42,287 research outputs found
Thermal entanglement in the nanotubular system Na_2V_3O_7
Macroscopic entanglement witnesses have been put forward recently to reveal
nonlocal quantum correlations between individual constituents of the solid at
nonzero temperatures. Here we apply a recently proposed universal entanglement
witness, the magnetic susceptibility [New J. Phys. {\bf 7}, 258 (2005)] for the
estimation of the critical temperature in the nanotubular system below which thermal entanglement is present. As a result of an
analysis based on the experimental data for dc-magnetic susceptibility, we show
that K, which is approximately three times higher than the
critical temperature corresponding to the bipartite entanglement.Comment: 6 pages, 3 figures, REVTeX
Fast partial decoherence of a superconducting flux qubit in a spin bath
The superconducting flux qubit has two quantum states with opposite magnetic
flux. Environment of nuclear spins can find out the direction of the magnetic
flux after a decoherence time inversely proportional to the magnitude
of the flux and the square root of the number of spins. When the Hamiltonian of
the qubit drives fast coherent Rabi oscillations between the states with
opposite flux, then flux direction is flipped at a constant rate and
the decoherence time is much longer than .
However, on closer inspection decoherence actually takes place on two
timescales. The long time is a time of full decoherence but a part of
quantum coherence is lost already after the short time . This fast
partial decoherence biases coherent flux oscillations towards the initial flux
direction and it can affect performance of the superconducting devices as
qubits.Comment: 7 page
Dimension minimization of a quantum automaton
A new model of a Quantum Automaton (QA), working with qubits is proposed. The
quantum states of the automaton can be pure or mixed and are represented by
density operators. This is the appropriated approach to deal with measurements
and dechorence. The linearity of a QA and of the partial trace super-operator,
combined with the properties of invariant subspaces under unitary
transformations, are used to minimize the dimension of the automaton and,
consequently, the number of its working qubits. The results here developed are
valid wether the state set of the QA is finite or not. There are two main
results in this paper: 1) We show that the dimension reduction is possible
whenever the unitary transformations, associated to each letter of the input
alphabet, obey a set of conditions. 2) We develop an algorithm to find out the
equivalent minimal QA and prove that its complexity is polynomial in its
dimension and in the size of the input alphabet.Comment: 26 page
Mixed State Entanglement of Assistance and the Generalized Concurrence
We consider the maximum bipartite entanglement that can be distilled from a
single copy of a multipartite mixed entangled state, where we focus mostly on
-dimensional tripartite mixed states. We show that this {\em
assisted entanglement}, when measured in terms of the generalized concurrence
(named G-concurrence) is (tightly) bounded by an entanglement monotone, which
we call the G-concurrence of assistance. The G-concurrence is one of the
possible generalizations of the concurrence to higher dimensions, and for pure
bipartite states it measures the {\em geometric mean} of the Schmidt numbers.
For a large (non-trivial) class of -dimensional mixed states, we are
able to generalize Wootters formula for the concurrence into lower and upper
bounds on the G-concurrence. Moreover, we have found an explicit formula for
the G-concurrence of assistance that generalizes the expression for the
concurrence of assistance for a large class of dimensional
tripartite pure states.Comment: 7 page
Maintaining Quantum Coherence in the Presence of Noise through State Monitoring
Unsharp POVM measurements allow the estimation and tracking of quantum
wavefunctions in real-time with minimal disruption of the dynamics. Here we
demonstrate that high fidelity state monitoring, and hence quantum control, is
possible even in the presence of classical dephasing and amplitude noise, by
simulating such measurements on a two-level system undergoing Rabi
oscillations. Finite estimation fidelity is found to persist indefinitely long
after the decoherence times set by the noise fields in the absence of
measurement.Comment: 5 pages, 4 figure
An observable entanglement measure for unknown mixed quantum states
We show how an unknown mixed quantum state's entanglement can be quantified
by a suitable, local parity measurement on its two-fold copy.Comment: in press in PR
Precision measurement with an optical Josephson junction
We study a new type of Josephson device, the so-called "optical Josephson
junction" as proposed in Phys. Rev. Lett. {\bf 95}, 170402 (2005). Two
condensates are optically coupled through a waveguide by a pair of Bragg beams.
This optical Josephson junction is analogous to the usual Josephson junction of
two condensates weakly coupled via tunneling. We discuss the use of this
optical Josephson junction, for making precision measurements.Comment: 6 pages, 1 figur
Scattering fidelity in elastodynamics
The recent introduction of the concept of scattering fidelity, causes us to
revisit the experiment by Lobkis and Weaver [Phys. Rev. Lett. 90, 254302
(2003)]. There, the ``distortion'' of the coda of an acoustic signal is
measured under temperature changes. This quantity is in fact the negative
logarithm of scattering fidelity. We re-analyse their experimental data for two
samples, and we find good agreement with random matrix predictions for the
standard fidelity. Usually, one may expect such an agreement for chaotic
systems only. While the first sample, may indeed be assumed chaotic, for the
second sample, a perfect cuboid, such an agreement is more surprising. For the
first sample, the random matrix analysis yields a perturbation strength
compatible with semiclassical predictions. For the cuboid the measured
perturbation strength is much larger than expected, but with the fitted values
for this strength, the experimental data are well reproduced.Comment: 4 page
A simple formula for pooling knowledge about a quantum system
When various observers obtain information in an independent fashion about a
classical system, there is a simple rule which allows them to pool their
knowledge, and this requires only the states-of-knowledge of the respective
observers. Here we derive an equivalent quantum formula. While its realm of
applicability is necessarily more limited, it does apply to a large class of
measurements, and we show explicitly for a single qubit that it satisfies the
intuitive notions of what it means to pool knowledge about a quantum system.
This analysis also provides a physical interpretation for the trace of the
product of two density matrices.Comment: 5 pages, Revtex
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