33 research outputs found
Decoherence in a quantum harmonic oscillator monitored by a Bose-Einstein condensate
We investigate the dynamics of a quantum oscillator, whose evolution is
monitored by a Bose-Einstein condensate (BEC) trapped in a symmetric double
well potential. It is demonstrated that the oscillator may experience various
degrees of decoherence depending on the variable being measured and the state
in which the BEC is prepared. These range from a `coherent' regime in which
only the variances of the oscillator position and momentum are affected by
measurement, to a slow (power law) or rapid (Gaussian) decoherence of the mean
values themselves.Comment: 4 pages, 3 figures, lette
Many-body symbolic dynamics of a classical oscillator chain
We study a certain type of the celebrated Fermi-Pasta-Ulam particle chain,
namely the inverted FPU model, where the inter-particle potential has a form of
a quartic double well. Numerical evidence is given in support of a simple
symbolic description of dynamics (in the regime of sufficiently high potential
barrier between the wells) in terms of an (approximate) Markov process. The
corresponding transition matrix is formally identical to a ferromagnetic
Heisenberg quantum spin-1/2 chain with long range coupling, whose
diagonalization yields accurate estimates for a class of time correlation
functions of the model.Comment: 22 pages in LaTeX with 14 figures; submitted to Nonlinearity ;
corrected page offset proble
Can quantum chaos enhance stability of quantum computation?
We consider stability of a general quantum algorithm with respect to a fixed
but unknown residual interaction between qubits, and show a surprising fact,
namely that the average fidelity of quantum computation increases by decreasing
average time correlation function of the perturbing operator in sequences of
consecutive quantum gates. Our thinking is applied to the quantum Fourier
transformation where an alternative 'less regular' quantum algorithm is devised
which is qualitatively more robust against static random residual n-qubit
interaction.Comment: 4 pages, 5 eps figures (3 color
Linear dynamical entropy and free-independence for quantized maps on the torus
We study the relations between the averaged linear entropy production in
periodically measured quantum systems and ergodic properties of their classical
counterparts. Quantized linear automorphisms of the torus, both classically
chaotic and regular ones, are used as examples. Numerical calculations show
different entropy production regimes depending on the relation between the
Kolmogorov-Sinai entropy and the measurement entropy. The hypothesis of free
independence relations between the dynamics and measurement proposed to explain
the initial constant and maximal entropy production is tested numerically for
those models.Comment: 7 pages, 5 figure
Fidelity and Purity Decay in Weakly Coupled Composite Systems
We study the stability of unitary quantum dynamics of composite systems (for
example: central system + environment) with respect to weak interaction between
the two parts. Unified theoretical formalism is applied to study different
physical situations: (i) coherence of a forward evolution as measured by purity
of the reduced density matrix, (ii) stability of time evolution with respect to
small coupling between subsystems, and (iii) Loschmidt echo measuring dynamical
irreversibility. Stability has been measured either by fidelity of pure states
of a composite system, or by the so-called reduced fidelity of reduced density
matrices within a subsystem. Rigorous inequality among fidelity,
reduced-fidelity and purity is proved and a linear response theory is developed
expressing these three quantities in terms of time correlation functions of the
generator of interaction. The qualitatively different cases of regular
(integrable) or mixing (chaotic in the classical limit) dynamics in each of the
subsystems are discussed in detail. Theoretical results are demonstrated and
confirmed in a numerical example of two coupled kicked tops.Comment: 21 pages, 12 eps figure
Entangled random pure states with orthogonal symmetry: exact results
We compute analytically the density of Schmidt
eigenvalues, distributed according to a fixed-trace Wishart-Laguerre measure,
and the average R\'enyi entropy for reduced
density matrices of entangled random pure states with orthogonal symmetry
. The results are valid for arbitrary dimensions of the
corresponding Hilbert space partitions, and are in excellent agreement with
numerical simulations.Comment: 15 pages, 5 figure
Intrinsic Decoherence Dynamics in Smooth Hamiltonian Systems: Quantum-classical Correspondence
A direct classical analog of the quantum dynamics of intrinsic decoherence in
Hamiltonian systems, characterized by the time dependence of the linear entropy
of the reduced density operator, is introduced. The similarities and
differences between the classical and quantum decoherence dynamics of an
initial quantum state are exposed using both analytical and computational
results. In particular, the classicality of early-time intrinsic decoherence
dynamics is explored analytically using a second-order perturbative treatment,
and an interesting connection between decoherence rates and the stability
nature of classical trajectories is revealed in a simple approximate classical
theory of intrinsic decoherence dynamics. The results offer new insights into
decoherence, dynamics of quantum entanglement, and quantum chaos.Comment: 12 pages, 7 figures, to appear in Physical Review
Decoherence by engineered quantum baths
We introduce, and determine decoherence for, a wide class of non-trivial
quantum spin baths which embrace Ising, XY and Heisenberg universality classes
coupled to a two-level system. For the XY and Ising universality classes we
provide an exact expression for the decay of the loss of coherence beyond the
case of a central spin coupled uniformly to all the spins of the baths which
has been discussed so far in the literature. In the case of the Heisenberg spin
bath we study the decoherence by means of the time-dependent density matrix
renormalization group. We show how these baths can be engineered, by using
atoms in optical lattices.Comment: 4 pages, 4 figure
Environmental sensing and response genes in cnidaria : the chemical defensome in the sea anemone Nematostella vectensis
Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Cell Biology and Toxicology 24 (2008): 483-502, doi:10.1007/s10565-008-9107-5.The starlet sea anemone Nematostella vectensis has been recently established as a
new model system for the study of the evolution of developmental processes, as cnidaria
occupy a key evolutionary position at the base of the bilateria. Cnidaria play important
roles in estuarine and reef communities, but are exposed to many environmental stressors.
Here I describe the genetic components of a âchemical defensomeâ in the genome of
N. vectensis, and review cnidarian molecular toxicology. Gene families that defend
against chemical stressors and the transcription factors that regulate these genes have
been termed a âchemical defensome,â and include the cytochromes P450 and other
oxidases, various conjugating enyzymes, the ATP-dependent efflux transporters,
oxidative detoxification proteins, as well as various transcription factors. These genes
account for about 1% (266/27200) of the predicted genes in the sea anemone genome,
similar to the proportion observed in tunicates and humans, but lower than that observed
in sea urchins. While there are comparable numbers of stress-response genes, the stress
sensor genes appear to be reduced in N. vectensis relative to many model protostomes
and deuterostomes. Cnidarian toxicology is understudied, especially given the important
ecological roles of many cnidarian species. New genomic resources should stimulate the
study of chemical stress sensing and response mechanisms in cnidaria, and allow us to
further illuminate the evolution of chemical defense gene networks.WHOI Ocean Life Institute and NIH R01-ES01591