64 research outputs found
Voltage-Current curves for small Josephson junction arrays
We compute the current voltage characteristic of a chain of identical
Josephson circuits characterized by a large ratio of Josephson to charging
energy that are envisioned as the implementation of topologically protected
qubits. We show that in the limit of small coupling to the environment it
exhibits a non-monotonous behavior with a maximum voltage followed by a
parametrically large region where . We argue that its
experimental measurement provides a direct probe of the amplitude of the
quantum transitions in constituting Josephson circuits and thus allows their
full characterization.Comment: 12 pages, 4 figure
Quantum two level systems and Kondo-like traps as possible sources of decoherence in superconducting qubits
We discuss the origin of decoherence in Josephson junction qubits. We find
that two level systems in the surrounding insulator cannot be the dominant
source of noise in small qubits. We argue that electron traps in the Josephson
barrier with large Coulomb repulsion would give noise that agrees both in
magnitude and in temperature dependence with experimental data.Comment: 4 pages, no figure
Microscopic model of quantum butterfly effect: out-of-time-order correlators and traveling combustion waves
We extend the Keldysh technique to enable the computation of out-of-time
order correlators. We show that the behavior of these correlators is described
by equations that display initially an exponential instability which is
followed by a linear propagation of the decoherence between two initially
identically copies of the quantum many body systems with interactions. At large
times the decoherence propagation (quantum butterfly effect) is described by a
diffusion equation with non-linear dissipation known in the theory of
combustion waves. The solution of this equation is a propagating non-linear
wave moving with constant velocity despite the diffusive character of the
underlying dynamics. Our general conclusions are illustrated by the detailed
computations for the specific models describing the electrons interacting with
bosonic degrees of freedom (phonons, two-level-systems etc.) or with each
other
Non-ergodic extended phase of the Quantum Random Energy model
The concept of non-ergodicity in quantum many body systems can be discussed
in the context of the wave functions of the many body system or as a property
of the dynamical observables, such as time-dependent spin correlators. In the
former approach the non-ergodic delocalized states is defined as the one in
which the wave functions occupy a volume that scales as a non-trivial power of
the full phase space. In this work we study the simplest spin glass model and
find that in the delocalized non-ergodic regime the spin-spin correlators decay
with the characteristic time that scales as non-trivial power of the full
Hilbert space volume. The long time limit of this correlator also scales as a
power of the full Hilbert space volume. We identify this phase with the glass
phase whilst the many body localized phase corresponds to a 'hyperglass' in
which dynamics is practically absent. We discuss the implications of these
finding to quantum information problems.Comment: Changes with respect to the first version (Dec. 2018): titles is
slightly changed, abstract is extended, discussion of previous papers on
similar subject is considerably augmente
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