71,191 research outputs found

    Effective Operator Treatment of the Lipkin Model

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    We analyze the Lipkin Model using effective operator techniques. We present both analytical and numerical results for effective Hamiltonians. The accuracy of the cluster approximation is investigated.Comment: To appear in Phys.Rev.

    Raw-data attacks in quantum cryptography with partial tomography

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    We consider a variant of the BB84 protocol for quantum cryptography, the prototype of tomographically incomplete protocols, where the key is generated by one-way communication rather than the usual two-way communication. Our analysis, backed by numerical evidence, establishes thresholds for eavesdropping attacks on the raw data and on the generated key at quantum bit error rates of 10% and 6.15%, respectively. Both thresholds are lower than the threshold for unconditional security in the standard BB84 protocol.Comment: 11 pages, 2 figure

    Aging dynamics of ferromagnetic and reentrant spin glass phases in stage-2 Cu0.80_{0.80}C0.20_{0.20}Cl2_{2} graphite intercalation compound

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    Aging dynamics of a reentrant ferromagnet stage-2 Cu0.8_{0.8}Co0.2_{0.2}Cl2_{2} graphite intercalation compound has been studied using DC magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures TcT_{c} (8.7\approx 8.7 K) and TRSGT_{RSG} (3.3\approx 3.3 K). The relaxation rate SZFC(t)S_{ZFC}(t) exhibits a characteristic peak at tcrt_{cr} below TcT_{c}. The peak time tcrt_{cr} as a function of temperature TT shows a local maximum around 5.5 K, reflecting a frustrated nature of the ferromagnetic phase. It drastically increases with decreasing temperature below TRSGT_{RSG}. The spin configuration imprinted at the stop and wait process at a stop temperature TsT_{s} (<Tc<T_{c}) during the field-cooled aging protocol, becomes frozen on further cooling. On reheating, the memory of the aging at TsT_{s} is retrieved as an anomaly of the thermoremnant magnetization at TsT_{s}. These results indicate the occurrence of the aging phenomena in the ferromagnetic phase (TRSG<T<TcT_{RSG}<T<T_{c}) as well as in the reentrant spin glass phase (T<TRSGT<T_{RSG}).Comment: 9 pages, 9 figures; submitted to Physical Review

    p-Wave superfluid and phase separation in atomic Bose-Fermi mixture

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    We consider a system of repulsively interacting Bose-Fermi mixtures of spin polarized uniform atomic gases at zero temperature. We examine possible realization of p-wave superfluidity of fermions due to an effective attractive interaction via density fluctuations of Bose-Einstein condensate within mean-field approximation. We find the ground state of the system by direct energy comparison of p-wave superfluid and phase-separated states, and suggest an occurrence of the p-wave superfluid for a strong boson-fermion interaction regime. We study some signatures in the p-wave superfluid phase, such as anisotropic energy gap and quasi-particle energy in the axial state, that have not been observed in spin unpolarized superfluid of atomic fermions. We also show that a Cooper pair is a tightly bound state like a diatomic molecule in the strong boson-fermion coupling regime and suggest an observable indication of the p-wave superfluid in the real experiment.Comment: 7 pages, 6 figur
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