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Pseudogap phenomena in ultracold atomic Fermi gases
Qijin Chen, Jibiao Wang
Frontiers of Physics. 2014, 9 (5 ): 539-570.
https://doi.org/10.1007/s11467-014-0448-7
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 Tc superconductors. 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 Tc superconductivity. One obstacle to the ultimate understanding of high Tc superconductivity, 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 Tc superconductivity.
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An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems
C. S. Hofmann, G. Günter, H. Schempp, N. L. M. Müller, A. Faber, H. Busche, M. Robert-de-Saint-Vincent, S. Whitlock, M. Weidemüller
Frontiers of Physics. 2014, 9 (5 ): 571-586.
https://doi.org/10.1007/s11467-013-0396-7
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.
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Quantum information processing and metrology with color centers in diamonds
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
Frontiers of Physics. 2014, 9 (5 ): 587-597.
https://doi.org/10.1007/s11467-014-0421-5
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.
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Security analysis on some experimental quantum key distribution systems with imperfect optical and electrical devices
Lin-Mei Liang(梁林海), Shi-Hai Sun(孙仕海), Mu-Sheng Jiang(江木生), Chun-Yan Li(李春燕)
Frontiers of Physics. 2014, 9 (5 ): 613-628.
https://doi.org/10.1007/s11467-014-0420-6
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].
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Temporal inequalities for sequential multi-time actions in quantum information processing
Marek Zukowski
Frontiers of Physics. 2014, 9 (5 ): 629-633.
https://doi.org/10.1007/s11467-013-0400-2
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.
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Modulation of four-wave mixing via photonic band gap
Zhen-Kun Wu, Kai-Ge Chang, Yi Hu, Yun-Zhe Zhang, Zi-Hai Jiang, Yan-Peng Zhang
Frontiers of Physics. 2014, 9 (5 ): 665-670.
https://doi.org/10.1007/s11467-014-0434-0
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.
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