1,282 research outputs found
Steady state entanglement in open and noisy quantum systems at high temperature
We show that quantum mechanical entanglement can prevail even in noisy open
quantum systems at high temperature and far from thermodynamical equilibrium,
despite the deteriorating effect of decoherence. The system consists of a
number N of interacting quantum particles, and it can interact and exchange
particles with some environment. The effect of decoherence is counteracted by a
simple mechanism, where system particles are randomly reset to some standard
initial state, e.g. by replacing them with particles from the environment. We
present a master equation that describes this process, which we can solve
analytically for small N. If we vary the interaction strength and the reset
against decoherence rate, we find a threshold below which the equilibrium state
is classically correlated, and above which there is a parameter region with
genuine entanglement.Comment: 5 pages, 3 figure
A spin foam model for general Lorentzian 4-geometries
We derive simplicity constraints for the quantization of general Lorentzian
4-geometries. Our method is based on the correspondence between coherent states
and classical bivectors and the minimization of associated uncertainties. For
spacelike geometries, this scheme agrees with the master constraint method of
the model by Engle, Pereira, Rovelli and Livine (EPRL). When it is applied to
general Lorentzian geometries, we obtain new constraints that include the EPRL
constraints as a special case. They imply a discrete area spectrum for both
spacelike and timelike surfaces. We use these constraints to define a spin foam
model for general Lorentzian 4-geometries.Comment: 27 pages, 1 figure; v4: published versio
OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution
We present a newly-developed, three-dimensional spatial classification
method, designed to analyze the spatial distribution of early type stars within
the 1 kpc sphere around the Sun. We propose a distribution model formed by two
intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)-
defined by their fundamental geometric parameters. Then, using a sample of
about 550 stars of spectral types earlier than B6 and luminosity classes
between III and V, with precise photometric distances of less than 1 kpc, we
estimate for some spectral groups the parameters of our model, as well as
single membership probabilities of GB and LGD stars, thus drawing a picture of
the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color.
Accepted for publication in The Astronomical Journal, 30 January 200
Quantum bath refrigeration towards absolute zero: unattainability principle challenged
A minimal model of a quantum refrigerator (QR), i.e. a periodically
phase-flipped two-level system permanently coupled to a finite-capacity bath
(cold bath) and an infinite heat dump (hot bath), is introduced and used to
investigate the cooling of the cold bath towards the absolute zero (T=0).
Remarkably, the temperature scaling of the cold-bath cooling rate reveals that
it does not vanish as T->0 for certain realistic quantized baths, e.g. phonons
in strongly disordered media (fractons) or quantized spin-waves in ferromagnets
(magnons). This result challenges Nernst's third-law formulation known as the
unattainability principle
Quantum Approach to a Derivation of the Second Law of Thermodynamics
We re-interprete the microcanonical conditions in the quantum domain as
constraints for the interaction of the "gas-subsystem" under consideration and
its environment ("container"). The time-average of a purity-measure is found to
equal the average over the respective path in Hilbert-space. We then show that
for typical (degenerate or non-degenerate) thermodynamical systems almost all
states within the allowed region of Hilbert-space have a local von
Neumann-entropy S close to the maximum and a purity P close to its minimum,
respectively. Typically thermodynamical systems should therefore obey the
second law.Comment: 4 pages. Accepted for publication in Phys. Rev. Let
Spin foams with timelike surfaces
Spin foams of 4d gravity were recently extended from complexes with purely
spacelike surfaces to complexes that also contain timelike surfaces. In this
article, we express the associated partition function in terms of vertex
amplitudes and integrals over coherent states. The coherent states are
characterized by unit 3--vectors which represent normals to surfaces and lie
either in the 2--sphere or the 2d hyperboloids. In the case of timelike
surfaces, a new type of coherent state is used and the associated completeness
relation is derived. It is also shown that the quantum simplicity constraints
can be deduced by three different methods: by weak imposition of the
constraints, by restriction of coherent state bases and by the master
constraint.Comment: 22 pages, no figures; v2: remarks on operator formalism added in
discussion; correction: the spin 1/2 irrep of the discrete series does not
appear in the Plancherel decompositio
Optimal refrigerator
We study a refrigerator model which consists of two -level systems
interacting via a pulsed external field. Each system couples to its own thermal
bath at temperatures and , respectively ().
The refrigerator functions in two steps: thermally isolated interaction between
the systems driven by the external field and isothermal relaxation back to
equilibrium. There is a complementarity between the power of heat transfer from
the cold bath and the efficiency: the latter nullifies when the former is
maximized and {\it vice versa}. A reasonable compromise is achieved by
optimizing the product of the heat-power and efficiency over the Hamiltonian of
the two system. The efficiency is then found to be bounded from below by
(an analogue of the Curzon-Ahlborn
efficiency), besides being bound from above by the Carnot efficiency
. The lower bound is reached in the
equilibrium limit . The Carnot bound is reached (for a finite
power and a finite amount of heat transferred per cycle) for . If
the above maximization is constrained by assuming homogeneous energy spectra
for both systems, the efficiency is bounded from above by and
converges to it for .Comment: 12 pages, 3 figure
Optimal Control of Quantum Dissipative Dynamics: Analytic solution for cooling the three level system
We study the problem of optimal control of dissipative quantum dynamics.
Although under most circumstances dissipation leads to an increase in entropy
(or a decrease in purity) of the system, there is an important class of
problems for which dissipation with external control can decrease the entropy
(or increase the purity) of the system. An important example is laser cooling.
In such systems, there is an interplay of the Hamiltonian part of the dynamics,
which is controllable and the dissipative part of the dynamics, which is
uncontrollable. The strategy is to control the Hamiltonian portion of the
evolution in such a way that the dissipation causes the purity of the system to
increase rather than decrease. The goal of this paper is to find the strategy
that leads to maximal purity at the final time. Under the assumption that
Hamiltonian control is complete and arbitrarily fast, we provide a general
framework by which to calculate optimal cooling strategies. These assumptions
lead to a great simplification, in which the control problem can be
reformulated in terms of the spectrum of eigenvalues of , rather than
itself. By combining this formulation with the Hamilton-Jacobi-Bellman
theorem we are able to obtain an equation for the globaly optimal cooling
strategy in terms of the spectrum of the density matrix. For the three-level
system, we provide a complete analytic solution for the optimal
cooling strategy. For this system it is found that the optimal strategy does
not exploit system coherences and is a 'greedy' strategy, in which the purity
is increased maximally at each instant.Comment: 9 pages, 3 fig
Optimal control of entanglement via quantum feedback
It has recently been shown that finding the optimal measurement on the
environment for stationary Linear Quadratic Gaussian control problems is a
semi-definite program. We apply this technique to the control of the
EPR-correlations between two bosonic modes interacting via a parametric
Hamiltonian at steady state. The optimal measurement turns out to be nonlocal
homodyne measurement -- the outputs of the two modes must be combined before
measurement. We also find the optimal local measurement and control technique.
This gives the same degree of entanglement but a higher degree of purity than
the local technique previously considered [S. Mancini, Phys. Rev. A {\bf 73},
010304(R) (2006)].Comment: 10 pages, 5 figure
Positive Quantum Brownian Evolution
Using the independent oscillator model with an arbitrary system potential, we
derive a quantum Brownian equation assuming a correlated total initial state.
Although not of Lindblad form, the equation preserves positivity of the density
operator on a restricted set of initial states
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