The effects of the N atom collective Lamb shift on single photon superradiance

The problem of single photon collective spontaneous emission, a.k.a. superradiance, from N atoms prepared by a single photon pulse of wave vector k₀ has been the subject of recent interest. It has been shown that a single photon absorbed uniformly by the N atoms will be followed by spontaneous emission in the same direction [M. Scully, E. Fry, C.H.R. Ooi, K. Wodkiewicz, Phys. Rev. Lett. 96 (2006) 010501; M. Scully, Laser Phys. 17 (2007) 635]; and in extensions of this work we have found a new kind of cavity QED in which the atomic cloud acts as a cavity containing the photon [A.A. Svidzinsky, J.T. Chang, M.O. Scully, Phys. Rev. Lett. 100 (2008) 160504]. In most of our studies, we have neglected virtual photon (“Lamb shift”) contributions. However, in a recent interesting paper, Friedberg and Mannassah [R. Friedberg, J.T. Manassah, Phys. Lett. A 372 (2008) 2514] study the effect of virtual photons investigating ways in which such effects can modify the time dependence and angular distributions of collective single photon emission. In the present Letter, we show that such virtual transitions play no essential role in our problem. The conclusions of [M. Scully, E. Fry, C.H.R. Ooi, K. Wodkiewicz, Phys. Rev. Lett. 96 (2006) 010501; M. Scully, Laser Phys. 17 (2007) 635; A.A. Svidzinsky, J.T. Chang, M.O. Scully, Phys. Rev. Lett. 100 (2008) 160504] stand as published. However, the N atom Lamb shift is an interesting problem in its own right and we here extend previous work both analytically and numerically.     

Ab initio quantum mechanics of a cluster of SiH4 and two H2 molecules, together with its dimer and trimer

Recent experimental work by Strobel et al. [T.A. Strobel, M. Somayazulu, R.J. Hemley, Phys. Rev. Lett. 103 (2009) 065701] on a molecular solid subjected to high pressures has motivated the present study. We first investigate a molecular cluster containing one silane molecule plus two H₂ molecules, by ab initio quantum mechanics. Geometrical and electronic structures for the ground state of such a cluster are thereby proposed. The di-cluster is next examined, and the coming together of two H₂ molecules is predicted. Finally, the tri-cluster is studied and important aspects of the di-cluster bonding are found to remain largely intact.     

Re-study of Nucleon Pole Contribution in J/

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An all-silicon linear chain NMR quantum computer

An updated version of our all-silicon quantum computing scheme [T.D. Ladd, J.R. Goldman, F. Yamaguchi, Y. Yamamoto, E. Abe, K.M. Itoh, Phys. Rev. Lett. 89 (2002) 017901. [3]] and the experimental progress towards its realization are discussed. We emphasize the importance of revisiting a wide range of isotope effects which have been explored over the past several decades for the construction of solid-state silicon quantum computers. Using RF decoupling techniques [T.D. Ladd, D. Maryenko, Y. Yamamoto, E. Abe, K.M. Itoh, Phys. Rev. B. 71 (2005) 014401] phase decoherence times T₂=25 s of ²⁹Si nuclear spins in single-crystal Si have been obtained at room temperature. We show that a linear chain of ²⁹Si stable isotopes with nuclear spin I=1/2 embedded in a spin free ²⁸Si stable isotope matrix can form an ideal building block for solid-state quantum information processors, especially, in the form of a quantum memory which requires a large number of operations within T₂ for the continuous error correction.     

Excess current in point contacts on two-band superconductor MgB2 in magnetic field

A series of I(V) characteristics and bias-dependent differential resistance dV/dI(V) curves of point contacts made between a single crystal of two-band superconductor MgB₂ and Cu were measured in magnetic fields up to 9 T. The magnetic field dependences of the excess current in the I(V) curves were obtained and analyzed using Koshelev and Golubov's [Phys. Rev. Lett. 90, 177002 (2003)] theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state, our data can be qualitatively described using the theoretical magnetic field dependence of the superconducting order parameter of the σ and π-bands and the averaged electronic density of states in MgB₂.     

Theoretical study of CuxAg1−xI alloys

We have performed first-principles calculations using full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT) to investigate the fundamental properties of Cu_xAg_1−xI alloys. We used both GGA96 [J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.] and EVGGA [E. Engel, S.H. Vosko, Phys. Rev. B. 47 (1993) 13164.] generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential. Quantities such as lattice constants, bulk modulus, band gap, density of occupied states and effective mass were calculated as a function of copper molar fraction x. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter, which follows Vegard's law. The microscopic origins of the gap bowing were explained using the approach of Zunger and co-workers; we have concluded that the band-gap energy bowing was mainly caused by the chemical charge-transfer effect and the volume deformation , while the structural relaxation contribute to the gap bowing parameter at smaller magnitude. The calculated phase diagram shows a broad miscibility gap for this alloy with a high critical temperature.     

Pair-Breaking Critical Current Density of Two-Band Superconductor MgB2

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On the integrability of the SU(N) Hubbard model

We exhibit explicitly the intertwiner operator for the monodromy matrices of the SU(N) Hubbard model recently proposed by Maassarani [Phys. Lett. A 239 (1998) 187]. This produces a new family of non-additive R-matrices and generalizes an earlier result by Shastry [Phys. Rev. Lett. 56 (1986) 2453; J. Stat. Phys. 30 (1988) 57].     

On the power-law behavior of the AC conductivity of the mixed crystal (NH4)3H(SO4)1.42(SeO4)0.58

A power law used to describe the AC conductivity from 299 to 393 K of the mixed crystal (NH₄)₃H(SO₄)_1.42(SeO₄)_0.58 led to fractional exponent values ranging from 1.08 to 0.91, depending on structural changes induced on temperature variation [B. Louati, M. Gargouri, K. Guidara and T. Mhiri, J. Phys. Chem. Solids 66 (2005) 762]. In the present note, we suggest that the fractional law exhibits features of lattice relaxation. Despite the structural changes, the parameters of the power law are mutually interconnected to yield a temperature independent phenomenon. Such behavior is probably of general validity and characterizes the universal fractional dispersion of the AC conductivity, as it was also observed in glasses of different composition.     

Phonon dynamics of hexagonal Y Mn1−xFexO3

Phonon dynamics was investigated on Y Mn_1−xFe_xO₃(0≤x≤0.20) hexagonal manganite polycrystals. Phonon modes were properly assigned using results obtained on Y MnO₃ single crystal, by mixing the ab plane and the c axis optical responses. Upon increasing the Fe content, most of the phonon mode frequencies do not vary drastically. However, some modes involving Y and O atom displacements along the c axis are affected by doping. Indeed, a redshift of their transverse optical frequency was interpreted as an elongation of Y-O distance along the c-axis, resulting in a change in yttrium coordination.     

Method for generating a new class of multipartite entangled state in cavity quantum electrodynamics

Recently, Yeo and Chua introduced a genuine four-qubit entangled state |χ〉 which can implement perfect teleportation of an arbitrary two-qubit state [Y. Yeo, W.K. Chua, Phys. Rev. Lett. 96 (2006) 060502]. It has been shown that the state |χ〉 is inequivalent to the well-known Greenberger-Horne-Zeilinger state, W state, and linear cluster state, in terms of stochastic local operations and classical communication [C.F. Wu, Y. Yeo, L.C. Kwek, C.H. Oh, Phys. Rev. A 75 (2007) 032332]. This “new” class of state has many interesting entanglement properties and possible applications in quantum-information processing and fundamental tests of quantum mechanics. Here, we propose a simple scheme to generate the state |χ〉 in cavity quantum electrodynamics. Our idea may be helpful for in-depth study on such a class of state and its practical applications.     

Angular Distributions for

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Three-level systems as amplifiers and attenuators: a thermodynamic analysis

Thermodynamics of a three-level maser was studied in the pioneering work of Scovil-Schulz-DuBois [Phys. Rev. Lett. 2, 262 (1959)10.1103/PhysRevLett.2.262]. In this Letter we consider the same three-level model, but we give a full thermodynamic analysis based on Hamiltonian and dissipative Lindblad superoperators. The first law of thermodynamics is obtained using a recently developed alternative [Phys. Rev. A 74, 063823 (2006)10.1103/PhysRevA.74.063823] to Alicki's definitions for heat flux and power [J. Phys. A 12, L103 (1979)10.1088/0305-4470/12/5/007]. Using a novel variation on Spohn's entropy production function [J. Math. Phys. (N.Y.) 19, 1227 (1978)10.1063/1.523789], we obtain Carnot's efficiency inequality and the Scovil-Schulz-DuBois maser efficiency formula when the three-level system is operated as a heat engine (amplifier). Finally, we show that the three-level system has two other modes of operation--a refrigerator mode and a squanderer mode--both of which attenuate the electric field.     

A genuine 3N-qubit entanglement and controlled teleportation

A genuine 3N-qubit entanglement is derived based on N GHZ trios and controlled teleportation. The state is a complementarity to the genuine 2N-qubit entangled state constructed with N Bell states by Yeo-Chua for N = 2 [Phys. Rev. Lett. 96, 060502 (2006)] and by Chen-Zhu-Guo for any N [Phys. Rev. A 74, 032324 (2006)]. By means of the measures proposed in Phys. Rev. A 74, 022314 (2006), the entanglement of the constructed state is quantified and classified with the well-known GHZ, W and Chen-Zhu-Guo's 2N-qubit entangled state.     

Derivation of diffusion coefficient of a Brownian particle in tilted periodic potential from the coordinate moments

In this research, diffusion of an overdamped Brownian particle in the tilted periodic potential is investigated. Using the one-dimensional hopping model, the formulations of the mean velocity VN and effective diffusion coefficient DN of the Brownian particle have been obtained [B. Derrida, J. Stat. Phys. 31 (1983) 433]. Based on the relation between the effective diffusion coefficient and the moments of the mean first passage time, the formulation of effective diffusion coefficient Deff of the Brownian particle also has been obtained [P. Reimann, et al., Phys. Rev. E 65 (2002) 031104]. In this research, we'll give another analytical expression of the effective diffusion coefficient Deff from the moments of the particle's coordinate.     

Top Quark Flavor Changing Decay t

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Temperature variation of the fundamental absorption edge in AgGaSe2

Optical absorption measurements were made in the temperature range 9–300 K on the chalcopyrite semiconductor compound AgGaSe₂ and the optical energy gap E_G determined as a function of temperature T. In order to obtain the values of E_G as a function of T, the Elliot-Toyozawa model [R.J. Elliot, J. Phys. Rev. 108 (1957) 1384; D.D. Sell, P. Lawaets, Phys. Rev. Lett. 26 (1971) 311] was employed to perform the analysis of the optical absorption spectra. The resulting E_G vs. T curve was fitted to a semi-empirical model that takes into account both the thermal expansion and the electron–phonon interaction contributions. The results have been used to estimate values of the deformation potentials of the valence and conduction bands of the compound.     

Stiffening of nanoscale anatase Ti0.9Zr0.1O2 upon multiple compression cycles

The compression behavior of nanoscale Zr-doped anatase was studied by means of a diamond anvil cell experiment with alternating cycles of compression and decompression in the stability field of anatase (up to 13 GPa). We found that multiple cycles of compression lead to stiffening of the material: Precompressed samples of nanoanatase Ti_0.9Zr_0.1O₂ have a higher bulk modulus (K₀=249(9) and 266(6) GPa) compared with the sample when compressed for the first time (K₀=211(7) GPa). Upon compression, the crystallite size remains the same and the crystalline areas are free of defects. After the experiment, the crystallites are surrounded by amorphous rims, confirming the theoretical prediction by Pischedda et al. [Ultrastability and enhanced stiffness of similar to 6 nm TiO₂ nanoanatase and eventual pressure-induced disorder on the nanometer scale, Phys. Rev. Let. 96 (2006) 035509] for nanoscale anatase, but yielding much lower pressures (12 GPa) for the onset of partial amorphization.     

Exposed-key weakness of αη

The αη protocol given by Barbosa et al. [G.A. Barbosa, E. Corndorf, P. Kumar, H.P. Yuen, Phys. Rev. Lett. 90 (2003) 227901, quant-ph/0212018] claims to be a secure way of encrypting messages using mesoscopic coherent states. We show that transmission under αη exposes information about the secret key to an eavesdropper, and we estimate the rate at which an eavesdropper can learn about the key. We also consider the consequences of using further randomization to protect the key and how our analysis applies to this case. We conclude that αη is not informationally secure.     

The Berry phase: A topological test for the spectrum structure of frustrated quantum spin systems

The nature of the low energy spectrum of frustrated quantum spin systems is investigated by means of a topological test introduced by Hatsugai [Y. Hatsugai, J. Phys. Soc. Jpn. 73 (2004) 2604; Y. Hatsugai, J. Phys. Soc. Jpn. 74 (2005) 1374; Y. Hatsugai, J. Phys. Soc. Jpn. 75 (2006) 123601] which enables to infer the possible existence or absence of a gap between the ground state and excited states of these systems. The test relies on the determination of an order parameter which is a Berry phase. The structure of the spectra of even and odd-legged systems in 2d and 3d is analysed. Results are confronted with previous work.