192 research outputs found
Discrimination between evolution operators
Under broad conditions, evolutions due to two different Hamiltonians are
shown to lead at some moment to orthogonal states. For two spin-1/2 systems
subject to precession by different magnetic fields the achievement of
orthogonalization is demonstrated for every scenario but a special one. This
discrimination between evolutions is experimentally much simpler than
procedures proposed earlier based on either sequential or parallel application
of the unknown unitaries. A lower bound for the orthogonalization time is
proposed in terms of the properties of the two Hamiltonians.Comment: 7 pages, 2 figures, REVTe
Mixing quantum and classical mechanics and uniqueness of Planck's constant
Observables of quantum or classical mechanics form algebras called quantum or
classical Hamilton algebras respectively (Grgin E and Petersen A (1974) {\it J
Math Phys} {\bf 15} 764\cite{grginpetersen}, Sahoo D (1977) {\it Pramana} {\bf
8} 545\cite{sahoo}). We show that the tensor-product of two quantum Hamilton
algebras, each characterized by a different Planck's constant is an algebra of
the same type characterized by yet another Planck's constant. The algebraic
structure of mixed quantum and classical systems is then analyzed by taking the
limit of vanishing Planck's constant in one of the component algebras. This
approach provides new insight into failures of various formalisms dealing with
mixed quantum-classical systems. It shows that in the interacting mixed
quantum-classical description, there can be no back-reaction of the quantum
system on the classical. A natural algebraic requirement involving restriction
of the tensor product of two quantum Hamilton algebras to their components
proves that Planck's constant is unique.Comment: revised version accepted for publication in J.Phys.A:Math.Phy
The rise and fall of quantum and classical correlations in open-system dynamics
Interacting quantum systems evolving from an uncorrelated composite initial
state generically develop quantum correlations -- entanglement. As a
consequence, a local description of interacting quantum system is impossible as
a rule. A unitarily evolving (isolated) quantum system generically develops
extensive entanglement: the magnitude of the generated entanglement will
increase without bounds with the effective Hilbert space dimension of the
system. It is conceivable, that coupling of the interacting subsystems to local
dephasing environments will restrict the generation of entanglement to such
extent, that the evolving composite system may be considered as approximately
disentangled. This conjecture is addressed in the context of some common models
of a bipartite system with linear and nonlinear interactions and local coupling
to dephasing environments. Analytical and numerical results obtained imply that
the conjecture is generally false. Open dynamics of the quantum correlations is
compared to the corresponding evolution of the classical correlations and a
qualitative difference is found.Comment: 35 pages, 10 figures. Revised according to comments of the referees.
Accepted for publication in Phys. Rev.
C++QED: An object-oriented framework for wave-function simulations of cavity QED systems
We present a framework for efficiently performing Monte Carlo wave-function
simulations in cavity QED with moving particles. It relies heavily on the
object-oriented programming paradigm as realised in C++, and is extensible and
applicable for simulating open interacting quantum dynamics in general. The
user is provided with a number of ``elements'', eg pumped moving particles,
pumped lossy cavity modes, and various interactions to compose complex
interacting systems, which contain several particles moving in electromagnetic
fields of various configurations, and perform wave-function simulations on such
systems. A number of tools are provided to facilitate the implementation of new
elements.Comment: 31 pages, 8 figures, 3 table
Temporal and Spatial Dependence of Quantum Entanglement from a Field Theory Perspective
We consider the entanglement dynamics between two Unruh-DeWitt detectors at
rest separated at a distance . This simple model when analyzed properly in
quantum field theory shows many interesting facets and helps to dispel some
misunderstandings of entanglement dynamics. We find that there is spatial
dependence of quantum entanglement in the stable regime due to the phase
difference of vacuum fluctuations the two detectors experience, together with
the interference of the mutual influences from the backreaction of one detector
on the other. When two initially entangled detectors are still outside each
other's light cone, the entanglement oscillates in time with an amplitude
dependent on spatial separation . When the two detectors begin to have
causal contact, an interference pattern of the relative degree of entanglement
(compared to those at spatial infinity) develops a parametric dependence on
. The detectors separated at those with a stronger relative degree of
entanglement enjoy longer disentanglement times. In the cases with weak
coupling and large separation, the detectors always disentangle at late times.
For sufficiently small , the two detectors can have residual entanglement
even if they initially were in a separable state, while for a little
larger, there could be transient entanglement created by mutual influences.
However, we see no evidence of entanglement creation outside the light cone for
initially separable states.Comment: 21 pages, 8 figures. Minor changes. Some plots are re-expressed in
logarithmic negativity. No change in the overall result
Efficient simulation of quantum evolution using dynamical coarse-graining
A novel scheme to simulate the evolution of a restricted set of observables
of a quantum system is proposed. The set comprises the spectrum-generating
algebra of the Hamiltonian. The idea is to consider a certain open-system
evolution, which can be interpreted as a process of weak measurement of the
distinguished observables performed on the evolving system of interest. Given
that the observables are "classical" and the Hamiltonian is moderately
nonlinear, the open system dynamics displays a large time-scales separation
between the dephasing of the observables and the decoherence of the evolving
state in the basis of the generalized coherent states (GCS), associated with
the spectrum-generating algebra. The time scale separation allows the unitary
dynamics of the observables to be efficiently simulated by the open-system
dynamics on the intermediate time-scale.The simulation employs unraveling of
the corresponding master equations into pure state evolutions, governed by the
stochastic nonlinear Schroedinger equantion (sNLSE). It is proved that GCS are
globally stable solutions of the sNLSE, if the Hamilonian is linear in the
algebra elements.Comment: The version submitted to Phys. Rev. A, 28 pages, 3 figures, comments
are very welcom
An Example of the Decoherence Approach to Quantum Dissipative Chaos
Quantum chaos---the study of quantized nonintegrable Hamiltonian systems---is
an extremely well-developed and sophisticated field. By contrast, very little
work has been done in looking at quantum versions of systems which classically
exhibit {\it dissipative} chaos. Using the decoherence formalism of Gell-Mann
and Hartle, I find a quantum mechanical analog of one such system, the forced
damped Duffing oscillator. I demonstrate the classical limit of the system, and
discuss its decoherent histories. I show that using decoherent histories, one
can define not only the quantum map of an entire density operator, but can find
an analog to the Poincar\'e map of the individual trajectory. Finally, I argue
the usefulness of this model as an example of quantum dissipative chaos, as
well as of a practical application of the decoherence formalism to an
interesting problem.Comment: Standard LaTeX, 15 pages + 2 figures (uuencoded postscript), to
appear in Physics Letters A, October 199
Finite-Time Disentanglement via Spontaneous Emission
We show that under the influence of pure vacuum noise two entangled qubits
become completely disentangled in a finite time, and in a specific example we
find the time to be given by times the
usual spontaneous lifetime.Comment: revtex, 4 pages, 2 figure
Rapid state purification protocols for a Cooper pair box
We propose techniques for implementing two different rapid state purification
schemes, within the constraints present in a superconducting charge qubit
system. Both schemes use a continuous measurement of charge (z) measurements,
and seek to minimize the time required to purify the conditional state. Our
methods are designed to make the purification process relatively insensitive to
rotations about the x-axis, due to the Josephson tunnelling Hamiltonian. The
first proposed method, based on the scheme of Jacobs [Phys. Rev. A 67,
030301(R) (2003)] uses the measurement results to control bias (z) pulses so as
to rotate the Bloch vector onto the x-axis of the Bloch sphere. The second
proposed method, based on the scheme of Wiseman and Ralph [New J. Phys. 8, 90
(2006)] uses a simple feedback protocol which tightly rotates the Bloch vector
about an axis almost parallel with the measurement axis. We compare the
performance of these and other techniques by a number of different measures.Comment: 14 pages, 14 figures. v2: Revised version after referee comments.
Accepted for publication by Physical Review
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