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 $|\psi >_A\otimes|\psi >_R$ 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 $\rho_{A}(t)$ 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 $|\psi >_{A\otimes R}$.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 ${Er}^{3+}$ ions that produce single photons at the telecommunication wavelength of $1.55 \mu$m.Comment: 4 pages, 4 EPS figure

Measurement-induced localization of relative degrees of freedom

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