35,975 research outputs found
Decoherence suppression by uncollapsing
We show that the qubit decoherence due to zero-temperature energy relaxation
can be almost completely suppressed by using the quantum uncollapsing
procedure. To protect a qubit state, a partial quantum measurement moves it
towards the ground state, where it is kept during the storage period, while the
second partial measurement restores the initial state. This procedure
preferentially selects the cases without energy decay events. Stronger
decoherence suppression requires smaller selection probability; a desired point
in this trade-off can be chosen by varying the measurement strength. The
experiment can be realized in a straightforward way using the superconducting
phase qubit.Comment: 4 page
Teleportation of an atomic ensemble quantum state
We propose a protocol to achieve high fidelity quantum state teleportation of
a macroscopic atomic ensemble using a pair of quantum-correlated atomic
ensembles. We show how to prepare this pair of ensembles using quasiperfect
quantum state transfer processes between light and atoms. Our protocol relies
on optical joint measurements of the atomic ensemble states and magnetic
feedback reconstruction
Quantum discord in finite XY chains
We examine the quantum discord between two spins in the exact ground state of
finite spin 1/2 arrays with anisotropic XY couplings in a transverse field B.
It is shown that in the vicinity of the factorizing field B_s, the discord
approaches a common finite non-negligible limit which is independent of the
pair separation and the coupling range. An analytic expression of this limit is
provided. The discord of a mixture of aligned pairs in two different
directions, crucial for the previous results, is analyzed in detail, including
the evaluation of coherence effects, relevant in small samples and responsible
for a parity splitting at B_s. Exact results for finite chains with first
neighbor and full range couplings and their interpretation in terms of such
mixtures are provided.Comment: 9 pages, 6 figure
Mode-Field Radius of Photonic Crystal Fibers Expressed by the V-parameter
We numerically calculate the equivalent mode-field radius of the fundamental
mode in a photonic crystal fiber (PCF) and show that this is a function of the
V-parameter only and not the relative hole size. This dependency is similar to
what is found for graded-index standard fibers and we furthermore show that the
relation for the PCF can be excellently approximated with the same general
mathematical expression. This is to our knowledge the first semi-analytical
description of the mode-field radius of a PCF.Comment: Accepted for Opt. Let
Dynamical Entanglement in Particle Scattering
This paper explores the connections between particle scattering and quantum
information theory in the context of the non-relativistic, elastic scattering
of two spin-1/2 particles. An untangled, pure, two-particle in-state is evolved
by an S-matrix that respects certain symmetries and the entanglement of the
pure out-state is measured. The analysis is phrased in terms of unitary,
irreducible representations (UIRs) of the symmetry group in question, either
the rotation group for the spin degrees of freedom or the Galilean group for
non-relativistic particles. Entanglement may occurs when multiple UIRs appear
in the direct sum decomposition of the direct product in-state, but it also
depends of the scattering phase shifts. \keywords{dynamical entanglement,
scattering, Clebsch-Gordan methods}Comment: 6 pages, submitted to Int. J. Mod. Phys. A as part of MRST 2005
conference proceeding
Simple quantum feedback of a solid-state qubit
We propose an experiment on quantum feedback control of a solid-state qubit,
which is almost within the reach of the present-day technology. Similar to the
earlier proposal, the feedback loop is used to maintain the coherent (Rabi)
oscillations in a qubit for an arbitrary long time; however, this is done in a
significantly simpler way, which requires much smaller bandwidth of the control
circuitry. The main idea is to use the quadrature components of the noisy
detector current to monitor approximately the phase of qubit oscillations.
The price for simplicity is a less-than-ideal operation: the fidelity is
limited by about 95%. The feedback loop operation can be experimentally
verified by appearance of a positive in-phase component of the detector current
relative to an external oscillating signal used for synchronization.Comment: 5 page
Modulated phases in magnetic models frustrated by long-range interactions
We study an Ising model in one dimension with short range ferromagnetic and
long range (power law) antiferromagnetic interactions. We show that the zero
temperature phase diagram in a (longitudinal) field H involves a sequence of up
and down domains whose size varies continuously with H, between -H_c and H_c
which represent the edge of the ferromagnetic up and down phases. The
implications of long range interaction in many body systems are discussed.Comment: 5 pages, 3 figure
Factorization and entanglement in general XYZ spin arrays in non-uniform transverse fields
We determine the conditions for the existence of a pair of degenerate parity
breaking separable eigenstates in general arrays of arbitrary spins connected
through couplings of arbitrary range and placed in a transverse field,
not necessarily uniform. Sufficient conditions under which they are ground
states are also provided. It is then shown that in finite chains, the
associated definite parity states, which represent the actual ground state in
the immediate vicinity of separability, can exhibit entanglement between any
two spins regardless of the coupling range or separation, with the reduced
state of any two subsystems equivalent to that of pair of qubits in an
entangled mixed state. The corresponding concurrences and negativities are
exactly determined. The same properties persist in the mixture of both definite
parity states. These effects become specially relevant in systems close to the
limit. The possibility of field induced alternating separable solutions
with controllable entanglement side limits is also discussed. Illustrative
numerical results for the negativity between the first and the
spin in an open spin chain for different values of and are as well
provided.Comment: 6 pages, figures adde
A Note on Asymptotic Freedom at High Temperatures
This short note considers, within the external field approach outlined in
hep-ph/0202026, the role of the lowest lying gluon Landau mode in QCD in the
high temperature limit. Its influence on a temperature- and field-dependent
running coupling constant is examined. The thermal imaginary part of the mode
is temperature-independent in our approach and exactly cancels the well-known
zero temperature imaginary part, thus rendering the Savvidy vacuum stable.
Combining the real part of the mode with the contributions from the higher
lying Landau modes and the vacuum contribution, a field-independent coupling
alpha_s(T) is obtained. It can be interpreted as the ordinary zero temperature
running coupling constant with average thermal momenta \approx 2pi T for
gluons and \approx pi T for quarks.Comment: 4 pages; minor changes, version to appear in Phys. Rev.
Weak measurement of quantum dot spin qubits
The theory of weak quantum measurements is developed for quantum dot spin
qubits. Building on recent experiments, we propose a control cycle to prepare,
manipulate, weakly measure, and perform quantum state tomography. This is
accomplished using a combination of the physics of electron spin resonance,
spin blockade, and Coulomb blockade, resulting in a charge transport process.
We investigate the influence of the surrounding nuclear spin environment, and
find a regime where this environment significantly simplifies the dynamics of
the weak measurement process, making this theoretical proposal realistic with
existing experimental technology. We further consider spin-echo refocusing to
combat dephasing, as well as discuss a realization of "quantum undemolition",
whereby the effects of quantum state disturbance are undone.Comment: 8 pages, 2 figure
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