Oct 2014, Volume 9 Issue 5

Cover illustration

  • A well-controlled single atom provides an idea quantum qubit and quantum node for quantum information processing. The cover illustrates the setup of trapping single Cesium atoms in a micro-size 1064 nm red-detuned optical tweezer and the corresponding state manipulation as a qubit in Shanxi University. In the experiment the qubit is encoded in Cesium “clock states” and the Rabi flopping is realized via a two-photon Raman process. For more details, please refer to [Detail] ...

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    Qijin Chen, Jibiao Wang

    The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tcsuperconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum simulator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tcsuperconductivity. One obstacle to the ultimate understanding of high Tcsuperconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics community, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fluctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tcsuperconductivity.

    C. S. Hofmann, G. Günter, H. Schempp, N. L. M. Müller, A. Faber, H. Busche, M. Robert-de-Saint-Vincent, S. Whitlock, M. Weidemüller

    Recent developments in the study of ultracold Rydberg gases demand an advanced level of experimental sophistication, in which high atomic and optical densities must be combined with excellent control of external fields and sensitive Rydberg atom detection. We describe a tailored experimental system used to produce and study Rydberg-interacting atoms excited from dense ultracold atomic gases. The experiment has been optimized for fast duty cycles using a high flux cold atom source and a three beam optical dipole trap. The latter enables tuning of the atomic density and temperature over several orders of magnitude, all the way to the Bose–Einstein condensation transition. An electrode structure surrounding the atoms allows for precise control over electric fields and single-particle sensitive field ionization detection of Rydberg atoms. We review two experiments which highlight the influence of strong Rydberg–Rydberg interactions on different many-body systems. First, the Rydberg blockade effect is used to pre-structure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called dark-state polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between dark-state polaritons. These experiments will ultimately provide a deeper understanding of many-body phenomena in strongly-interacting regimes, including the study of strongly-coupled plasmas and interfaces between atoms and light at the quantum level.

    Jing-Wei Zhou, Peng-Fei Wang, Fa-Zhan Shi, Pu Huang, Xi Kong, Xiang-Kun Xu, Qi Zhang, Zi-Xiang Wang, Xing Rong, Jiang-Feng Du

    The Nitrogen–Vacancy (NV) center is becoming a promising qubit for quantum information processing. The defect has a long coherence time at room temperature and it allows spin state initialized and read out by laser and manipulated by microwave pulses. It has been utilized as a ultra sensitive probe for magnetic fields and remote spins as well. Here, we review the recent progresses in experimental demonstrations based on NV centers. We first introduce our work on implementation of the Deutsch–Jozsa algorithm with a single electronic spin in diamond. Then the quantum nature of the bath around the center spin is revealed and continuous wave dynamical decoupling has been demonstrated. By applying dynamical decoupling, a multi-pass quantum metrology protocol is realized to enhance phase estimation. In the final, we demonstrated NV center can be regarded as a ultra-sensitive sensor spin to implement nuclear magnetic resonance (NMR) imaging at nanoscale.

    Peng-Jun Wang, Jing Zhang

    We review our recent experimental realization and investigation of a spin–orbit (SO) coupled Bose–Einstein condensate (BEC) and quantum degenerate Fermi gas. By using two counter-propagating Raman lasers and controlling the different frequency of two Raman lasers to engineer the atom–light interaction, we first study the SO coupling in BEC. Then we study SO coupling in Fermi gas. We observe the spin dephasing in spin dynamics and momentum distribution asymmetry of the equilibrium state as hallmarks of SO coupling in a Fermi gas. To clearly reveal the property of SO coupling Fermi gas, we also study the momentum-resolved radio-frequency spectroscopy which characterizes the energy–momentum dispersion and spin composition of the quantum states. We observe the change of fermion surfaces in different helicity branches with different atomic density, which indicates that a Lifshitz transition of the Fermi surface topology change can be found by further cooling the system. At last, we study the momentum-resolved Raman spectroscopy of an ultracold Fermi gas.

    Lin-Mei Liang(梁林梅), Shi-Hai Sun(孙仕海), Mu-Sheng Jiang(江木生), Chun-Yan Li(李春燕)

    In general, quantum key distribution (QKD) has been proved unconditionally secure for perfect devices due to quantum uncertainty principle, quantum noncloning theorem and quantum nondividing principle which means that a quantum cannot be divided further. However, the practical optical and electrical devices used in the system are imperfect, which can be exploited by the eavesdropper to partially or totally spy the secret key between the legitimate parties. In this article, we first briefly review the recent work on quantum hacking on some experimental QKD systems with respect to imperfect devices carried out internationally, then we will present our recent hacking works in details, including passive faraday mirror attack, partially random phase attack, wavelength-selected photon-number-splitting attack, frequency shift attack, and single-photon-detector attack. Those quantum attack reminds people to improve the security existed in practical QKD systems due to imperfect devices by simply adding countermeasure or adopting a totally different protocol such as measurement-device independent protocol to avoid quantum hacking on the imperfection of measurement devices [Lo, et al., Phys. Rev. Lett., 2012, 108: 130503].

    Marek Zukowski

    A new kind of temporal inequalities are discussed, which apply to algorithmic processes, involving a finite memory processing unit. They are an alternative to the Leggett–Grag ones, as well as to the modified ones by Brukner et al. If one considers comparison of quantum and classical processes involving systems of finite memory (of the same capacity in both cases), the inequalities give a clear message why we can expect quantum speed-up. In a classical process one always has clearly defined values of possible measurements, or in terms of the information processing language, if we have a sequential computations of some function depending on data arriving at each step on an algorithm, the function always has a clearly defined value. In the quantum case only the final value, after the end of the algorithm, is defined. All intermediate values, in agreement with Bohr’s complementarity, cannot be ascribed a definite value.

    Zhi-Hui Wang, Gang Li, Ya-Li Tian, Tian-Cai Zhang

    Based on single Cesium atoms trapped in a 1064 nm microscopic optical trap we have exhibited a single qubit encoded in the Cesium “clock states”. The single qubit initialization, detection and the fast state rotation with high efficiencies are demonstrated and this state manipulation is crucial for quantum information processing. The ground states Rabi flopping rate of 229.0±0.6 kHz is realized by a two-photon Raman process. A clock states dephasing time of 3.0±0.7 ms is measured, while an irreversible homogeneous dephasing time of 124±17 ms is achieved by using the spin-echo technique. This well-controlled single atom provides an ideal quantum qubit and quantum node for quantum information processing.

    Xiang Yan, Ya-Fei Yu, Zhi-Ming Zhang

    We present a scheme for locally concentrating a non-maximally entangled four-photon cluster state into a maximally-entangled four-photon cluster state. This scheme has a high success probability. The controlled-NOT (CNOT) gate is a crucial ingredient in this scheme, and we use a nearly deterministic CNOT gate, which is similar with that first introduced by Nemoto et al. (Phys. Rev. Lett., 2004, 93: 250502). This CNOT gate has a simple structure and does not need the strong nonlinearity.

    Xiao-Fei Cai, Xu-Tao Yu, Li-Hui Shi, Zai-Chen Zhang

    The traditional method for information transfer in a quantum communication system using partially entangled state resource is quantum distillation or direct teleportation. In order to reduce the waiting time cost in hop-by-hop transmission and execute independently in each node, we propose a quantum bridging method with partially entangled states to teleport quantum states from source node to destination node. We also prove that the designed specific quantum bridging circuit is feasible for partially entangled states teleportation across multiple intermediate nodes. Compared to two traditional ways, our partially entanglement quantum bridging method uses simpler logic gates, has better security, and can be used in less quantum resource situation.

    Ondrej Stranik, Jacqueline Jatschka, Andrea Csáki, Wolfgang Fritzsche

    Metal nanostructures exhibit special optical resonance modes originating from the subwavelength confinement of conductive electrons in the material. These resonance modes represent a strong research focus due to their application potential in optics and sensing application. In this short review recent achievements of our group in this field are highlighted. A wet-chemistry approach synthesis of advanced metallic nanostructures will be introduced and their exact positioning and manipulation by electric field is shown. Next, the application of these nanostructures for a detection of small molecules will be described in several examples. Also, it will be shown that metal nanostructures can be used for sub-wavelength light focusing and for efficient energy coupling into polymer chains.

    Zhen-Kun Wu, Kai-Ge Chang, Yi Hu, Yun-Zhe Zhang, Zi-Hai Jiang, Yan-Peng Zhang

    The dressed four-wave mixing (FWM) in a four-level 85Rb atomic system, experimentally demonstrated in this paper, is comprised by two coexisting processes. One is emission signal due to enhanced nonlinear via electromagnetically induced transparency (EIT). The other is the Bragg reflection of probe beam because of the created photonic band gap (PBG), which is affected by both linear and third-order nonlinear susceptibility. Moreover, we have demonstrated that different experimental parameters can significantly influence the measured signal with flexibly controlled PBG. These studies are found useful for understanding the fundamental mechanisms in generated FWM processing.