213 research outputs found
Full protection of superconducting qubit systems from coupling errors
Solid state qubits realized in superconducting circuits are potentially scalable. However, strong decoherence
may be transferred to the qubits by various elements of the circuits that couple individual qubits, particularly
when coupling is implemented over long distances. We propose here an encoding that provides full protection
against errors originating from these coupling elements, for a chain of superconducting qubits with a nearest
neighbor anisotropic XY-interaction. The encoding is also seen to provide partial protection against errors
deriving from general electronic noise
Nonequilibrium stabilization of charge states in double quantum dots
We analyze the decoherence of charge states in double quantum dots due to
cotunneling. The system is treated using the Bloch-Redfield generalized master
equation for the Schrieffer-Wolff transformed Hamiltonian. We show that the
decoherence, characterized through a relaxation and a dephasing time
, can be controlled through the external voltage and that the
optimum point, where these times are maximum, is not necessarily in
equilibrium. We outline the mechanism of this nonequilibrium-induced
enhancement of lifetime and coherence. We discuss the relevance of our results
for recent charge qubit experiments.Comment: 5 pages, 5 figure
Observation of a controllable PI-junction in a 3-terminal Josephson device
Recently Baselmans et al. [Nature, 397, 43 (1999)] showed that the direction
of the supercurrent in a superconductor/normal/superconductor Josephson
junction can be reversed by applying, perpendicularly to the supercurrent, a
sufficiently large control current between two normal reservoirs. The novel
behavior of their 4-terminal device (called a controllable PI-junction) arises
from the nonequilibrium electron energy distribution established in the normal
wire between the two superconductors. We have observed a similar supercurrent
reversal in a 3-terminal device, where the control current passes from a single
normal reservoir into the two superconductors. We show theoretically that this
behavior, although intuitively less obvious, arises from the same
nonequilibrium physics present in the 4-terminal device. Moreover, we argue
that the amplitude of the PI-state critical current should be at least as large
in the 3-terminal device as in a comparable 4-terminal device.Comment: 4 pages, 4 figures, to appear in Physical Review B Rapid
Communication
Suppression and enhancement of the critical current in multiterminal S/N/S mesoscopic structures
We analyse the measured critical current in a mesoscopic
4-terminal S/N/S structure. The current through the S/N interface is shown to
consist not only of the Josephson component but also a
phase-coherent part of the subgap current. The current
is determined by the both components and and depends
in a nonmonotonic way on the voltage between superconductors and normal
reservoirs reaching a maximum at . The obtained theoretical
resultas are in qualitative agreement with recent experimental data.Comment: 4 page, 3 figures. To be puplished in PRB Rapid co
Quantum Dissipative Dynamics of the Magnetic Resonance Force Microscope in the Single-Spin Detection Limit
We study a model of a magnetic resonance force microscope (MRFM) based on the
cyclic adiabatic inversion technique as a high-resolution tool to detect single
electron spins. We investigate the quantum dynamics of spin and cantilever in
the presence of coupling to an environment. To obtain the reduced dynamics of
the combined system of spin and cantilever, we use the Feynman-Vernon influence
functional and get results valid at any temperature as well as at arbitrary
system-bath coupling strength. We propose that the MRFM can be used as a
quantum measurement device, i.e., not only to detect the modulus of the spin
but also its direction
Proximity Effect and Multiple Andreev Reflections in Chaotic Josephson junctions
We study the dc-current transport in a voltage biased superconductor-chaotic
dot-superconductor junction with an induced proximity effect(PE) in the dot. It
is found that for a Thouless energy of the dot smaller than the
superconducting energy gap , the PE is manifested as peaks in the
differential conductance at voltages of order away from the even
subharmonic gap structures . These peaks are
insensitive to temperatures but are suppressed by a weak
magnetic field. The current for suppressed PE is independent of and
magnetic field and is shown to be given by the Octavio-Tinkham-Blonder-Klapwijk
theory.Comment: 4 pages, 3 figure
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