57 research outputs found
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
Quantum Logical States and Operators for Josephson-like Systems
We give a formal algebraic description of Josephson-type quantum dynamical
systems, i.e., Hamiltonian systems with a cos theta-like potential term. The
two-boson Heisenberg algebra plays for such systems the role that the h(1)
algebra does for the harmonic oscillator. A single Josephson junction is
selected as a representative of Josephson systems. We construct both logical
states (codewords) and logical (gate) operators in the superconductive regime.
The codewords are the even and odd coherent states of the two-boson algebra:
they are shift-resistant and robust, due to squeezing. The logical operators
acting on the qubit codewords are expressed in terms of operators in the
enveloping of the two-boson algebra. Such a scheme appears to be relevant for
quantum information applications.Comment: 12 pages in RevTex. In press, Journal of Physics A/Letter
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
Models of environment and T_1 relaxation in Josephson Charge Qubits
A theoretical interpretation of the recent experiments of Astafiev et. al. on
the T_1-relaxation rate in Josephson Charge Qubits is proposed. The
experimentally observed reproducible nonmonotonic dependence of T_1 on the
splitting E_J of the qubit levels suggests further specification of the
previously proposed models of the background charge noise. From our point of
view the most promising is the ``Andreev fluctuator'' model of the noise. In
this model the fluctuator is a Cooper pair that tunnels from a superconductor
and occupies a pair of localized electronic states. Within this model one can
naturally explain both the average linear T_1(E_J) dependence and the irregular
fluctuations. The role of fluctuators in the formation of strong resonant peaks
in this dependence is also discussed.Comment: 4 pages, 3 figure
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
Dissipationless dynamics of randomly coupled spins at high temperatures
We develop a technique to compute the high-frequency asymptotics of spin
correlators in weakly interacting disordered spin systems. We show that the
dynamical spin correlator decreases exponentially at high frequencies, and compute the characteristic time
of this dependence. In a typical random configuration, some fraction
of spins form strongly coupled pairs, which behave as two-level systems. Their
switching dynamics is driven by the high-frequency noise from the surrounding
spins, resulting in low-frequency noise in the magnetic susceptibility
and other physical quantities. We discuss application of these results to the
problem of susceptibility and flux noise in superconducting circuits at mK
temperatures
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
Quantum entanglement and classical communication through a depolarising channel
We analyse the role of entanglement for transmission of classical information
through a memoryless depolarising channel. Using the isotropic character of
this channel we prove analytically that the mutual information cannot be
increased by encoding classical bits into entangled states of two qubits.Comment: 6 pages, 2 figures; contribution to special issue of JMO on the
physics of quantum information; 2nd version: slight modifications and
improved presentatio
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