15,847 research outputs found
Longitudinal vortices in a transitioning boundary layer
Naturally occurring spanwise variations of the streamwise velocity component, characteristic of longitudinal vortices embedded in a transitioning boundary layer were explored using hot-wire anemometers. A vibrating ribbon introduced stable or unstable Tollmien-Schlichting waves into the laminar boundary layer. These damped or growing disturbances always developed a strong three dimensional pattern even though no spanwise perturbations were artificially induced. Changing the radius of the leading edge and other modifications to the flat plate, wind tunnel and boundary layer did not alter the spanwise wavelength of the vortices
Extending the memory times of trapped-ion qubits with error correction and global entangling operations
The technical demands to perform quantum error correction are considerable.
The task requires the preparation of a many-body entangled state, together with
the ability to make parity measurements over subsets of the physical qubits of
the system to detect errors. Here we propose two trapped-ion experiments to
realise error-correcting codes of variable size to protect a single encoded
qubit from dephasing errors. Novel to our schemes is the use of a global
entangling phase gate, which could be implemented in both Penning traps and
Paul traps. We make use of this entangling operation to significantly reduce
the experimental complexity of state preparation and syndrome measurements. We
also show, in our second scheme, that storage times can be increased further by
repeatedly teleporting the logical information between two codes supported by
the same ion Coulomb crystal to learn information about the locations of
errors. We estimate that a logical qubit encoded in such a crystal will
maintain high coherence for times more than an order of magnitude longer than
each physical qubit would.Comment: 18 pages, 8 figures. The authors list has changed since the first
version of this draf
Macroscopic Entanglement and Phase Transitions
This paper summarises the results of our research on macroscopic entanglement
in spin systems and free Bosonic gases. We explain how entanglement can be
observed using entanglement witnesses which are themselves constructed within
the framework of thermodynamics and thus macroscopic observables. These
thermodynamical entanglement witnesses result in bounds on macroscopic
parameters of the system, such as the temperature, the energy or the
susceptibility, below which entanglement must be present. The derived bounds
indicate a relationship between the occurrence of entanglement and the
establishment of order, possibly resulting in phase transition phenomena. We
give a short overview over the concepts developed in condensed matter physics
to capture the characteristics of phase transitions in particular in terms of
order and correlation functions. Finally we want to ask and speculate whether
entanglement could be a generalised order concept by itself, relevant in
(quantum induced) phase transitions such as BEC, and that taking this view may
help us to understand the underlying process of high-T superconductivity.Comment: 9 pages, 7 figures (color), Submitted to special OSID issue,
Proceedings of the 38th Symposium on Mathematical Physics - Quantum
Entanglement & Geometry, Torun (Poland), June 200
Influence of Correlated Hybridization on the Conductance of Molecular Transistors
We study the spin-1/2 single-channel Anderson impurity model with correlated
(occupancy dependent) hybridization for molecular transistors using the
numerical renormalization-group method. Correlated hybridization can induce
nonuniversal deviations in the normalized zero-bias conductance and, for some
parameters, modestly enhance the spin polarization of currents in applied
magnetic field. Correlated hybridization can also explain a gate-voltage
dependence to the Kondo scale similar to what has been observed in recent
experiments.Comment: 4 pages, 5 figure
A Numerical Renormalization Group approach to Green's Functions for Quantum Impurity Models
We present a novel technique for the calculation of dynamical correlation
functions of quantum impurity systems in equilibrium with Wilson's numerical
renormalization group. Our formulation is based on a complete basis set of the
Wilson chain. In contrast to all previous methods, it does not suffer from
overcounting of excitation. By construction, it always fulfills sum rules for
spectral functions. Furthermore, it accurately reproduces local thermodynamic
expectation values, such as occupancy and magnetization, obtained directly from
the numerical renormalization group calculations.Comment: 13 pages, 7 figur
Half-Filled Lowest Landau Level on a Thin Torus
We solve a model that describes an interacting electron gas in the
half-filled lowest Landau level on a thin torus, with radius of the order of
the magnetic length. The low energy sector consists of non-interacting,
one-dimensional, neutral fermions. The ground state, which is homogeneous, is
the Fermi sea obtained by filling the negative energy states and the excited
states are gapless neutral excitations out of this one-dimensional sea.
Although the limit considered is extreme, the solution has a striking
resemblance to the composite fermion description of the bulk
state--the ground state is homogeneous and the excitations are neutral and
gapless. This suggests a one-dimensional Luttinger liquid description, with
possible observable effects in transport experiments, of the bulk state where
it develops continuously from the state on a thin torus as the radius
increases.Comment: 4 pages, 1 figur
Quantum phase transitions in disordered dimerized quantum spin models and the Harris criterion
We use quantum Monte Carlo simulations to study effects of disorder on the
quantum phase transition occurring versus the ratio g=J/J' in square-lattice
dimerized S=1/2 Heisenberg antiferromagnets with intra- and inter-dimer
couplings J and J'. The dimers are either randomly distributed (as in the
classical dimer model), or come in parallel pairs with horizontal or vertical
orientation. In both cases the transition violates the Harris criterion,
according to which the correlation-length exponent should satisfy nu >= 1. We
do not detect any deviations from the three-dimensional O(3) universality class
obtaining in the absence of disorder (where nu = 0.71). We discuss special
circumstances which allow nu<1 for the type of disorder considered here.Comment: 4+ pages, 3 figure
Enhanced energy transfer to an optomechanical piston from indistinguishable photons
Thought experiments involving gases and pistons, such as Maxwell’s demon and Gibbs’ mixing, are central to our understanding of thermodynamics. Here we present a quantum thermodynamic thought experiment in which the energy transfer from two photonic gases to a piston membrane grows quadratically with the number of photons for indistinguishable gases, while linearly for distinguishable gases. This signature of Bosonic bunching may be observed in optomechanical experiments, highlighting the potential of these systems for the realization of thermodynamic thought experiments in the quantum realm
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