105 research outputs found
Extreme sub-wavelength atom localization via coherent population trapping
We demonstrate an atom localization scheme based on monitoring of the atomic
coherences. We consider atomic transitions in a Lambda configuration where the
control field is a standing wave field. The probe field and the control field
produce coherence between the two ground states. We show that this coherence
has the same fringe pattern as produced by a Fabry-Perot interferometer and
thus measurement of the atomic coherence would localize the atom. Interestingly
enough the role of the cavity finesse is played by the ratio of the intensities
of the pump and probe. This is in fact the reason for obtaining extreme
subwavelenth localization. We suggest several methods to monitor the produced
localization.Comment: 6 pages, 5 figure
Cavity-mediated long-range interaction for fast multiqubit quantum logic operations
Interactions among qubits are essential for performing two-qubit quantum
logic operations. However, nature gives us only nearest neighbor interactions
in simple and controllable settings. Here we propose a strategy to induce
interactions among two atomic entities that are not necessarily neighbors of
each other through their common coupling with a cavity field. This facilitates
fast multiqubit quantum logic operations through a set of two-qubit operations.
The ideas presented here are applicable to various quantum computing proposals
for atom based qubits such as, trapped ions, atoms trapped in optical cavities
and optical lattices.Comment: 10 pages, 3 figure
Phase control of electromagnetically induced transparency and its applications to tunable group velocity and atom localization
We show that, by simple modifications of the usual three-level -type
scheme used for obtaining electromagnetically induced transparency (EIT), phase
dependence in the response of the atomic medium to a weak probe field can be
introduced. This gives rise to phase dependent susceptibility. By properly
controlling phase and amplitudes of the drive fields we obtain variety of
interesting effects. On one hand we obtain phase control of the group velocity
of a probe field passing through medium to the extent that continuous tuning of
the group velocity from subluminal to superluminal and back is possible. While
on the other hand, by choosing one of the drive fields to be a standing wave
field inside a cavity, we obtain sub-wavelength localization of moving atoms
passing through the cavity field.Comment: To Appear in SPIE Proceedings Volume 573
Performance of dynamical decoupling in bosonic environments and under pulse-timing fluctuations
We study the suppression of qubit dephasing through Uhrig dynamical
decoupling (UDD) in nontrivial environments modeled within the spin-boson
formalism. In particular, we address the case of (i) a qubit coupled to a
bosonic bath with power-law spectral density, and (ii) a qubit coupled to a
single harmonic oscillator that dissipates energy into a bosonic bath, which
embodies an example of a structured bath for the qubit. We then model the
influence of random time jitter in the UDD protocol by sorting
pulse-application times from Gaussian distributions centered at appropriate
values dictated by the optimal protocol. In case (i) we find that, when few
pulses are applied and a sharp cutoff is considered, longer coherence times and
robust UDD performances (against random timing errors) are achieved for a
super-Ohmic bath. On the other hand, when an exponential cutoff is considered a
super-Ohmic bath is undesirable. In case (ii) the best scenario is obtained for
an overdamped harmonic motion. Our study provides relevant information for the
implementation of optimized schemes for the protection of quantum states from
decoherence.Comment: 8 pages, 5 figure
Generation of Large Number-Path Entanglement Using Linear Optics and Feed-Forward
We show how an idealised measurement procedure can condense photons from two
modes into one, and how, by feeding forward the results of the measurement, it
is possible to generate efficiently superpositions of components for which only
one mode is populated, commonly called ``N00N states''. For the basic
procedure, sources of number states leak onto a beam splitter, and the output
ports are monitored by photodetectors. We find that detecting a fixed fraction
of the input at one output port suffices to direct the remainder to the same
port with high probability, however large the initial state. When instead
photons are detected at both ports, Schr\"{o}dinger cat states are produced. We
describe a circuit for making the components of such a state orthogonal, and
another for subsequent conversion to a N00N state. Our approach scales
exponentially better than existing proposals. Important applications include
quantum imaging and metrology
A Bootstrapping Approach for Generating Maximally Path-Entangled Photon States
We propose a bootstrapping approach to generation of maximally path-entangled
states of photons, so called ``NOON states''. Strong atom-light interaction of
cavity QED can be employed to generate NOON states with about 100 photons;
which can then be used to boost the existing experimental Kerr nonlinearities
based on quantum coherence effects to facilitate NOON generation with
arbitrarily large number of photons all within the current experimental state
of the art technology. We also offer an alternative scheme that uses an
atom-cavity dispersive interaction to obtain sufficiently high
Kerr-nonlinearity necessary for arbitrary NOON generation
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