40 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
Microscopic origin of low frequency flux noise in Josephson circuits
We analyze the data and discuss their implications for the microscopic origin
of the low frequency flux noise in superconducting circuits. We argue that this
noise is produced by spins at the superconductor insulator boundary whose
dynamics is due to RKKY interaction. We show that this mechanism explains size
independence of the noise, different frequency dependences of the spectra
reported in large and small SQUIDs and gives the correct intensity for
realistic parameters.Comment: 4 pages, no figure
Quasiparticle poisoning and Josephson current fluctuations induced by Kondo impurities
We introduce a toy model that allows us to study the physical properties of a
spin impurity coupled to the electrons in the superconducting island. We show
that when the coupling of the spin is of the order of the superconducting gap
two almost degenerate subgap states are formed. By computing the Berry phase
that is associated with the superconducting phase rotations in this model, we
prove that these subgap states are characterized by a different charge and
demonstrate that the switching between these states has the same effect as
quasiparticle poisoning (unpoisoning) of the island. We also show that an
impurity coupled to both the island and the lead generates Josepshon current
fluctuations.Comment: 5 pages, 1 figur