Preservation of energy-time entanglement in a slow light medium

We demonstrate the preservation of entanglement of an energy-time entangled biphoton through a slow light medium. Using the D(1) and D(2) fine structure resonances of Rubidium, we delay one photon of the 1.5 THz biphoton by approximately 1.3 correlation lengths and measure the fourth order correlation fringes. After the group delay the fringe visibility is reduced from 97.0+/-4.4% to 80.0+/-4.8%, but is still sufficient to violate a Bell inequality. We show that temporal broadening is the primary mechanism for reducing the fringe visibility and that smaller bandwidths lead to greatly reduced broadening.     

六光子量子纠缠态的制备

我们提出了一个完全基于量子擦除概念上的六光子纠缠态的制备方案。首先，提出了利用一组四纠缠光子来制备六光子纠缠态的方案。接着，运用同样的技术，通过五粒子纠缠和Bell态的相互作用来制备六光子纠缠态。我们的实验方案可以应用于以测量为基础的量子计算和多体量子通信领域的研究。我们发现该方案成功的概率取决于纠缠态的系数。     

Proposal of an experimentally accessible measure of many-fermion entanglement

We propose an experimentally accessible measure of entanglement in many-fermion systems that characterizes interaction-induced ground state correlations. It is formulated in terms of cross correlations of currents through resonant fermion levels weakly coupled to the probed system. The proposed entanglement measure vanishes in the absence of many-body interactions at zero temperature and it is related to measures of occupation number entanglement. We evaluate it for two examples of interacting electronic nanostructures.     

Proposal of the nano-sensing device and system using a nano-waveguide transducer for distributed sensors

We propose a new concept of a distributed sensing system using a nano-waveguide and an array waveguide. The small change in physical quantity affects the change in device parameters such as refractive index or length, which is relatively absorbed and observed by the resonant wavelength. In principle, the dense wavelength separation is generated by using a soliton pulse propagating within a ring resonator system, whereas a resonant signal can be stored within the nano-waveguide, i.e. a transducer, which is formed by the sensing device. Induced change in the resonant signal at each wavelength occurs, and can be detected by using the optical spectrum analyzer. Such a proposed device is suitable to perform the measurements in the nano-scale regime such as force, stress and temperature. Moreover, the distributed or multiplexed sensing applications are also available using the nano-waveguide sensing device incorporating the array waveguide, which is discussed in details. Quantum measurement using the same system is also described.     

Localization of quasiparticles in a disordered vortex

We study the diffusive motion of low-energy normal quasiparticles along the core of a single vortex in a dirty, type-II, s-wave superconductor. The physics of this system is argued to be described by a one-dimensional supersymmetric non-linear σ model, which differs from the σ models known for disordered metallic wires. For an isolated vortex and quasiparticle energies less than the Thouless energy E_Th, we recover the spectral correlations that are predicted by random matrix theory for the universality class C. We then consider the transport problem of transmission of quasiparticles through a vortex connected to particle reservoirs at both ends. The transmittance at zero energy exhibits a weak localization correction reminiscent of quasi-one-dimensional metallic systems with symmetry index β = 1. Weak localization disappears with increasing energy over a scale set by E_Th This crossover should be observable in measurements of the longitudinal heat conductivity of an ensemble of vortices under mesoscopic conditions. In the regime of strong localization, the localization length is shown to decrease by a factor of 8 as the quasiparticle energy goes to zero.     

On a quantum statistics of quasiparticles

An example of quasiparticles whose statistics is neither Bose nor Fermi is presented. Experiments which permit the determination of the statistics are proposed. The collision integral for quasiparticles of arbitrary statistics is presented.     

Lifetime of quasiparticles in a hybrid electron-exciton system

The lifetimes (damping) of one-body and collective excitations in a hybrid system consisting of spatially separated layers of a two-dimensional electron gas and a gas of indirect excitons have been calculated at zero temperature in the presence of the Bose-Einstein condensate of excitons. It has been shown that the electron-exciton interaction leads to a considerable shortening of the lifetime of electrons as compared to the electron-electron interaction and to the appearance of a nonzero (linear in the wave vector) damping of plasmons. The interaction of the exciton Bose gas with the electron layer induces damping of Bogoliubov phonons in the exciton Bose gas, which is, however, much lower than their intrinsic (Belyaev) damping.     

Beliaev damping of quasiparticles in a Bose-Einstein condensate

We report a measurement of the suppression of collisions of quasiparticles with ground state atoms within a Bose-Einstein condensate at low momentum. These collisions correspond to Beliaev damping of the excitations, in the previously unexplored regime of the continuous quasiparticle energy spectrum. We use a hydrodynamic simulation of the expansion dynamics, with the Beliaev damping cross section, in order to confirm the assumptions of our analysis.     

Relaxation of hot quasiparticles in a d-wave superconductor

Motivated by recent pump-probe experiments we consider the processes by which "hot" quasiparticles produced near the antinodes of a d-wave superconductor can relax. We show that in a large region of momentum space processes which break Cooper pairs are forbidden by energy and momentum conservation. Equilibration then occurs by scattering with thermal quasiparticles: Umklapp scattering is exponentially suppressed at low temperatures, but small-angle scattering leads to power-law behavior. By solving the Boltzmann equation analytically we make detailed predictions for the temperature and intensity dependence of these processes, which we compare with experiment.     

Effective mass of quasiparticles in a Wigner liquid

The exchange interaction and effective mass of fermionic excitation in a low-density (r ^S ? 1) system of two-dimensional electrons are estimated from simple considerations. For the ratio of effective (renormalized due to interaction) to band mass, the dependence ${{m^* } \mathord{\left/ {\vphantom {{m^* } m}} \right. \kern-0em} m} = ({A \mathord{\left/ {\vphantom {A {\sqrt {r_S } }}} \right. \kern-0em} {\sqrt {r_S } }})\exp (\alpha \sqrt {r_S } )$ is obtained, where A and α are constants on the order of unity. The effective g factor is independent of r ^S and is larger than its bare value in the two-valley case (silicon). Comparison with experimental data shows a qualitative agreement with silicon.     

Number fluctuations of sparse quasiparticles in a superconductor

We have directly measured quasiparticle number fluctuations in a thin film superconducting Al resonator in thermal equilibrium. The spectrum of these fluctuations provides a measure of both the density and the lifetime of the quasiparticles. We observe that the quasiparticle density decreases exponentially with decreasing temperature, as theoretically predicted, but saturates below 160 mK to 25-55/μm(3). We show that this saturation is consistent with the measured saturation in the quasiparticle lifetime, which also explains similar observations in qubit decoherence times.     

Polaronic quasiparticles in a strongly correlated electron band

We show that a strongly renormalized band of polaronic quasiparticle excitations is induced at the Fermi level of an interacting many-electron system on increasing the coupling of the electrons to local phonons. We give results for the local density of states at zero temperature both for the electrons and phonons. The polaronic quasiparticles satisfy Luttinger's theorem for all regimes considered, and their dispersion shows a kink similar to that observed experimentally in copper oxides. Our calculations are based on the dynamical mean field theory and the numerical renormalization group for the hole-doped Holstein-Hubbard model and large on-site repulsion.     

Two types of quasiparticles excited in a Bose gas

We present a novel approach for investigating the superfluid liquid 4He based on the proper use of the nonideal Bose gas model for dilute hard spheres. The results show that the presence of a macroscopic number of condensate atoms leads to the existence of a nonphysical branch, corresponding to density excitations, in addition to the continuous branch of the phonon-maxon-roton excitation spectrum.     

Three-wave mixing of bogoliubov quasiparticles in a bose-einstein condensate

A dressed basis is used to calculate the dynamics of three-wave mixing between Bogoliubov quasiparticles in a Bose condensate. Because of the observed oscillations between different momentum modes, an energy splitting, analogous to the optical Mollow triplet, appears in the Beliaev damping spectrum of the excitations from the oscillating modes.     

Coherent heavy quasiparticles in a CePt5 surface alloy

We report on the results of a high-resolution angle-resolved photoemission study on the ordered surface alloy CePt(5). The temperature dependence of the spectra show the formation of the coherent low-energy heavy-fermion band near the Fermi level. These experimental data are supported by a multiband model calculation in the framework of the dynamical mean-field theory.     

Induced entanglement revival and entanglement protection in a two qubit system

A system of two initially entangled qubits interacting with a bosonic environment is considered. The interaction induces a loss of the initial entanglement of the two qubits, and for specific initial states it causes entanglement sudden death. An investigation of the modifications on the entanglement dynamics by a single pulse control field, performed in the two qubit system, shows that the control field can not only protect entangled states against sudden death but also induce a revival of entanglement in the two qubit system.     

Quantum information transfer and entanglement with SQUID qubits in cavity QED: a dark-state scheme with tolerance for nonuniform device parameter

We investigate the experimental feasibility of realizing quantum information transfer (QIT) and entanglement with SQUID qubits in a microwave cavity via dark states. Realistic system parameters are presented. Our results show that QIT and entanglement with two-SQUID qubits can be achieved with a high fidelity. The present scheme is tolerant to device parameter nonuniformity. We also show that the strong coupling limit can be achieved with SQUID qubits in a microwave cavity. Thus, cavity-SQUID systems provide a new way for production of nonclassical microwave source and quantum communication.