36,662 research outputs found
Instanton constraints and renormalization
The renormalization is investigated of one-loop quantum fluctuations around a
constrained instanton in -theory with negative coupling. It is found
that the constraint should be renormalized also. This indicates that in general
only renormalizable constraints are permitted.Comment: 29 page
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
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
Quantum Coherence and Correlations of optical radiation by atomic ensembles interacting with a two-level atom in microwave cavity
We examine quantum statistics of optical photons emitted from atomic
ensembles which are classically driven and simultaneously coupled to a
two-level atom via microwave photon exchange. Quantum statistics and
correlations are analyzed by calculating second order coherence degree, von
Neumann entropy, spin squeezing for multi-particle entanglement, as well as
genuine two and three-mode entanglement parameters for steady state and
non-equilibrium situations. Coherent transfer of population between the
radiation modes and quantum state mapping between the two-level atom and the
optical modes are discussed. A potential experimental realization of the
theoretical results in a superconducting coplanar waveguide resonator
containing diamond crystals with Nitrogen-Vacancy color centers and a
superconducting artificial two-level atom is discussed.Comment: 15 pages, 17 figures, submitted to Phys. Rev.
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
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
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
Detecting separable states via semidefinite programs
We introduce a new technique to detect separable states using semidefinite
programs. This approach provides a sufficient condition for separability of a
state that is based on the existence of a certain local linear map applied to a
known separable state. When a state is shown to be separable, a proof of this
fact is provided in the form of an explicit convex decomposition of the state
in terms of product states. All states in the interior of the set of separable
states can be detected in this way, except maybe for a set of measure zero.
Even though this technique is more suited for a numerical approach, a new
analytical criterion for separability can also be derived.Comment: 8 pages, accepted for publication in Physical Review
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