ANNUAL REPORT - MTA SzFKI

T. NONLINEAR AND QUANTUM OPTICS
P. Ádám, J. Asbóth # , P. Domokos, A. Gábris, J. Janszky, O. Kálmán, A. Kárpáti, Zs. Kis,
T. Kiss, M. Koniorczyk, Z. Kurucz, D. Nagy # , V. Szalay, G. Szirmai, G. Tóth, A. Vukics
Laser-induced dynamics of atoms, cavity QED. — The effect of the dipole-dipole
interaction on the far-off-resonance optical dipole trapping scheme was calculated by a
mean-field approach. The trapping laser field polarizes the atoms and the accompanying
dipole-dipole energy shift deepens the attractive potential minimum in a pancake-shaped
cloud. At high density the thermal motion cannot stabilize the gas against self-contraction
and an instability occurs. We calculated the boundary of the stable and unstable
equilibrium regions on a two-dimensional phase diagram of the atom number and the ratio
of the trap depth to the temperature. We discussed the limitations imposed by the dipoledipole
instability on the parameters needed to reach Bose-Einstein condensation in an
optical dipole trap.
We have investigated the dynamics of 1-dimensional optical lattices in the strong
collective coupling regime of light-matter interaction. Using the transfer matrix method,
we have taken scattering of the lattice light by the trapped particles into account to all
orders. For asymmetric pumping, we have found that traveling density waves arise in the
lattice, which can ultimately destabilize it, even in the overdamped limit. This shows
explicitly that in this regime the dynamics cannot be described in terms of an "optical
potential", an approach often encountered in the literature. We have discussed the
shortcomings of the potential energy approach, and shown different derivations of the
optical force, in this 1-dimensional model. We have fully characterized the traveling
density waves analytically and numerically.
Laser-illuminated atoms in an optical resonator exhibit a phase transition between the
homogenous distribution and two possible ordered configurations in the optical lattice
formed by the cavity and pump fields. At zero temperature, atom-field entanglement plays
a crucial role in the spatial reordering of the atoms from a homogeneous towards the two
ordered states, where all atoms occupy either only even or only odd lattice sites.
Concurrent with the buildup of atom-field entanglement, the homogeneous atomic cloud
evolves immediately into the superposition of the two stable patterns entangled with
opposite cavity field amplitudes. This possibility is absent in a factorized (classical)
treatment of atoms and field and should be generic for spontaneous symmetry breaking in
quantum phase transitions in optical potentials.
Quantum information, entanglement and teleportation. — We have analyzed the
realization of a quantum-walk search algorithm in a passive, linear optical network. We
have shown that deviations from directionally uniform photon losses lead to the
enhancement of the search efficiency compared to uniform loss with the same average. In
certain cases even increasing loss in some of the directions can improve search efficiency.
We have shown that while we approach the classical limit of the general search algorithm
by introducing random phase fluctuations, its utility for searching is lost. Using numerical
methods, we found that for static phase errors the averaged search efficiency displays a
damped oscillatory behaviour that asymptotically tends to a non-zero value.
The rapid development of quantum control makes it now possible to create large scale
quantum entanglement in various physical systems, e.g., in trapped ions or cold atoms.
Since full quantum tomography in these cases is not possible, for the detection of
# Ph.D student
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