3,380 research outputs found
Quantum MERA Channels
Tensor networks representations of many-body quantum systems can be described
in terms of quantum channels. We focus on channels associated with the
Multi-scale Entanglement Renormalization Ansatz (MERA) tensor network that has
been recently introduced to efficiently describe critical systems. Our approach
allows us to compute the MERA correspondent to the thermodynamic limit of a
critical system introducing a transfer matrix formalism, and to relate the
system critical exponents to the convergence rates of the associated channels.Comment: 4 pages, 2 figure
Electron-electron interactions in decoupled graphene layers
Multi-layer graphene on the carbon face of silicon carbide is an intriguing
electronic system which typically consists of a stack of ten or more layers.
Rotational stacking faults in this system dramatically reduce inter-layer
coherence. In this article we report on the influence of inter-layer
interactions, which remain strong even when coherence is negligible, on the
Fermi liquid properties of charged graphene layers. We find that inter-layer
interactions increase the magnitudes of correlation energies and decrease
quasiparticle velocities, even when remote-layer carrier densities are small,
and that they lessen the influence of exchange and correlation on the
distribution of carriers across layers.Comment: 8 pages, 4 figures, submitte
Theory of integer quantum Hall polaritons in graphene
We present a theory of the cavity quantum electrodynamics of the graphene
cyclotron resonance. By employing a canonical transformation, we derive an
effective Hamiltonian for the system comprised of two neighboring Landau levels
dressed by the cavity electromagnetic field (integer quantum Hall polaritons).
This generalized Dicke Hamiltonian, which contains terms that are quadratic in
the electromagnetic field and respects gauge invariance, is then used to
calculate thermodynamic properties of the quantum Hall polariton system.
Finally, we demonstrate that the generalized Dicke description fails when the
graphene sheet is heavily doped, i.e. when the Landau level spectrum of 2D
massless Dirac fermions is approximately harmonic. In this case we `integrate
out' the Landau levels in valence band and obtain an effective Hamiltonian for
the entire stack of Landau levels in conduction band, as dressed by strong
light-matter interactions.Comment: 20 pages, 7 figure
Superconducting Fluctuation Corrections to the Thermal Current in Granular Metals
The first-order superconducting fluctuation corrections to the thermal
conductivity of a granular metal are calculated. A suppression of thermal
conductivity proportional to is observed in a region not too
close to the critical temperature . As , a saturation of the
correction is found, and its sign depends on the ratio between the barrier
transparency and the critical temperature. In both regimes, the Wiedemann-Franz
law is violated.Comment: 9 pages, 7 figures. Replaced with published version. Important
change
Robust optimal quantum gates for Josephson charge qubits
Quantum optimal control theory allows to design accurate quantum gates. We
employ it to design high-fidelity two-bit gates for Josephson charge qubits in
the presence of both leakage and noise. Our protocol considerably increases the
fidelity of the gate and, more important, it is quite robust in the disruptive
presence of 1/f noise. The improvement in the gate performances discussed in
this work (errors of the order of 10^{-3}-10^{-4} in realistic cases) allows to
cross the fault tolerance threshold.Comment: 4 pages, 4 figure
4e-condensation in a fully frustrated Josephson junction diamond chain
Fully frustrated one-dimensional diamond Josephson chains have been shown [B.
Dou\c{c}ot and J. Vidal, Phys. Rev. Lett. {\bf 88}, 227005 (2002)] to posses a
remarkable property: The superfluid phase occurs through the condensation of
pairs of Cooper pairs. By means of Monte Carlo simulations we analyze
quantitatively the Insulator to -Superfluid transition. We determine the
location of the critical point and discuss the behaviour of the phase-phase
correlators. For comparison we also present the case of a diamond chain at zero
and 1/3 frustration where the standard -condensation is observed.Comment: 5 pages, 7 figure
Dynamics of entanglement in quantum computers with imperfections
The dynamics of the pairwise entanglement in a qubit lattice in the presence
of static imperfections exhibits different regimes. We show that there is a
transition from a perturbative region, where the entanglement is stable against
imperfections, to the ergodic regime, in which a pair of qubits becomes
entangled with the rest of the lattice and the pairwise entanglement drops to
zero. The transition is almost independent of the size of the quantum computer.
We consider both the case of an initial maximally entangled and separable
state. In this last case there is a broad crossover region in which the
computer imperfections can be used to create a significant amount of pairwise
entanglement.Comment: 4 pages, 4 figure
Phase Diagram of the Bose-Hubbard Model with T_3 symmetry
In this paper we study the quantum phase transition between the insulating
and the globally coherent superfluid phases in the Bose-Hubbard model with T_3
structure, the "dice lattice". Even in the absence of any frustration the
superfluid phase is characterized by modulation of the order parameter on the
different sublattices of the T_3 structure. The zero-temperature critical point
as a function of a magnetic field shows the characteristic "butterfly" form. At
fully frustration the superfluid region is strongly suppressed. In addition,
due to the existence of the Aharonov-Bohm cages at f=1/2, we find evidence for
the existence of an intermediate insulating phase characterized by a zero
superfluid stiffness but finite compressibility. In this intermediate phase
bosons are localized due to the external frustration and the topology of the
T_3 lattice. We name this new phase the Aharonov-Bohm (AB) insulator. In the
presence of charge frustration the phase diagram acquires the typical
lobe-structure. The form and hierarchy of the Mott insulating states with
fractional fillings, is dictated by the particular topology of the T_3 lattice.
The results presented in this paper were obtained by a variety of analytical
methods: mean-field and variational techniques to approach the phase boundary
from the superconducting side, and a strongly coupled expansion appropriate for
the Mott insulating region. In addition we performed Quantum Monte Carlo
simulations of the corresponding (2+1)D XY model to corroborate the analytical
calculations with a more accurate quantitative analysis. We finally discuss
experimental realization of the T_3 lattice both with optical lattices and with
Josephson junction arrays.Comment: 16 pages, 17 figure
Continuous measurements of two qubits
We develop a theory of coherent quantum oscillations in two, in general
interacting, qubits measured continuously by a mesoscopic detector with
arbitrary non-linearity and discuss an example of SQUID magnetometer that can
operate as such a detector. Calculated spectra of the detector output show that
the detector non-linearity should lead to mixing of the oscillations of the two
qubits. For non-interacting qubits oscillating with frequencies and
, the mixing manifests itself as spectral peaks at the combination
frequencies . Additional nonlinearity introduced by the
qubit-qubit interaction shifts all the frequencies. In particular, for
identical qubits, the interaction splits coherent superposition of the
single-qubit peaks at . Quantum mechanics of the measurement
imposes limitations on the height of the spectral peaks.Comment: 14 pages, 6 figure
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