279 research outputs found
Decoherence-free manipulation of photonic memories for quantum computation
We present a protocol to construct an arbitrary quantum circuit. The quantum
bits (qubits) are encoded in polarisation states of single photons. They are
stored in spatially separated dense media deposed in an optical cavity.
Specific sequences of pulses address individually the storage media to encode
the qubits and to implement a universal set of gates. The proposed protocol is
decoherence-free in the sense that spontaneous emission and cavity damping are
avoided. We discuss a coupling scheme for experimental implementation in Neon
atoms.Comment: 5 pages, 4 figures, submitted to Phys. Re
Stimulated Raman adiabatic passage into continuum
We propose a technique which produces nearly complete ionization of the
population of a discrete state coupled to a continuum by a two-photon
transition via a lossy intermediate state whose lifetime is much shorter than
the interaction duration. We show that using counterintuitively ordered pulses,
as in stimulated Raman adiabatic passage (STIRAP), wherein the pulse coupling
the intermediate state to the continuum precedes and partly overlaps the pulse
coupling the initial and intermediate states, greatly increases the ionization
signal and strongly reduces the population loss due to spontaneous emission
through the lossy state. For strong spontaneous emission from that state,
however, the ionization is never complete because the dark state required for
STIRAP does not exist. We demonstrate that this drawback can be eliminated
almost completely by creating a laser-induced continuum structure (LICS) by
embedding a third discrete state into the continuum with a third control laser.
This LICS introduces some coherence into the continuum, which enables a
STIRAP-like population transfer into the continuum. A highly accurate analytic
description is developed and numerical results are presented for Gaussian pulse
shapes
Measuring the Density Matrix by Local Addressing
We introduce a procedure to measure the density matrix of a material system.
The density matrix is addressed locally in this scheme by applying a sequence
of delayed light pulses. The procedure is based on the stimulated Raman
adiabatic passage (STIRAP) technique. It is shown that a series of population
measurements on the target state of the population transfer process yields
unambiguous information about the populations and coherences of the addressed
states, which therefore can be determined.Comment: 4 pages, 1 figur
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