Boundary effects in transverse spin dynamics of spin-polarized quantum gases

Spin dynamics of spin-polarized quantum gases near nonmagnetic walls is discussed. We consider boundary-induced line shifts and attenuation of spin-waves, and a possible macroscopic boundary condition for systems close to a Knudsen ballistic regime. By a coordinate transformation, we reduce the problem of collisions with a rough wall to an equivalent problem with a specular wall but with stochastic bulk perturbations. The boundary effects are described by a bulk-like transverse spin-diffusion coefficient inversely proportional to the correlation function of surface inhomogeneities. This leads to an effective macroscopic boundary condition applied for the spin-wave resonances. The situation is changed at low temperatures by an appearance of absorbed boundary layers which lead to additional exchange processes and renormalize the molecular field near the walls. The experimental implications for helium and hydrogen systems are discussed, including the signs of surface spin modes.     

Pressure diffusion and sound absorption in spin-polarized quantum systems

Pressure diffusion and related phenomena are studied in the cases of Fermi liquids and dilute gases with arbitary degree of quantum degeneracy. An equation is derived expressing the (spin) pressure diffusion ratio through partial viscosities of (spin) components of systems. The exact values of corresponding transport coefficients are given for the cases of spin-polarized Boltzmann or degenerate quantum gases and spin-polarized Fermi liquids. The influence of surface slip effects on diffusion properties of spin-polarized quantum systems is discussed. The results may be used for gaseous and liquid³He,³He -⁴He solutions, gases H and D , and other spin-polarized or binary quantum systems.     

Spin filtering and quantum phase transition in double quantum dots attached to spin-polarized leads

We study the spin filtering and quantum phase transition (QPT) in double quantum dots attached to spin-polarized leads. For spin-independent leads, we observe a Kosterlitz-Thouless transition between the local triplet and doublet. For spin-polarized leads, the above QPT becomes first order, and Kondo splitting, gate-controlled spin reversal and a perfect spin filtering are observed. The breaking of spin-rotation SU(2) symmetry and the interdot transport mediated by the conduction electron are responsible for the fully spin-polarized conductance. Because spin-polarized leads suppress the Kondo effect, in order to obtain a large conductance with perfect spin filtering, one should choose leads with small spin polarization, such as Rashba spin-orbital coupling leads.     

Decoherence in quantum systems

We discuss various definitions of decoherence and how it can be measured. We compare and contrast decoherence in quantum systems with an infinite number of eigenstates (such as the free particle and the oscillator) and spin systems. In the former case, we point out the essential difference between assuming "entanglement at all times" and entanglement with the reservoir occuring at some initial time. We also discuss optimum calculational techniques in both arenas.     

Dislocation movement and slip systems in β-SiC

Plastic-carbon replicas taken off the fracture surfaces of self-bonded SiC were found to have transparent flakes of-SiC attached. Certain of the flakes contained dislocations which were shown to have moved during the fracture process. The dislocations were shown to move on a {111} slip plane and to have a Burgers vectora/2 110.     

Decoherence rates in large-scale quantum computers and macroscopic quantum systems

Markovian regime decoherence effects in quantum computers are studied in terms of the fidelity for the situation where the number of qubits N becomes large. A general expression giving the decoherence time scale in terms of Markovian relaxation elements and expectation values of products of system fluctuation operators is obtained, which could also be applied to study decoherence in other macroscopic systems such as Bose condensates and superconductors. A standard circuit model quantum computer involving three-state lambda system ionic qubits is considered, with qubits localized around well-separated positions via trapping potentials. The centre of mass vibrations of the qubits act as a reservoir. Coherent one and two qubit gating processes are controlled by time-dependent localized classical electromagnetic fields that address specific qubits, the two qubit gating processes being facilitated by a cavity mode ancilla, which permits state interchange between qubits. With a suitable choice of parameters, it is found that the decoherence time can be made essentially independent of N.     

Energy transfer and phase slip by quantum vortex motion in superfluid4He

The purpose of the present article is to emphasize the usefulness of the ideas of E. R. Huggins in thinking about vortex motion and phase slip in superfluid⁴He, and is primarily pedagogical. Several explicit illustrations of vortex motion and phase-slip processes are considered. In addition, it is shown that Huggins's results lead to a generalization and a more complete understanding of the familiar expression E+v_s · p for the energy in the rest system of an excitation in the flowing superfluid, as applied to vortex excitations. Here, E is the energy and p is the momentum of the excitation in the moving system, and v_s is the superfluid velocity.     

Boundary effects in fluid flow. Application to quantum liquids

The effect of boundaries on the flow of rarefied gases is considered. For an excitation gas of arbitrary statistics and energy-momentum relationship we determine the magnitude of the slip length and the flow between parallel plates mostly by variational methods. Our approximate method avoids the need to solve integral equations numerically and yields in the stationary case better than 1% agreement with known exact results for the classical Maxwell-Boltzmann gas. Our general results are primarily applied to normal and superfluid Fermi liquids. We calculate the surface impedance of an oscillating plate and determine the frequency-dependent slip length for frequencies ranging from the hydrodynamic to the collisionless limit. Our results are applied to the analysis of viscosity measurements based on a torsional oscillator or a vibrating wire. The slip effects are shown to be very important for realistic experimental parameters, especially at low temperatures in the superfluid B phase of liquid ³He.     

Nano-optics with single quantum systems

This paper reviews the recent progress in using single quantum systems, here mainly single fluorescent molecules, as local probes for nano-optical field distributions. We start by discussing the role of the absorption cross-section for the spatial resolution attainable in such experiments and its behaviour for different environmental conditions. It is shown that the spatial distribution of field components in a high-numerical aperture laser focus can be mapped with high precision using single fluorescent molecules embedded in a thin polymer film on glass. With this proof-of-principle experiment as a starting point, the possibility of mapping strongly confined and enhanced nano-optical fields close to material structures, e.g. sharp metal tips, is discussed. The mapping of the spatial distribution of the enhanced field at an etched gold tip using a single molecule is presented as an example. Energy transfer effects and quenching are identified as possible artefacts in this context. Finally, it is demonstrated that the local quenching at a sharp metal structure nevertheless can be exploited as a novel contrast mechanism for ultrahigh-resolution optical microscopy with single-molecule sensitivity.     

Magnetic relaxation rates in spin-polarized hydrogen

A self-contained discussion is presented of the longitudinal (T ₁ ^–1 ) and transverse (T ₂ ^–1 ) relaxation rates in bulk and surface samples of spin-polarized atomic hydrogen (H), at sufficiently low temperatures that only the lowest two atomic hyperfine levels are thermally populated. The nonhydrodynamic contribution to the rates, due to binary collisions between hydrogen atoms, in both normal and Bose condensed samples of H are emphasized. However, the approach is general and is equally well suited for treating long-wavelength, hydrodynamic relaxation processes. Most of the discussion pertains to samples close to thermodynamic equilibrium. The calculation of the longitudinal relaxation rate for some states far from equilibrium, particularly relevant for real samples of H, is also presented. Some of the interesting results are:(1) the potentially long surface longitudinal relaxation time (T ₁) in samples with most of the available surface area oriented perpendicular to the direction of the stabilizing field; (2) the possibility of extracting the condensate fractionn _o(T)/n from aT ₁ measurement in the Bose condensed state, and finally (3) an amusingGedanken experiment that would allow us to detect the onset of Bose condensation in aT ₁ measurement in the absence of recombination.     

Zero-temperature relaxation in spin-polarized fermi liquids

We discuss the effect of zero-temperature attenuation, which has been recently observed in spin dynamics of spin-polarized Fermi liquids, on other Fermi-liquid processes. The transfer of this attenuation mechanism from transverse spin dynamics to longitudinal processes can be caused by the magnetic dipole interaction, namely, by the direct dipole processes and the dipole coupling between the transverse spin dynamics and the longitudinal transport and relaxation processes. We calculated the zero-temperature sound attenuation in spin-polarized Fermi liquids, corrections to the threshold of spin-wave (Castaing) instability, and the effective zero-temperature viscosity and longitudinal relaxation time in low- and high-frequency regimes.     

Transverse spin dynamics in spin-polarized Fermi liquids

A microscopic framework for a generalized Landau theory is established in the form of two coupled equations in partial transverse densities. The 4-component effective interaction is related to the irreducible vertex, and not to the full vertex. The results explain the zero-temperature transverse relaxation and attenuation of spin waves. The spectrum of spin waves is expressed via harmonics of the interaction operator and its derivatives at arbitrary polarization. Our exact equations differ from previous semi-phenomenological ones, and reproduce all proper limiting cases like spin-polarized quantum gases or the Silin-Leggett low field equations.     

Electrical transport phenomena in systems of semiconductor quantum dots

While a fairly good understanding of optical and transport properties that are associated with single quantum dots has emerged in recent years the understanding of the relation between these properties and the observed macroscopic optical and electrical properties of solid ensembles of such dots is still at a very rudimentary level. This is in particular so in regard to the transport properties where the interplay between inter-dot conduction and the connectivity of the dots network determines the macroscopic observations. Reviewing the basic concepts and issues associated with these two essential ingredients, and considering some recent experimental observations on quantum dot ensembles of CdSe and Si, an effort is made here to derive a whole-but-simple physical basis for the understanding of the transport and the optoelectronic properties of solid state ensemble of semiconductor quantum dots.     

Electrons at the surface of quantum systems

Electrons can be trapped at the surfaces and interfaces of the condensed phases of quantum matter (in particular hydrogen and helium), where they form classical two-dimensional Coulomb systems. Apart from studying the intrinsic properties of these nearly ideal systems, like the transition from an electron gas to a Wigner solid, one can use the electrons also as a sensitive probe to investigate the surface of quantum liquids and solids. The examples presented here include the surface of solid H₂, He films, and the interface between liquid and solid He, where phenomena such as annealing, layering, and crystal growth are studied. In addition, the interaction between the electrons and long-wavelength interfacial modes leads to an electrohydrodynamic instability, which bears interesting similarities with other critical phenomena.     

Comparing the states of many quantum systems

Abstract

We investigate how to determine whether the states of a set of quantum systems are identical or not. This paper treats both error-free comparison, and comparison where errors in the result are allowed. Error-free comparison means that we aim to obtain definite answers, which are known to be correct, as often as possible. In general, we will also have to accept inconclusive results, giving no information. To obtain a definite answer that the states of the systems are not identical is always possible, whereas in the situation considered here, a definite answer that they are identical will not be possible. The optimal universal error-free comparison strategy is a projection onto the totally symmetric and the different non-symmetric subspaces, invariant under permutations and unitary transformations. We also show how to construct optimal comparison strategies when allowing for some errors in the result, minimizing either the error probability, or the average cost of making an error. We point out that it is possible to realize universal error-free comparison strategies using only linear elements and particle detectors, albeit with less than ideal efficiency. Also minimum-error and minimum-cost strategies may sometimes be realized in this way. This is of great significance for practical applications of quantum comparison.