78 research outputs found
Quantum Phase in Nanoscopic Superconductors
Using the pseudospin representation and the SU(2) phase operators we
introduce a complex parameter to characterize both infinite and finite
superconducting systems. While in the bulk limit the parameter becomes
identical to the conventional order parameter, in the nanoscopic limit its
modulus reduces to the number parity effect parameter and its phase takes
discrete values. We evaluate the Josephson coupling energy and show that in
bulk superconductor it reproduces the conventional expression and in the
nanoscopic limit it leads to quantized Josephson effect. Finally, we study the
phase flow or dual Josephson effect in a superconductor with fixed number of
electrons.Comment: 11 page
Quantum phase in nanoscopic superconductors
Using the pseudospin representation and the SU(2) phase operators we introduce a complex parameter to characterize both infinite and finite superconducting systems.While in the bulk limit the parameter becomes identical to the conventional order parameter, in the nanoscopic limit its modulus reduces to the number parity effect parameter and its phase takes discrete values. We evaluate the Josephson coupling energy and show that in bulk superconductor it reproduces the conventional expression and in the nanoscopic limit it leads to quantized Josephson effect. Finally, we study the phase flow or dual Josephson effect in a superconductor with fixed number of electrons
Spin Bath Decoherence Mediated by Phonons
We introduce an exactly solvable model to study decoherence of a central spin
interacting with a spin bath where the coupling is mediated by phonons which we
assume to be in a coherent state or thermal distribution. For the coherent
state case, we find that the decoherence factor decays in a Gaussian fashion
and it becomes independent of the phonon frequencies at short times. If the
phonon energies are much larger than spin-phonon coupling or bath spins are
fully polarized, decoherence time becomes independent of the initial phonon
state. For the thermal state case, phonons play more important role in
decoherence with increasing temperature. We also discuss possible effects of
the temperature on spin bath contribution to decoherence.Comment: 8 pages, no figures, to appear in Solid State Communication
Qubit state transfer via discrete-time quantum walks
We propose a scheme for perfect transfer of an unknown qubit state via the
discrete-time quantum walk on a line or a circle. For this purpose, we
introduce an additional coin operator which is applied at the end of the walk.
This operator does not depend on the state to be transferred. We show that
perfect state transfer over an arbitrary distance can be achieved only if the
walk is driven by an identity or a flip coin operator. Other biased coin
operators and Hadamard coin allow perfect state transfer over finite distances
only. Furthermore, we show that quantum walks ending with a perfect state
transfer are periodic.Comment: 13 pages, 5 figure
Two-dimensional quantum walk under artificial magnetic field
We introduce the Peierls substitution to a two-dimensional discrete-time
quantum walk on a square lattice to examine the spreading dynamics and the
coin-position entanglement in the presence of an artificial gauge field. We use
the ratio of the magnetic flux through the unit cell to the flux quantum as a
control parameter. For a given flux ratio, we obtain faster spreading for a
small number of steps and the walker tends to be highly localized around the
origin. Moreover, the spreading of the walk can be suppressed and decreased
within a limited time interval for specific rational values of flux ratio. When
the flux ratio is an irrational number, even for a large number of steps, the
spreading exhibit diffusive behavior rather than the well-known ballistic one
as in the classical random walk and there is a significant probability of
finding the walker at the origin. We also analyze the coin-position
entanglement and show that the asymptotic behavior vanishes when the flux ratio
is different from zero and the coin-position entanglement become nearly maximal
in a periodic manner in a long time range.Comment: 7 pages, 5 figures, sections 3 and 4 revise
Optimal Local Transformations of Flip and Exchange Symmetric Entangled States
Local quantum operations relating multiqubit flip (0-1) and exchange
symmetric (FES) states, with the maximum possible probability of success, have
been determined by assuming that the states are converted via one-shot FES
transformations. It has been shown that certain entangled states are more
robust than others, in the sense that the optimum probability of converting
these robust states to the states lying in the close neighborhood of separable
ones vanish under local FES operations.Comment: 11 pages, 2 figure
Quantum tunnelling and decoherence in nanomagnets
Starting from the observation that at very low temperatures an isolated nanomagnet can be treated as an effective two-state system, we introduce a model to study quantum tunnelling and decoherence phenomena. We assume that the total spin of the magnet interacts with a spin bath. We show that depending upon the interaction strengths the molecule can exhibit tunnelling with or without decoherence. We also investigate the case where the spin-spin interaction is mediated by phonons
Application of the no-signaling principle to obtain quantum cloners for any allowed value of fidelity
Special relativity forbids superluminal influences. Using only the
no-signaling principle and an assumption about the form of the Schmidt
decomposition, we show that for "any" allowed fidelity there is a "unique"
approximate qubit cloner which can be written explicitly. We introduce the
prime cloners whose fidelities have multiplicative property and show that the
fidelity of the prime cloners for the infinite copy limit is 1/2.Comment: 8 pages, no figure
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