196 research outputs found
Reply to "Comment on `Quantum linear Boltzmann equation with finite intercollision time' ''
Hornberger and Vacchini [Phys. Rev. A82, 036101 (2010); arxiv:0907.3018]
claim that the specific collisional momentum decoherence, pointed out in my
recent work [Phys. Rev. A80, 064104 (2009); arXiv:0905.3908], is already
described by their theory. However, I have performed a calculation whereby I
disprove the authors' claim and refute their conclusion that my recent work had
no advantage over theirs.Comment: 2p
Entanglement Sudden Death as an Indicator of Fidelity in a Four-Qubit Cluster State
I explore the entanglement evolution of a four qubit cluster state in a
dephasing environment concentrating on the phenomenon of entanglement sudden
death (ESD). Specifically, I ask whether the onset of ESD has an effect on the
utilization of this cluster state as a means of implementing a single qubit
rotation in the measurement based cluster state model of quantum computation.
To do this I compare the evolution of the entanglement to the fidelity, a
measure of how accurately the desired state (after the measurement based
operations) is achieved. I find that ESD does not cause a change of behavior or
discontinuity in the fidelity but may indicate when the fidelity of certain
states goes to .5.Comment: 8 pages, 9 figure
Efficient quantum computation of high harmonics of the Liouville density distribution
We show explicitly that high harmonics of the classical Liouville density
distribution in the chaotic regime can be obtained efficiently on a quantum
computer [1,2]. As was stated in [1], this provides information unaccessible
for classical computer simulations, and replies to the questions raised in
[3,4].Comment: revtex, 2 pages, 1 figure; related to [1] quant-ph/0101004, [2]
quant-ph/0102082, [8] quant-ph/0105149, [4] quant-ph/0110019, [3]
quant-ph/011002
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
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
Entanglement Sudden Death in a Quantum Memory
I explore entanglement dynamics in examples of quantum memories, decoherence
free subspaces (DFS) and noiseless subsystems (NS), to determine how a complete
loss of entanglement affects the ability of these techniques to protect quantum
information. Using negativity and concurrence as entanglement measures, I find
that in general there is no correlation between the complete loss of
entanglement in the system and the fidelity of the stored quantum information.
These results complement previous results in which quantum protocols not
explictly based on entanglement exhibit little correlation between ESD and the
accuracy of the given protocol.Comment: 6 pages, 3 composite figure
Superoperator Analysis of Entanglement in a Four-Qubit Cluster State
In this paper we utilize superoperator formalism to explore the entanglement
evolution of four-qubit cluster states in a number of decohering environments.
A four-qubit cluster state is a resource for the performance of an arbitrary
single logical qubit rotation via measurement based cluster state quantum
computation. We are specifically interested in the relationship between
entanglement evolution and the fidelity with which the arbitrary single logical
qubit rotation can be implemented in the presence of decoherence as this will
have important experimental ramifications. We also note the exhibition of
entanglement sudden death (ESD) and ask how severely its onset affects the
utilization of the cluster state as a means of implementing an arbitrary single
logical qubit rotation.Comment: 9 pages, 9 composite figures, presentation of results completely
rewritte
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
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