81,780 research outputs found
Signatures of the superfluid to Mott-insulator transition in the excitation spectrum of ultracold atoms
We present a detailed analysis of the dynamical response of ultra-cold
bosonic atoms in a one-dimensional optical lattice subjected to a periodic
modulation of the lattice depth. Following the experimental realization by
Stoferle et al [Phys. Rev. Lett. 92, 130403 (2004)] we study the excitation
spectrum of the system as revealed by the response of the total energy as a
function of the modulation frequency Omega. By using the Time Evolving Block
Decimation algorithm, we are able to simulate one-dimensional systems
comparable in size to those in the experiment, with harmonic trapping and
across many lattice depths ranging from the Mott-insulator to the superfluid
regime. Our results produce many of the features seen in the experiment, namely
a broad response in the superfluid regime, and narrow discrete resonances in
the Mott-insulator regime. We identify several signatures of the
superfluid-Mott insulator transition that are manifested in the spectrum as it
evolves from one limit to the other.Comment: 18 pages and 12 figures; Some improved results and additional
references. To appear in a special issue of New J. Phy
Charged spin 1/2 particle in an arbitrary magnetic field in two spatial dimensions: a supersymmetric quantum mechanical system
It is shown that the 2 X 2 matrix Hamiltonian describing the dynamics of a
charged spin 1/2 particle with g-factor 2 moving in an arbitrary, spatially
dependent, magnetic field in two spatial dimensions can be written as the
anticommuator of a nilpotent operator and its hermitian conjugate.
Consequently, the Hamiltonians for the two different spin projections form
partners of a supersymmetric quantum mechanical system. The resulting
supersymmetry algebra can then be exploited to explicitly construct the exact
zero energy ground state wavefunction for the system. Modulo this ground state,
the remainder of the eigenstates and eigenvalues of the two partner
Hamiltonians form positive energy degenerate pairs. We also construct the
spatially asymptotic form of the magnetic field which produces a finite
magnetic flux and associated zero energy normalizable ground state
wavefunction.Comment: 10 pages, LaTe
What is a quantum simulator?
Quantum simulators are devices that actively use quantum effects to answer
questions about model systems and, through them, real systems. Here we expand
on this definition by answering several fundamental questions about the nature
and use of quantum simulators. Our answers address two important areas. First,
the difference between an operation termed simulation and another termed
computation. This distinction is related to the purpose of an operation, as
well as our confidence in and expectation of its accuracy. Second, the
threshold between quantum and classical simulations. Throughout, we provide a
perspective on the achievements and directions of the field of quantum
simulation.Comment: 13 pages, 2 figure
Formulation and evaluation of C-Ether fluids as lubricants useful to 260 C
Three base stocks were evaluated in bench and bearing tests to determine their suitability for use at bulk oil temperatures (BOT) from -40 C to +260 C. A polyol ester gave good bearing tests at a bulk temperature of 218 C, but only a partially successful run at 274 C. These results bracket the fluid's maximum operating temperature between these values. An extensive screening program selected lubrication additives for a C-ether (modified polyphenyl ether) base stock. One formulation lubricated a bearing for 111 hours at 274 C (BOT), but this fluid gave many deposit related problems. Other C-ether blends produced cage wear or fatigue failures. Studies of a third fluid, a C-ether/disiloxane blend, consisted of bench oxidation and lubrication tests. These showed that some additives react differently in the blend than in pure C-ethers
Entanglement consumption of instantaneous nonlocal quantum measurements
Relativistic causality has dramatic consequences on the measurability of
nonlocal variables and poses the fundamental question of whether it is
physically meaningful to speak about the value of nonlocal variables at a
particular time. Recent work has shown that by weakening the role of the
measurement in preparing eigenstates of the variable it is in fact possible to
measure all nonlocal observables instantaneously by exploiting entanglement.
However, for these measurement schemes to succeed with certainty an infinite
amount of entanglement must be distributed initially and all this entanglement
is necessarily consumed. In this work we sharpen the characterisation of
instantaneous nonlocal measurements by explicitly devising schemes in which
only a finite amount of the initially distributed entanglement is ever
utilised. This enables us to determine an upper bound to the average
consumption for the most general cases of nonlocal measurements. This includes
the tasks of state verification, where the measurement verifies if the system
is in a given state, and verification measurements of a general set of
eigenstates of an observable. Despite its finiteness the growth of entanglement
consumption is found to display an extremely unfavourable exponential of an
exponential scaling with either the number of qubits needed to contain the
Schmidt rank of the target state or total number of qubits in the system for an
operator measurement. This scaling is seen to be a consequence of the
combination of the generic exponential scaling of unitary decompositions
combined with the highly recursive structure of our scheme required to overcome
the no-signalling constraint of relativistic causality.Comment: 32 pages and 14 figures. Updated to published versio
Heat generation in aircraft tires under free rolling conditions
A method was developed for calculating the internal temperature distribution in an aircraft tire while free rolling under load. The method uses an approximate stress analysis of each point in the tire as it rolls through the contact patch, and from this stress change the mechanical work done on each volume element may be obtained and converted into a heat release rate through a knowledge of material characteristics. The tire cross-section is then considered as a body with internal heat generation, and the diffusion equation is solved numerically with appropriate boundary conditions of the wheel and runway surface. Comparison with data obtained with buried thermocouples in tires shows good agreement
Efficient generation of graph states for quantum computation
We present an entanglement generation scheme which allows arbitrary graph
states to be efficiently created in a linear quantum register via an auxiliary
entangling bus. The dynamics of the entangling bus is described by an effective
non-interacting fermionic system undergoing mirror-inversion in which qubits,
encoded as local fermionic modes, become entangled purely by Fermi statistics.
We discuss a possible implementation using two species of neutral atoms stored
in an optical lattice and find that the scheme is realistic in its requirements
even in the presence of noise.Comment: 4 pages, 3 figures, RevTex 4; v2 - Major changes and new result
A comparison of some static and dynamic mechanical properties of 18 x 5.5 and 49 x 17 type 7 aircraft tires as measured by three test facilities
The properties were measured during static, slow rolling, and high-speed tests, and comparisons were made between data as acquired on indoor drum dynamometers and on an outdoor test track. In addition, mechanical properties were also obtained from scale model tires and compared with corresponding properties from full-size tires. While the tests covered a wide range of tire properties, results seem to indicate that speed effects are not large, scale models may be used for obtaining some but not all tire properties, and that predictive equations developed in NASA TR R-64 are still useful in estimating most mechanical properties
Definition of Tire Properties Required for Landing System Analysis
The data bank constructed provided two basic advantages for the user of aircraft tire information. First, computerization of the data bank allowed mechanical property data to be stored, corrected, updated, and revised quickly and easily as more reliable tests and measurements were carried out. Secondly, the format of the book which can be printed from the computerized data bank can be easily adjusted to suit the needs of the users without the great expense normally associated with reprinting and editing books set by ordinary typography
Generation of twin Fock states via transition from a two-component Mott insulator to a superfluid
We propose the dynamical creation of twin Fock states, which exhibit
Heisenberg limited interferometric phase sensitivities, in an optical lattice.
In our scheme a two-component Mott insulator with two bosonic atoms per lattice
site is melted into a superfluid. This process transforms local correlations
between hyperfine states of atom pairs into multi-particle correlations
extending over the whole system. The melting time does not scale with the
system size which makes our scheme experimentally feasible.Comment: 4 pages, 4 figure
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