322 research outputs found
Multiqubit Spin
It is proposed that the state space of a quantum object with a complicated
discrete spectrum can be used as a basis for multiqubit recording and
processing of information in a quantum computer. As an example, nuclear spin
3/2 is considered. The possibilities of writing and reading two quantum bits of
information, preparation of the initial state, implementation of the "rotation"
and "controlled negation" operations, which are sufficient for constructing any
algorithms, are demonstrated.Comment: 7 pages, PostScript, no figures; translation of Pis'ma Zh. Eksp.
Teor. Fiz. 70, No. 1, pp. 59-63, 10 July 1999; (Submitted 29 April 1999;
resubmitted 2 June 1999
Characterizing heralded single-photon sources with imperfect measurement devices
Any characterization of a single-photon source is not complete without
specifying its second-order degree of coherence, i.e., its function.
An accurate measurement of such coherence functions commonly requires
high-precision single-photon detectors, in whose absence, only time-averaged
measurements are possible. It is not clear, however, how the resulting
time-averaged quantities can be used to properly characterize the source. In
this paper, we investigate this issue for a heralded source of single photons
that relies on continuous-wave parametric down-conversion. By accounting for
major shortcomings of the source and the detectors--i.e., the multiple-photon
emissions of the source, the time resolution of photodetectors, and our chosen
width of coincidence window--our theory enables us to infer the true source
properties from imperfect measurements. Our theoretical results are
corroborated by an experimental demonstration using a PPKTP crystal pumped by a
blue laser, that results in a single-photon generation rate about 1.2 millions
per second per milliwatt of pump power. This work takes an important step
toward the standardization of such heralded single-photon sources.Comment: 18 pages, 9 figures; corrected Eq. (11) and the description follows
Eq. (22
The Origin of Time Asymmetry
It is argued that the observed Thermodynamic Arrow of Time must arise from
the boundary conditions of the universe. We analyse the consequences of the no
boundary proposal, the only reasonably complete set of boundary conditions that
has been put forward. We study perturbations of a Friedmann model containing a
massive scalar field but our results should be independent of the details of
the matter content. We find that gravitational wave perturbations have an
amplitude that remains in the linear regime at all times and is roughly time
symmetric about the time of maximum expansion. Thus gravitational wave
perturbations do not give rise to an Arrow of Time. However density
perturbations behave very differently. They are small at one end of the
universe's history, but grow larger and become non linear as the universe gets
larger. Contrary to an earlier claim, the density perturbations do not get
small again at the other end of the universe's history. They therefore give
rise to a Thermodynamic Arrow of Time that points in a constant direction while
the universe expands and contracts again. The Arrow of Time does not reverse at
the point of maximum expansion. One has to appeal to the Weak Anthropic
Principle to explain why we observe the Thermodynamic Arrow to agree with the
Cosmological Arrow, the direction of time in which the universe is expanding.Comment: 41 pages, DAMTP R92/2
Protecting Quantum Information Encoded in Decoherence Free States Against Exchange Errors
The exchange interaction between identical qubits in a quantum information
processor gives rise to unitary two-qubit errors. It is shown here that
decoherence free subspaces (DFSs) for collective decoherence undergo Pauli
errors under exchange, which however do not take the decoherence free states
outside of the DFS. In order to protect DFSs against these errors it is
sufficient to employ a recently proposed concatenated DFS-quantum error
correcting code scheme [D.A. Lidar, D. Bacon and K.B. Whaley, Phys. Rev. Lett.
{\bf 82}, 4556 (1999)].Comment: 7 pages, no figures. Discussion in section V.A. significantly
expanded. Several small changes. Two authors adde
Macroscopically distinct quantum superposition states as a bosonic code for amplitude damping
We show how macroscopically distinct quantum superposition states
(Schroedinger cat states) may be used as logical qubit encodings for the
correction of spontaneous emission errors. Spontaneous emission causes a bit
flip error which is easily corrected by a standard error correction circuit.
The method works arbitrarily well as the distance between the amplitudes of the
superposed coherent states increases.Comment: 4 pages, 2 postscript figures, LaTeX2e, RevTeX, minor changes, 1
reference adde
Analysis of radiatively stable entanglement in a system of two dipole-interacting three-level atoms
We explore the possibilities of creating radiatively stable entangled states
of two three-level dipole-interacting atoms in a configuration by
means of laser biharmonic continuous driving or pulses. We propose three
schemes for generation of entangled states which involve only the lower states
of the system, not vulnerable to radiative decay. Two of them employ
coherent dynamics to achieve entanglement in the system, whereas the third one
uses optical pumping, i.e., an essentially incoherent process.Comment: Replaced with the final version; 14 pages, 6 figures; to appear in
Phys. Rev. A, vol. 61 (2000
The Generalized Hartle-Hawking Initial State: Quantum Field Theory on Einstein Conifolds
Recent arguments have indicated that the sum over histories formulation of
quantum amplitudes for gravity should include sums over conifolds, a set of
histories with more general topology than that of manifolds. This paper
addresses the consequences of conifold histories in gravitational functional
integrals that also include scalar fields. This study will be carried out
explicitly for the generalized Hartle-Hawking initial state, that is the
Hartle-Hawking initial state generalized to a sum over conifolds. In the
perturbative limit of the semiclassical approximation to the generalized
Hartle-Hawking state, one finds that quantum field theory on Einstein conifolds
is recovered. In particular, the quantum field theory of a scalar field on de
Sitter spacetime with spatial topology is derived from the generalized
Hartle-Hawking initial state in this approximation. This derivation is carried
out for a scalar field of arbitrary mass and scalar curvature coupling.
Additionally, the generalized Hartle-Hawking boundary condition produces a
state that is not identical to but corresponds to the Bunch-Davies vacuum on
de Sitter spacetime. This result cannot be obtained from the original
Hartle-Hawking state formulated as a sum over manifolds as there is no Einstein
manifold with round boundary.Comment: Revtex 3, 31 pages, 4 epsf figure
Nontrivial Dynamics in the Early Stages of Inflation
Inflationary cosmologies, regarded as dynamical systems, have rather simple
asymptotic behavior, insofar as the cosmic baldness principle holds.
Nevertheless, in the early stages of an inflationary process, the dynamical
behavior may be very complex. In this paper, we show how even a simple
inflationary scenario, based on Linde's ``chaotic inflation'' proposal,
manifests nontrivial dynamical effects such as the breakup of invariant tori,
formation of cantori and Arnol'd's diffusion. The relevance of such effects is
highlighted by the fact that even the occurrence or not of inflation in a given
Universe is dependent upon them.Comment: 26 pages, Latex, 9 Figures available on request, GTCRG-94-1
Thermodynamics of entanglement in Schwarzschild spacetime
Extending the analysis in our previous paper, we construct the entanglement
thermodynamics for a massless scalar field on the Schwarzschild spacetime.
Contrary to the flat case, the entanglement energy turns out to be
proportional to area radius of the boundary if it is near the horizon. This
peculiar behavior of can be understood by the red-shift effect caused
by the curved background. Combined with the behavior of the entanglement
entropy, this result yields, quite surprisingly, the entanglement
thermodynamics of the same structure as the black hole thermodynamics. On the
basis of these results, we discuss the relevance of the concept of entanglement
as the microscopic origin of the black hole thermodynamics.Comment: 27 pages, Latex file, 7 figures; revised to clarify our choice of the
state and to add references. Accepted for publication in Physical Review
Chaos in Quantum Cosmology
Much of the foundational work on quantum cosmology employs a simple
minisuperspace model describing a Friedmann-Robertson-Walker universe
containing a massive scalar field. We show that the classical limit of this
model exhibits deterministic chaos and explore some of the consequences for the
quantum theory. In particular, the breakdown of the WKB approximation calls
into question many of the standard results in quantum cosmology.Comment: 4 pages, 4 figures, RevTex two column style. Minor revisions and
clarifications to reflect version published in Phys. Rev. Let
- …