Coherence properties of a continuous atom laser

Abstract

We investigate the coherence properties of an atomic beam evaporatively cooled in a magnetic guide, assuming thermal equilibrium in the quantum degenerate regime. The gas experiences two-dimensional, transverse Bose-Einstein condensation rather than a full three-dimensional condensation because of the very elongated geometry of the magnetic guide. First order and second order correlation functions of the atomic field are used to characterize the coherence properties of the gas along the axis of the guide. The coherence length of the gas is found to be much larger than the thermal de Broglie wavelength in the strongly quantum degenerate regime. Large intensity fluctuations present in the ideal Bose gas model are found to be strongly reduced by repulsive atomic interactions; this conclusion is obtained with a one-dimensional classical field approximation valid when the temperature of the gas is much higher than its chemical potential, k _B T » |μ|.     

Reduction of Quantum Entropy by Reversible Extraction of Classical Information

Abstract

We inquire under what conditions some of the information in a quantum signal source, namely a set of pure states ψ_a emitted with probabilities p _a, can be extracted in classical form by a measurement leaving the quantum system with less entropy than it had before, but retaining the ability to regenerate the source state exactly from the classical measurement result and the after-measurement state of the quantum system. We show that this can be done if and only if the source states ψ_a fall into two or more mutually orthogonal subsets.     

Quantum mechanical law of corresponding states

The structure and thermodynamic properties of the quantum liquids ⁴He and H₂ near the liquid-gas critical point are calculated with the effective potential approximation, in which the two-body Slater sum replaces the Boltzmann factor; the statistical mechanics problem is formally equivalent to that of a fictitious classical fluid. Approximate integral equation methods from the theory of classical fluids are then used to calculate the critical point parameters and the radial distribution function g(r). The results from the approximate integral equations are compared both with experimental data and with the results of an exact Monte Carlo calculation for the fictitious fluid for a range of densities at the temperatures 7 and 8 K in ⁴He and 36 and 40 K in H₂.Work supported in part by the National Science Foundation.Work supported in part by the National Research Council of Canada.Work supported in part by the National Science Foundation Award No. SMI77-17458.     

Photoluminescence study of strain-induced GaInNAs/GaAs quantum dots

Strain-induced Ga_0.8In_0.2N _y As_1–y quantum dots on GaAs were fabricated by metal organic vapor phase epitaxy. Nitrogen concentration y was varied between 0% and 1.3%. The effect of nitrogen concentration on the optical properties of the quantum dots was investigated by continuous-wave and time-resolved photoluminescence measurements. Carrier localization in the states below the band edge of the nitrogen-containing quantum well has been observed. These states are thought to originate from the variation of the quantum well width or from the fluctuation of the composition. Such variations have been identified in GaInNAs quantum wells on GaAs without stressor islands. The measured energy difference of the quantum well and quantum dot ground-state peak energies increase with increasing temperature.     

Beyond the heisenberg uncertainty

Abstract

An entangled EPR two-particle system is characterized by Δ(p ₁ + p ₂) = 0 and Δ(x ₁ - x ₂) = 0, simultaneously, with the momentum and position of each individual particle completely undefined. Statistically, however, the uncertainty of the correlation measurements on the two individual particles must obey Δ(p1 + p ₂) > Δp _1,2 and Δ(x ₁ - x ₂) > Δx _1,2. While one sees the measurements of p ₁ + p ₂ and x ₁ - x ₂ of the two individual particles satisfying the EPR δ-function, and believes the classical statistical inequality, one might easily be trapped into concluding either there is a violation of the uncertainty principle or there exists action-at-a-distance. Through the analysis of three typical EPR experiments, we show that the measurement of p ₁ + p ₂ and x ₁ - x ₂ for an entangled system can indeed violate the classical statistical inequality. Based on the experimental results, we emphasize, again, the surprising physics of quantum coherent two-particle superposition. Although questions regarding fundamental issues of quantum theory still exist, quantum entanglement has brought us a novel concept in non-local timing and positioning measurements with ultra-high accuracy, even beyond the classical limit.     

Hydrogen Adsorption in Nanotubes

Model calculations are presented of the adsorption of hydrogen in carbon nanotubes. Using a phenomenological interaction potential, we compute the low coverage thermodynamic properties, showing explicitly the quantum effects in the Feynman (semiclassical) effective potential approximation. The effects of interactions are evaluated with a quasi-one dimensional classical Lennard-Jones approximation.     

Microscopic Structure in Liquid Hydrogen and Deuterium: An X-Ray Scattering Study

The static structure factors S(Q) of liquid normal H₂ and D₂ have been measured in the wave-vector region 5 nm^–1<Q<30 nm^–1 through X-ray scattering using two independent sets of data. While the first set has been derived using a standard liquid diffraction geometry, the second has been worked out integrating the dynamic structure factors S(Q, ), measured through inelastic X-ray scattering spectroscopy. In the latter case the first spectral-moment sum rule has been used to obtain directly absolute static structure factor values. A comparison with quantum and classical Monte Carlo simulations shows that a quantitative agreement with the experimental data for both H₂ and D₂ can be obtained only if quantum effects are properly taken into account.     

Quantum effects and the resistive transition in superconducting granular films

Static and dynamic properties of an array of Josephson junctions shunted by Ohmic resistors are discussed within a quantum Ginzburg-Landau theory. The phase diagram at zero temperature is calculated in mean field approximation. It shows that global superconductivity atT=0 is possible only if the normal-state film resistanceR _n is smaller than a critical valueR _n ^c which depends only logarithmically on the Josephson coupling and charging energies. The particular valueR _n ^c =6.5 k found in recent experiments on granular films is in reasonable agreement with estimates for these parameters. A phenomenological order parameter relaxation mechanism is introduced and the associated fluctuation-induced conductivity and diamagnetic susceptibility aboveT _c are determined. The resulting precursor conductivity does not explain the observed exponential decrease withR _n-R _n ^c of the residual resistance at low temperature. However, a very simple model for the resistance due to vortex flow, generalizing the classical Kosterlitz-Thousless picture, is in good agreement with the experimental data.     

Phase transitions and quantum effects in adsorbed monolayers

Phase transitions in absorbed (two-dimensional) fluids and in absorbed layers of linear molecules are studied with a combination of path integral Monte Carlo (PIMC), Gibbs ensemble Monte Carlo (GEMC), and finite size scaling techniques. For a classical (nonadditive) hard-disk fluid the critical nonadditivities, where the entropy-driven phase separations set in, are presented. For a fluid with internal quantum states the gas-liquid coexistence region, tricritical, and triple points can be determined, and a comparison with density functional (DFT) results shows good agreement for the freezing densities. LinearN ₂ molecules adsorbed on graphite (in the 3 × 3 structure) show a transition from a high-temperature phase to a low-temperature phase withherringbone ordering of the orientational degrees of freedom. The order of the transition is determined in the anisotropic planar rotor model as a weak first-order transition. The effect of quantum fluctuations on the herringbone transition is quantified by PIMC and classical simulational methods. The values of the order parameter at low temperatures and the transition temperature are both lowered by roughly 10% due to quantum effects. Rounding effects of the phase transition in adsorbed layers of (N₂) _x (CO)₁, for× < 7% are analyzed by Monte Carlo ( MC) methods, and the ground state ordering for the transition in the adsorbed pure CO system is discussed, from ab initio potentials.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Material design of plasma-enhanced chemical vapour deposition SiCH films for low-k cap layers in the further scaling of ultra-large-scale integrated devices-Cu interconnects

Abstract

Cap layers for Cu interconnects in ultra-large-scale integrated devices (ULSIs), with a low dielectric constant (k-value) and strong barrier properties against Cu and moisture diffusion, are required for the future further scaling of ULSIs. There is a trade-off, however, between reducing the k-value and maintaining strong barrier properties. Using quantum mechanical simulations and other theoretical computations, we have designed ideal dielectrics: SiCH films with Si–C₂H₄–Si networks. Such films were estimated to have low porosity and low k; thus they are the key to realizing a cap layer with a low k and strong barrier properties against diffusion. For fabricating these ideal SiCH films, we designed four novel precursors: isobutyl trimethylsilane, diisobutyl dimethylsilane, 1, 1-divinylsilacyclopentane and 5-silaspiro [4,4] noname, based on quantum chemical calculations, because such fabrication is difficult by controlling only the process conditions in plasma-enhanced chemical vapor deposition (PECVD) using conventional precursors. We demonstrated that SiCH films prepared using these newly designed precursors had large amounts of Si–C₂H₄–Si networks and strong barrier properties. The pore structure of these films was then analyzed by positron annihilation spectroscopy, revealing that these SiCH films actually had low porosity, as we designed. These results validate our material and precursor design concepts for developing a PECVD process capable of fabricating a low-k cap layer.     

The information role of entanglement and interference operators in Shor quantum algorithm gate dynamics

Abstract

Shor algorithm dynamics of quantum computation states are analysed from the classical and the quantum information theory points of view. The Shannon entropy is interpreted as the degree of information accessibility through measurement, while the von Neumann entropy is employed to measure the quantum information of entanglement. The intelligence of a state with respect to a subset of qubits is defined. The intelligence of a state is maximal if the gap between the Shannon and the von Neumann entropy for the chosen result qubits is minimal. We prove that the quantum Fourier transform creates maximally intelligent states with respect to the first n qubits for Shor's problem, since it annihilates the gap between the classical and quantum entropies for the first n qubits of every output state.     

Privacy-Preserving Quantum Two-Party Geometric Intersection

Privacy-preserving computational geometry is the research area on the intersection of the domains of secure multi-party computation (SMC) and computational geometry. As an important field, the privacy-preserving geometric intersection (PGI) problem is when each of the multiple parties has a private geometric graph and seeks to determine whether their graphs intersect or not without revealing their private information. In this study, through representing Alice’s (Bob’s) private geometric graph G_A (G_B) as the set of numbered grids S_A (S_B), an efficient privacy-preserving quantum two-party geometric intersection (PQGI) protocol is proposed. In the protocol, the oracle operation O_A (O_B) is firstly utilized to encode the private elements of S_A =(a₀,a₁,…,a_M-1) (S_B =(b₀,b₁,…,b_N-1)) into the quantum states, and then the oracle operation O_f is applied to obtain a new quantum state which includes the XOR results between each element of S_A and S_B. Finally, the quantum counting is introduced to get the amount (t) of the states |a_i⊕b_j| equaling to |0|, and the intersection result can be obtained by judging t >0 or not. Compared with classical PGI protocols, our proposed protocol not only has higher security, but also holds lower communication complexity.     

Perspectives of an Experiment for Detecting Macroscopic Quantum Coherence

In the second half of 1993 we presented to INFN a proposal of an experiment for detecting macroscopic quantum coherence with a system of SQUIDs. It was based essentially on ideas presented first by Leggett and collaborators and developed in many articles in the 1980s. The experimental work started in the 1994 just after the approval of the INFN. As an introduction, the experimental method and the setup we choose in order to perform a set of measurements on a system of SQUIDs is described. Then the measuring procedures and their purposes are explained and discussed.As a future perspective, a possible test on the existence of the quantum gravity is discussed. According to a theoretical proposal by John Ellis and collaborators, the quantum gravitational friction could induce the transition from QM behavior of microscopic states to the classical behavior of macroscopic states, i.e., states containing an Avogadro number (M _pl/M _e ) of elementary particles, as may be obtained in a SQUID.     

The one-way quantum computer--a non-network model of quantum computation

A one-way quantum computer (QC _C ) works by performing a sequence of one-qubit measurements on a particular entangled multi-qubit state, the cluster state. No non-local operations are required in the process of computation. Any quantum logic network can be simulated on the QC _C . On the other hand, the network model of quantum computation cannot explain all ways of processing quantum information possible with the QC _C . In this paper, two examples of the non-network character of the QC _C are given. First, circuits in the Clifford group can be performed in a single time step. Second, the QC _C -realization of a particular circuit—the bit-reversal gate—has no network interpretation.     

A New Quantum Formalism for Damping and Gain in Harmonic Oscillators

Abstract

A new formulation of loss or gain in the quantum theory of harmonic oscillators is put forward using a non-passive reactive circuit which can be readily quantized. The analysis is based on the electrical circuit theory and demonstrates how a circuit, with negative inductance ? L _n and negative capacitance ? C _n, coupled to a conventional harmonic oscillator circuit, of positive inductance L and positive capacitance C, can act as a source or sink of energy and allow for both gain and loss. Classically this series circuit is indistinguishable in its transients from either a + G or ? G conductance shunted across a main LC oscillator circuit. However, unlike the resistive circuit, this coupled circuit can be quantized, maintaining the uncertainty principle. A two-valued solution is found, dependent on whether the circuits are in a state to receive energy or a state to absorb energy. A full correspondence, including second-order frequency shifts, is found between the quantum and the classical solutions with states which are appropriate to thermodynamic equilibrium of a conductance at a temperature T as well as to the classical-like coherent states. While the accessible mode in the + L + C circuit does not exhibit any squeezing directly, the system as a whole is an example of two-mode squeezing discussed by other authors.     

Quantum Cavitation: A Comparison Between Superfluid Helium-4 and Normal Liquid Helium-3

Cavitation has now been studied in superfluid helium-4 and in normal liquid helium-3, both theoretically and experimentally. We compare the two cases and discuss the existence of a crossover from quantum cavitation, where bubbles are nucleated by tunneling, to classical cavitation where their nu-deation is thermally activated. In helium-3, where evidence for quantum nucleation is lacking, the interpretation of the experimental results leads to two related questions. The first one concerns the extrapolation of the properties of this Fermi liquid at negative pressure. The second one concerns the validity of present theories of quantum cavitation in a Fermi liquid.     

Macroscopic jumps of the axial angular momentum variance of a bidimensionally trapped ion

Abstract

The time evolution of the axial angular momentum [Lcirc] _z of an ion confined in a bidimensional trap is investigated. We find that, under suitable initial conditions, the interaction of the ion with two properly configured classical laser beams induces a peculiar dynamical behaviour of the axial angular momentum fluctuations. We show, in fact, that there exists an instant of time at which the variance of [Lcirc] _z undergoes variations proportional to N ² further to a change of one quantum only in the initial total number N ? 1 of vibrational quanta. The non-classical origin of these macroscopic jumps is brought to the light and carefully discussed.     

High‐Performance Inorganic Perovskite Quantum Dot–Organic Semiconductor Hybrid Phototransistors

All‐inorganic lead halide perovskite quantum dots (IHP QDs) have great potentials in photodetectors. However, the photoresponsivity is limited by the low charge transport efficiency of the IHP QD layers. High‐performance phototransistors based on IHP QDs hybridized with organic semiconductors (OSCs) are developed. The smooth surface of IHP QD layers ensures ordered packing of the OSC molecules above them. The OSCs significantly improve the transportation of the photoexcited charges, and the gate effect of the transistor structure significantly enhances the photoresponsivity while simultaneously maintaining high I _photo/I _dark ratio. The devices exhibit outstanding optoelectronic properties in terms of photoresponsivity (1.7 × 10⁴ A W^?1), detectivity (2.0 × 10¹⁴ Jones), external quantum efficiency (67000%), I _photo/I _dark ratio (8.1 × 10⁴), and stability (100 d in air). The overall performances of our devices are superior to state‐of‐the‐art IHP photodetectors. The strategy utilized here is general and can be easily applied to many other perovskite photodetectors.     

Axial Phase of Quantum Fluids in Nanotubes

We explore the equations of state and other properties of various quantum fluids (³He, ⁴He, their mixtures, and H₂) confined within individual carbon nanotubes. Above a threshold number of particles, N _a, the fluid density near the axis begins to grow above a negligibly small value. The properties of this axial fluid phase are sensitive to the tube size and hence to the transverse compression in the case of a bundle of nanotubes. We consider He at zero temperature and H₂ at low temperatures.     

Effect of Dissipation on Quantum Tunneling of Vortices in Tl2Ba2Ca2Cu3O10+δSuperconductors

The temperature and magnetic field dependence of the magnetic relaxation rate has been investigated at low temperatures (1.8 < T < 10 K) on two Tl₂Ba₂Ca₂Cu₃O₁₀₊ samples (epitaxial thin film and sintered pellet). The temperature dependence gives evidence of a crossover in the mechanism of vortex motion, from classical thermal activation to quantum tunneling as temperature decreases. The field dependence of the relaxation rate indicates a crossover in the dimensionality of vortices, from three-dimensional flux lines to two-dimensional pancake vortices as field increases. For the thin film, the temperature dependence of the rate has been fitted to the theoretically predicted expressions for finite-temperature enhancement of the quantum rate in different regimes of dissipation. In spite of the similarity of the fits, the estimate of the ratio of Hall to viscous drag terms in the equation of motion indicates that quantum tunneling in this system occurs in an intermediate dissipative regime, where both terms contribute to the motion of vortices.