73 research outputs found
A C++ library using quantum trajectories to solve quantum master equations
Quantum trajectory methods can be used for a wide range of open quantum
systems to solve the master equation by unraveling the density operator
evolution into individual stochastic trajectories in Hilbert space. This C++
class library offers a choice of integration algorithms for three important
unravelings of the master equation. Different physical systems are modeled by
different Hamiltonians and environment operators. The program achieves
flexibility and user friendliness, without sacrificing execution speed, through
the way it represents operators and states in Hilbert space. Primary operators,
implemented in the form of simple routines acting on single degrees of freedom,
can be used to build up arbitrarily complex operators in product Hilbert spaces
with arbitrary numbers of components. Standard algebraic notation is used to
build operators and to perform arithmetic operations on operators and states.
States can be represented in a local moving basis, often leading to dramatic
savings of computing resources. The state and operator classes are very general
and can be used independently of the quantum trajectory algorithms. Only a
rudimentary knowledge of C++ is required to use this package.Comment: 17 pages standard LaTeX + 3 figures (postscript). Submitted to
Computer Physics Communications. Web site:
http://galisteo.ma.rhbnc.ac.uk/applied/QSD.htm
A Simple Algorithm for Local Conversion of Pure States
We describe an algorithm for converting one bipartite quantum state into
another using only local operations and classical communication, which is much
simpler than the original algorithm given by Nielsen [Phys. Rev. Lett. 83, 436
(1999)]. Our algorithm uses only a single measurement by one of the parties,
followed by local unitary operations which are permutations in the local
Schmidt bases.Comment: 5 pages, LaTeX, reference adde
Properties of the frequency operator do not imply the quantum probability postulate
We review the properties of the frequency operator for an infinite number of
systems and disprove claims in the literature that the quantum probability
postulate can be derived from these properties.Comment: 21 pages, no figures, REVTEX. Only change in v.2 is change the title
of Sec. IIIC so that it doesn't have a \cite command in it. v.3 incorporates
changes that will be published as an erratum in Annals of Physic
Classical predictability and coarse-grained evolution of the quantum baker's map
We investigate how classical predictability of the coarse-grained evolution
of the quantum baker's map depends on the character of the coarse-graining. Our
analysis extends earlier work by Brun and Hartle [Phys. Rev. D 60, 123503
(1999)] to the case of a chaotic map. To quantify predictability, we compare
the rate of entropy increase for a family of coarse-grainings in the decoherent
histories formalism. We find that the rate of entropy increase is dominated by
the number of scales characterising the coarse-graining.Comment: 28 pages, 1 figur
Conditions for compatibility of quantum state assignments
Suppose N parties describe the state of a quantum system by N possibly
different density operators. These N state assignments represent the beliefs of
the parties about the system. We examine conditions for determining whether the
N state assignments are compatible. We distinguish two kinds of procedures for
assessing compatibility, the first based on the compatibility of the prior
beliefs on which the N state assignments are based and the second based on the
compatibility of predictive measurement probabilities they define. The first
procedure leads to a compatibility criterion proposed by Brun, Finkelstein, and
Mermin [BFM, Phys. Rev. A 65, 032315 (2002)]. The second procedure leads to a
hierarchy of measurement-based compatibility criteria which is fundamentally
different from the corresponding classical situation. Quantum mechanically none
of the measurement-based compatibility criteria is equivalent to the BFM
criterion.Comment: REVTEX 4, 19 pages, 1 postscript figur
Preparation information and optimal decompositions for mixed quantum states
Consider a joint quantum state of a system and its environment. A measurement
on the environment induces a decomposition of the system state. Using
algorithmic information theory, we define the preparation information of a pure
or mixed state in a given decomposition. We then define an optimal
decomposition as a decomposition for which the average preparation information
is minimal. The average preparation information for an optimal decomposition
characterizes the system-environment correlations. We discuss properties and
applications of the concepts introduced above and give several examples.Comment: 13 pages, latex, 2 postscript figure
A de Finetti Representation Theorem for Quantum Process Tomography
In quantum process tomography, it is possible to express the experimenter's
prior information as a sequence of quantum operations, i.e., trace-preserving
completely positive maps. In analogy to de Finetti's concept of exchangeability
for probability distributions, we give a definition of exchangeability for
sequences of quantum operations. We then state and prove a representation
theorem for such exchangeable sequences. The theorem leads to a simple
characterization of admissible priors for quantum process tomography and solves
to a Bayesian's satisfaction the problem of an unknown quantum operation.Comment: 10 page
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