1,926 research outputs found
Recycling of quantum information: Multiple observations of quantum clocks
How much information about the original state preparation can be extracted
from a quantum system which already has been measured? That is, how many
independent (non-communicating) observers can measure the quantum system
sequentially and give a nontrivial estimation of the original unknown state? We
investigate these questions and we show from a simple example that quantum
information is not entirely lost as a result of the measurement-induced
collapse of the quantum state, and that an infinite number of independent
observers who have no prior knowledge about the initial state can gain a
partial information about the original preparation of the quantum system.Comment: 4 page
Dissipation-assisted quantum computation in atom-cavity systems
The principal obstacle to quantum information processing with many qubits is
decoherence. One source of decoherence is spontaneous emission which causes
loss of energy and information. Inability to control system parameters with
high precision is another possible source of error. Strategies aimed at
overcoming one kind of error typically increase sensitivity to others. As a
solution we propose quantum computing with dissipation-assisted quantum gates.
These can be run relatively fast while achieving fidelities close to one. The
success rate of each gate operation can, at least in principle, be arbitrary
close to one.Comment: proceedings for the SPIE conference on Fluctuations and Noise, June
2003 in Santa Fe, 12 pages, minor change
Generating optimal states for a homodyne Bell test
Published versio
Cavity-free nondestructive detection of a single optical photon
Detecting a single photon without absorbing it is a long standing challenge
in quantum optics. All experiments demonstrating the nondestructive detection
of a photon make use of a high quality cavity. We present a cavity free scheme
for nondestructive single-photon detection. By pumping a nonlinear medium we
implement an inter-field Rabi-oscillation which leads to a ?pi phase shift on
weak probe coherent laser field in the presence of a single signal photon
without destroying the signal photon. Our cavity-free scheme operates with a
fast intrinsic time scale in comparison with similar cavity-based schemes. We
implement a full real-space multimode numerical analysis of the interacting
photonic modes and confirm the validity of our nondestructive scheme in the
multimode case.Comment: 4 figures, 5 page
On the Preparation of Pure States in Resonant Microcavities
We consider the time evolution of the radiation field (R) and a two-level
atom (A) in a resonant microcavity in terms of the Jaynes-Cummings model with
an initial general pure quantum state for the radiation field. It is then
shown, using the Cauchy-Schwarz inequality and also a Poisson resummation
technique, that {\it perfect} coherence of the atom can in general never be
achieved. The atom and the radiation field are, however, to a good
approximation in a pure state in the middle of what
has been traditionally called the ``collapse region'', independent of the
initial state of the atoms, provided that the initial pure state of the
radiation field has a photon number probability distribution which is
sufficiently peaked and phase differences that do not vary significantly around
this peak. An approximative analytic expression for the quantity
\Tr[\rho^2_{A}(t)], where is the reduced density matrix for the
atom, is derived. We also show that under quite general circumstances an
initial entangled pure state will be disentangled to the pure state .Comment: 14 pages and 3 figure
Complementarity and uncertainty relations for matter wave interferometry
We establish a rigorous quantitative connection between (i) the
interferometric duality relation for which-way information and fringe
visibility and (ii) Heisenberg's uncertainty relation for position and modular
momentum. We apply our theory to atom interferometry, wherein spontaneously
emitted photons provide which way information, and unambiguously resolve the
challenge posed by the metamaterial `perfect lens' to complementarity and to
the Heisenberg-Bohr interpretation of the Heisenberg microscope thought
experiment.Comment: nine pages, five figure
Single-qubit rotations in two-dimensional optical lattices with multiqubit addressing
Published versio
Single photons on demand from 3D photonic band-gap structures
We describe a practical implementation of a (semi-deterministic) photon gun
based on stimulated Raman adiabatic passage pumping and the strong enhancement
of the photonic density of states in a photonic band-gap material. We show that
this device allows {\em deterministic} and {\em unidirectional} production of
single photons with a high repetition rate of the order of 100kHz. We also
discuss specific 3D photonic microstructure architectures in which our model
can be realized and the feasibility of implementing such a device using
ions that produce single photons at the telecommunication
wavelength of m.Comment: 4 pages, 4 EPS figure
Measurement-induced localization of relative degrees of freedom
Published versio
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