44,628 research outputs found
Teleportation with a Mixed State of Four Qubits and the Generalized Singlet Fraction
Recently, an explicit protocol for faithfully teleporting
arbitrary two-qubit states using genuine four-qubit entangled states was
presented by us [Phys. Rev. Lett. {\bf 96}, 060502 (2006)]. Here, we show that
with an arbitrary four-qubit mixed state resource is
equivalent to a generalized depolarizing bichannel with probabilities given by
the maximally entangled components of the resource. These are defined in terms
of our four-qubit entangled states. We define the generalized singlet fraction
, and illustrate its physical significance with several
examples. We argue that in order to teleport arbitrary two-qubit states with
average fidelity better than is classically possible, we have to demand that
. In addition, we conjecture that when then no entanglement can be teleported. It is shown that to determine the
usefulness of for , it is necessary to analyze .Comment: 11 page
Spin squeezing in optical lattice clocks via lattice-based QND measurements
Quantum projection noise will soon limit the best achievable precision of
optical atomic clocks based on lattice-confined neutral atoms. Squeezing the
collective atomic pseudo-spin via measurement of the clock state populations
during Ramsey interrogation suppresses the projection noise. We show here that
the lattice laser field can be used to perform ideal quantum non-demolition
measurements without clock shifts or decoherence and explore the feasibility of
such an approach in theory with the lattice field confined in a ring-resonator.
Detection of the motional sideband due to the atomic vibration in the lattice
wells can yield signal sizes a hundredfold above the projection noise limit.Comment: Substantially expanded versio
Nature vs. Nurture: Predictability in Low-Temperature Ising Dynamics
Consider a dynamical many-body system with a random initial state
subsequently evolving through stochastic dynamics. What is the relative
importance of the initial state ("nature") vs. the realization of the
stochastic dynamics ("nurture") in predicting the final state? We examined this
question for the two-dimensional Ising ferromagnet following an initial deep
quench from to . We performed Monte Carlo studies on the
overlap between "identical twins" raised in independent dynamical environments,
up to size . Our results suggest an overlap decaying with time as
with ; the same exponent holds for a
quench to low but nonzero temperature. This "heritability exponent" may equal
the persistence exponent for the 2D Ising ferromagnet, but the two differ more
generally.Comment: 5 pages, 3 figures; new version includes results for nonzero
temperatur
Teleportation and Dense Coding with Genuine Multipartite Entanglement
We present an explicit protocol for faithfully teleporting an
arbitrary two-qubit state via a genunie four-qubit entangled state. By
construction, our four-partite state is not reducible to a pair of Bell states.
Its properties are compared and contrasted with those of the four-party GHZ and
W states. We also give a dense coding scheme involving our state
as a shared resource of entanglement. Both and
indicate that our four-qubit state is a likely candidate for the genunine
four-partite analogue to a Bell state.Comment: 9 pages, 0 figur
Prospects for a mHz-linewidth laser
We propose a new light source based on having alkaline-earth atoms in an
optical lattice collectively emit photons on an ultra-narrow clock transition
into the mode of a high Q-resonator. The resultant optical radiation has an
extremely narrow linewidth in the mHz range, even smaller than that of the
clock transition itself due to collective effects. A power level of order
is possible, sufficient for phase-locking a slave optical local
oscillator. Realizing this light source has the potential to improve the
stability of the best clocks by two orders of magnitude.Comment: minor revisions + shortening; factor 2 algebra mistake correcte
Extreme non-linear response of ultra-narrow optical transitions in cavity QED for laser stabilization
We explore the potential of direct spectroscopy of ultra-narrow optical
transitions of atoms localized in an optical cavity. In contrast to
stabilization against a reference cavity, which is the approach currently used
for the most highly stabilized lasers, stabilization against an atomic
transition does not suffer from Brownian thermal noise. Spectroscopy of
ultra-narrow optical transitions in a cavity operates in a very highly
saturated regime in which non-linear effects such as bistability play an
important role. From the universal behavior of the Jaynes-Cummings model with
dissipation, we derive the fundamental limits for laser stabilization using
direct spectroscopy of ultra-narrow atomic lines. We find that with current
lattice clock experiments, laser linewidths of about 1 mHz can be achieved in
principle, and the ultimate limitations of this technique are at the 1 Hz
level.Comment: 5 pages, 4 figure
Quantum state redistribution based on a generalized decoupling
We develop a simple protocol for a one-shot version of quantum state
redistribution, which is the most general two-terminal source coding problem.
The protocol is simplified from a combination of protocols for the fully
quantum reverse Shannon and fully quantum Slepian-Wolf problems, with its
time-reversal symmetry being apparent. When the protocol is applied to the case
where the redistributed states have a tensor power structure, more natural
resource rates are obtained
Generating sequential space-filling designs using genetic algorithms and Monte Carlo methods
In this paper, the authors compare a Monte Carlo method and an optimization-based approach using genetic algorithms for sequentially generating space-filling experimental designs. It is shown that Monte Carlo methods perform better than genetic algorithms for this specific problem
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