contribution of fermionic atoms acting as impurities in a Fermi-Bose quantum gas mixture. In particular, we study the dependence of this transition on the fermionic (40)K impurity concentration by a comparison to the corresponding superfluid to Mott-insulator transition in a pure bosonic (87)Rb gas and find a significant shift in the transition parameter. The observed shift is larger than expected based on a simple mean-field argument, which indicates that disorder-related effects play a significant role.     

A Bright Single-Photon Source from Nitrogen-Vacancy Centers in Diamond Nanowires

Single-photon flux is one of the crucial properties of nitrogen vacancy(NV) centers in diamond for its application in quantum information techniques. Here we fabricate diamond conical nanowires to enhance the single-photon count rate. Through the interaction between tightly confined optical mode in nanowires and NV centers, the single-photon lifetime is much shortened and the collection efficiency is enhanced. As a result, the detected single-photon rate can be at 564 kcps,and the total detection coefficient can be 0.8%,wich is much higher than that in bulk diamond. Such a nanowire single-photon device with high photon flux can be applied to improve the fidelity of quantum computation and the precision of quantum sensors.     

基于低Q腔光子Faraday旋转的远程态制备

基于低Q腔中单光子的输入与输出关系,提出了利用偏振光Faraday旋转分别遥远制备单原子态和两原子纠缠态的可行方案.研究结果表明,当初始原子态的系数为实数时,通过选择合适的偏振光、腔场与原子相互作用系统的参数,单原子态与两原子纠缠态的远程制备均可确定性地得以实现.与以前的原子态远程制备方案相比,本文方案采用光子作为飞行比特来传递量子信息,故原则上可实现原子态的真正长距离制备.由于原子态的信息编码在耗散单边腔囚禁的Λ型三能级原子的两个基态能级,且原子仅虚激发,因此本文方案对腔衰减和原子自发辐射不敏感.此外,本文所提出的两种方案不需要两体或多体正交测量,仅涉及单体直积态测量,而且两种方案都工作在低Q腔,不需要原子与光腔的强耦合,从而有效降低了实验难度.     

强耦合腔量子电动力学中单原子转移的实验及模拟

对于强耦合腔量子电动力学系统中以自由下落方式转移原子与腔模强耦合作用过程进行了实验研究,并在理论上利用蒙特卡罗方法对整个实验过程进行了模拟.根据模拟的高精度光学微腔实时记录的原子穿腔信号,获得了原子与腔模相互作用以及冷原子的参数等基本信息,包括不同初始条件下原子与腔模相互作用时腔的透射谱、单个原子在腔内的驻留时间、原子到达腔模时刻的概率分布以及原子到达腔模的动能分布等,并作为对比给出了相应的实验结果.基于模拟结果,实验上建立了腔内光学偶极阱来俘获单个原子,测量的单原子的腔内俘获寿命达到5 ms,比自由穿越时延长了约30倍.该研究对于原子-腔受限空间内,以自由下落方式转移原子以及原子与腔的耦合过程给出详细的分析,有助于对类似实验结果的分析和系统参数的优化.     

稀薄里德伯原子气体中的两体纠缠

量子纠缠是量子信息处理和量子计算中不可或缺的物理资源,制备稳定可操控的量子纠缠是研究的热点之一.里德伯原子具有不同于普通中性原子的特点,长寿命和原子之间强烈的偶极相互作用,使得它成为量子信息处理和量子计算的最优候选者.本文在稀薄里德伯原子气体中,构建了空间四面体排布的里德伯原子模型(空间等距的四个原子模型),通过数值求解主方程来研究两体纠缠和里德伯激发的稳态和瞬态动力学性质,发现偶极阻塞机制下的量子纠缠最大,其他满足反偶极阻塞条件的高阶激发引起的纠缠较小,进而从理论上分析了这两种机制下量子纠缠的物理实质.     

Ion trapping for quantum information processing

In this paper we have reviewed the recent progresses on the ion trapping for quantum information processing and quantum computation. We have first discussed the basic principle of quantum information theory and then focused on ion trapping for quantum information processing. Many variations, especially the techniques of ion chips, have been investigated since the original ion trap quantum computation scheme was proposed. Full two-dimensional control of multiple ions on an ion chip is promising for the realization of scalable ion trap quantum computation and the implementation of quantum networks.