3,190 research outputs found
Weak values are universal in von Neumann measurements
We refute the widely held belief that the quantum weak value necessarily
pertains to weak measurements. To accomplish this, we use the transverse
position of a beam as the detector for the conditioned von Neumann measurement
of a system observable. For any coupling strength, any initial states, and any
choice of conditioning, the averages of the detector position and momentum are
completely described by the real parts of three generalized weak values in the
joint Hilbert space. Higher-order detector moments also have similar weak value
expansions. Using the Wigner distribution of the initial detector state, we
find compact expressions for these weak values within the reduced system
Hilbert space. As an application of the approach, we show that for any
Hermite-Gauss mode of a paraxial beam-like detector these expressions reduce to
the real and imaginary parts of a single system weak value plus an additional
weak-value-like contribution that only affects the momentum shift.Comment: 7 pages, 3 figures, includes Supplementary Materia
Stochastic wave function method for non-Markovian quantum master equations
A generalization of the stochastic wave function method to quantum master
equations which are not in Lindblad form is developed. The proposed stochastic
unravelling is based on a description of the reduced system in a doubled
Hilbert space and it is shown, that this method is capable of simulating
quantum master equations with negative transition rates. Non-Markovian effects
in the reduced systems dynamics can be treated within this approach by
employing the time-convolutionless projection operator technique. This ansatz
yields a systematic perturbative expansion of the reduced systems dynamics in
the coupling strength. Several examples such as the damped Jaynes Cummings
model and the spontaneous decay of a two-level system into a photonic band gap
are discussed. The power as well as the limitations of the method are
demonstrated.Comment: RevTex, 14 pages, 9 figures, uses multico
Avoiding dark states in open quantum systems by tailored initial correlations
We study the transport of excitations on a V-shaped network of three coupled
two-level systems that are subjected to an environment that induces incoherent
hopping between the nodes. Two of the nodes are coupled to a source while the
third node is coupled to a drain. A common feature of these networks is the
existence of a dark-state that blocks the transport to the drain. Here we
propose a means to avoid this state by a suitable choice of initial
correlations, induced by a source that is common to both coupled nodes.Comment: 5 pages, 3 figure
New method to simulate quantum interference using deterministic processes and application to event-based simulation of quantum computation
We demonstrate that networks of locally connected processing units with a
primitive learning capability exhibit behavior that is usually only attributed
to quantum systems. We describe networks that simulate single-photon
beam-splitter and Mach-Zehnder interferometer experiments on a causal,
event-by-event basis and demonstrate that the simulation results are in
excellent agreement with quantum theory. We also show that this approach can be
generalized to simulate universal quantum computers.Comment: J. Phys. Soc. Jpn. (in press) http://www.compphys.net/dl
Formulae for partial widths derived from the Lindblad equation
A method for calculating partial widths of auto-ionizing states is proposed.
It combines either a complex absorbing potential or exterior complex scaling
with the Lindblad equation. The corresponding classical rate equations are
reproduced, and the trace conservation inherent in the Lindblad equation
ensures that the partial widths sums up to the total width of the initial
auto-ionizing state
The Accuracy of Perturbative Master Equations
We consider open quantum systems with dynamics described by master equations
that have perturbative expansions in the system-environment interaction. We
show that, contrary to intuition, full-time solutions of order-2n accuracy
require an order-(2n+2) master equation. We give two examples of such
inaccuracies in the solutions to an order-2n master equation: order-2n
inaccuracies in the steady state of the system and order-2n positivity
violations, and we show how these arise in a specific example for which exact
solutions are available. This result has a wide-ranging impact on the validity
of coupling (or friction) sensitive results derived from second-order
convolutionless, Nakajima-Zwanzig, Redfield, and Born-Markov master equations.Comment: 6 pages, 0 figures; v2 updated references; v3 updated references,
extension to full-time and nonlocal regime
Local in time master equations with memory effects: Applicability and interpretation
Non-Markovian local in time master equations give a relatively simple way to
describe the dynamics of open quantum systems with memory effects. Despite
their simple form, there are still many misunderstandings related to the
physical applicability and interpretation of these equations. Here we clarify
these issues both in the case of quantum and classical master equations. We
further introduce the concept of a classical non-Markov chain signified through
negative jump rates in the chain configuration.Comment: Special issue on loss of coherence and memory effects in quantum
dynamics, J. Phys. B., to appea
Stochastic wave function approach to the calculation of multitime correlation functions of open quantum systems
Within the framework of probability distributions on projective Hilbert space
a scheme for the calculation of multitime correlation functions is developed.
The starting point is the Markovian stochastic wave function description of an
open quantum system coupled to an environment consisting of an ensemble of
harmonic oscillators in arbitrary pure or mixed states. It is shown that matrix
elements of reduced Heisenberg picture operators and general time-ordered
correlation functions can be expressed by time-symmetric expectation values of
extended operators in a doubled Hilbert space. This representation allows the
construction of a stochastic process in the doubled Hilbert space which enables
the determination of arbitrary matrix elements and correlation functions. The
numerical efficiency of the resulting stochastic simulation algorithm is
investigated and compared with an alternative Monte Carlo wave function method
proposed first by Dalibard et al. [Phys. Rev. Lett. {\bf 68}, 580 (1992)]. By
means of a standard example the suggested algorithm is shown to be more
efficient numerically and to converge faster. Finally, some specific examples
from quantum optics are presented in order to illustrate the proposed method,
such as the coupling of a system to a vacuum, a squeezed vacuum within a finite
solid angle, and a thermal mixture of coherent states.Comment: RevTex, 19 pages, 3 figures, uses multico
Dissipation in a rotating frame: master equation, effective temperature and Lamb-shift
Motivated by recent realizations of microwave-driven nonlinear resonators in
superconducting circuits, the impact of environmental degrees of freedom is
analyzed as seen from a rotating frame. A system plus reservoir model is
applied to consistently derive in the weak coupling limit the master equation
for the reduced density in the moving frame and near the first bifurcation
threshold. It turns out that additional interactions between momenta of system
and bath appear which have been omitted in previous studies. Explicit
expressions for the effective temperature and the Lamb-shift are given which
for ohmic baths are in agreement with experimental findings, while for
structured environments population inversion is predicted that may
qualitatively explain recent observations.Comment: 7 pages, 5 figure
Long-lived qubit from three spin-1/2 atoms
A system of three spin-1/2 atoms allows the construction of a
reference-frame-free (RFF) qubit in the subspace with total angular momentum
. The RFF qubit stays coherent perfectly as long as the spins of the
three atoms are affected homogeneously. The inhomogeneous evolution of the
atoms causes decoherence, but this decoherence can be suppressed efficiently by
applying a bias magnetic field of modest strength perpendicular to the plane of
the atoms. The resulting lifetime of the RFF qubit can be many days, making RFF
qubits of this kind promising candidates for quantum information storage units.
Specifically, we examine the situation of three atoms trapped
in a -laser-generated optical lattice and find that, with
conservatively estimated parameters, a stored qubit maintains a fidelity of
0.9999 for two hours.Comment: 15 pages, 9 figures; version 2 reports a much improved analysis;
version 3 contains more details about the four-atom cas
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