5,128 research outputs found
Network Synthesis of Linear Dynamical Quantum Stochastic Systems
The purpose of this paper is to develop a synthesis theory for linear
dynamical quantum stochastic systems that are encountered in linear quantum
optics and in phenomenological models of linear quantum circuits. In
particular, such a theory will enable the systematic realization of
coherent/fully quantum linear stochastic controllers for quantum control,
amongst other potential applications. We show how general linear dynamical
quantum stochastic systems can be constructed by assembling an appropriate
interconnection of one degree of freedom open quantum harmonic oscillators and,
in the quantum optics setting, discuss how such a network of oscillators can be
approximately synthesized or implemented in a systematic way from some linear
and non-linear quantum optical elements. An example is also provided to
illustrate the theory.Comment: Revised and corrected version, published in SIAM Journal on Control
and Optimization, 200
Applying matrix product operators to model systems with long-range interactions
An algorithm is presented which computes a translationally invariant matrix
product state approximation of the ground state of an infinite 1D system; it
does this by embedding sites into an approximation of the infinite
``environment'' of the chain, allowing the sites to relax, and then merging
them with the environment in order to refine the approximation. By making use
of matrix product operators, our approach is able to directly model any
long-range interaction that can be systematically approximated by a series of
decaying exponentials. We apply our techniques to compute the ground state of
the Haldane-Shastry model and present results.Comment: 7 pages, 3 figures; manuscript has been expanded and restructured in
order to improve presentation of the algorith
Extremal Quantum Correlations and Cryptographic Security
We investigate a fundamental property of device independent security in
quantum cryptography by characterizing probability distributions which are
necessarily independent of the measurement results of any eavesdropper. We show
that probability distributions that are secure in this sense are exactly the
extremal quantum probability distributions. This allows us to give a
characterization of security in algebraic terms. We apply the method to common
examples for two-party as well as multi-party setups and present a scheme for
verifying security of probability distributions with two parties, two
measurement settings, and two outcomes.Comment: 7 pages, 2 figures, revised version, accepted for publication in
Phys. Rev. Let
Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements
The production of the elements heavier than iron via slow neutron captures
(the s process) is a main feature of the contribution of asymptotic giant
branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos.
However, our understanding of the main neutron source, the 13C(alpha,n)16O
reaction, is still incomplete. It is commonly assumed that in AGB stars mixing
beyond convective borders drives the formation of 13C pockets. However, there
is no agreement on the nature of such mixing and free parameters are present.
By means of a parametric model we investigate the impact of different mixing
functions on the final s-process abundances in low-mass AGB models. Typically,
changing the shape of the mixing function or the mass extent of the region
affected by the mixing produce the same results. Variations in the relative
abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are
generally within +/-0.2 dex, similar to the observational error bars. We
conclude that other stellar uncertainties - the effect of rotation and of
overshoot into the C-O core - play a more important role than the details of
the mixing function. The exception is at low metallicity, where the Pb
abundance is significantly affected. In relation to the composition observed in
stardust SiC grains from AGB stars, the models are relatively close to the data
only when assuming the most extreme variation in the mixing profile.Comment: 17 pages, 8 figures, 6 tables, accepted for publications on Monthly
Notices of the Royal Astronomical Societ
Entanglement of indistinguishable particles in condensed matter physics
The concept of entanglement in systems where the particles are
indistinguishable has been the subject of much recent interest and controversy.
In this paper we study the notion of entanglement of particles introduced by
Wiseman and Vaccaro [Phys. Rev. Lett. 91, 097902 (2003)] in several specific
physical systems, including some that occur in condensed matter physics. The
entanglement of particles is relevant when the identical particles are
itinerant and so not distinguished by their position as in spin models. We show
that entanglement of particles can behave differently to other approaches that
have been used previously, such as entanglement of modes (occupation-number
entanglement) and the entanglement in the two-spin reduced density matrix. We
argue that the entanglement of particles is what could actually be measured in
most experimental scenarios and thus its physical significance is clear. This
suggests entanglement of particles may be useful in connecting theoretical and
experimental studies of entanglement in condensed matter systems.Comment: 13 pages, 6 figures, comments welcome, published version (minor
changes, added references
Mechanical squeezing via parametric amplification and weak measurement
Nonlinear forces allow motion of a mechanical oscillator to be squeezed below the zero-point motion. Of existing methods, mechanical parametric amplification is relatively accessible, but previously thought to be limited to 3 dB of squeezing in the steady state. We consider the effect of applying continuous weak measurement and feedback to this system. If the parametric drive is optimally detuned from resonance, correlations between the quadratures of motion allow unlimited steady-state squeezing. Compared to backaction evasion, we demonstrate that the measurement strength, temperature and efficiency requirements for quantum squeezing are significantly relaxed
A lower bound on the dimension of a quantum system given measured data
We imagine an experiment on an unknown quantum mechanical system in which the
system is prepared in various ways and a range of measurements are performed.
For each measurement M and preparation rho the experimenter can determine,
given enough time, the probability of a given outcome a: p(a|M,rho). How large
does the Hilbert space of the quantum system have to be in order to allow us to
find density matrices and measurement operators that will reproduce the given
probability distribution? In this note, we prove a simple lower bound for the
dimension of the Hilbert space. The main insight is to relate this problem to
the construction of quantum random access codes, for which interesting bounds
on Hilbert space dimension already exist. We discuss several applications of
our result to hidden variable, or ontological models, to Bell inequalities and
to properties of the smooth min-entropy.Comment: 8 pages, revtex, v2: improved presentation. To appear in Phys. Rev.
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