119 research outputs found
Light propagation beyond the mean-field theory of standard optics
With ready access to massive computer clusters we may now study light
propagation in a dense cold atomic gas by means of basically exact numerical
simulations. We report on a direct comparison between traditional optics, that
is, electrodynamics of a polarizable medium, and numerical simulations in an
elementary problem of light propagating through a slab of matter. The standard
optics fails already at quite low atom densities, and the failure becomes
dramatic when the average interatomic separation is reduced to around ,
where is the wave number of resonant light. The difference between the two
solutions originates from correlations between the atoms induced by
light-mediated dipole-dipole interactions
Theoretical formalism for collective electromagnetic response of discrete metamaterial systems
We develop a general formalism to describe the propagation of a near-resonant
electromagnetic field in a medium composed of magnetodielectric resonators. As
the size and the spatial separation of nanofabricated resonators in a
metamaterial array is frequently less than the wavelength, we describe them as
discrete scatterers, supporting a single mode of current oscillation
represented by a single dynamic variable. We derive a Lagrangian and
Hamiltonian formalism for the coupled electromagnetic fields and oscillating
currents in the length gauge, obtained by the Power-Zienau-Woolley
transformation. The response of each resonator to electromagnetic field is then
described by polarization and magnetization densities that, to the lowest order
in a multipole expansion, generate electric and magnetic dipole excitations. We
derive a closed set of equations for the coherently scattered field and normal
mode amplitudes of current oscillations of each resonator both within the
rotating wave approximation, in which case the radiative decay rate is much
smaller than the resonance frequency, and without such an assumption. The set
of equations includes the radiative couplings between a discrete set of
resonators mediated by the electromagnetic field, fully incorporating recurrent
scattering processes to all orders. By considering an example of a
two-dimensional split ring resonator metamaterial array, we show that the
system responds cooperatively to near-resonant field, exhibiting collective
eigenmodes, resonance frequencies, and radiative linewidths that result from
strong radiative interactions between closely-spaced resonators.Comment: 34 pages, 6 figure
Classical stochastic measurement trajectories: Bosonic atomic gases in an optical cavity and quantum measurement backaction
We formulate computationally efficient classical stochastic measurement
trajectories for a multimode quantum system under continuous observation.
Specifically, we consider the nonlinear dynamics of an atomic Bose-Einstein
condensate contained within an optical cavity subject to continuous monitoring
of the light leaking out of the cavity. The classical trajectories encode
within a classical phase-space representation a continuous quantum measurement
process conditioned on a given detection record. We derive a Fokker-Planck
equation for the quasi-probability distribution of the combined
condensate-cavity system. We unravel the dynamics into stochastic classical
trajectories that are conditioned on the quantum measurement process of the
continuously monitored system, and that these trajectories faithfully represent
measurement records of individual experimental runs. Since the dynamics of a
continuously measured observable in a many-atom system can be closely
approximated by classical dynamics, the method provides a numerically efficient
and accurate approach to calculate the measurement record of a large multimode
quantum system. Numerical simulations of the continuously monitored dynamics of
a large atom cloud reveal considerably fluctuating phase profiles between
different measurement trajectories, while ensemble averages exhibit local
spatially varying phase decoherence. Individual measurement trajectories lead
to spatial pattern formation and optomechanical motion that solely result from
the measurement backaction. The backaction of the continuous quantum
measurement process, conditioned on the detection record of the photons,
spontaneously breaks the symmetry of the spatial profile of the condensate and
can be tailored to selectively excite collective modes.Comment: 22 pages, 11 figure
Energetically stable singular vortex cores in an atomic spin-1 Bose-Einstein condensate
We analyze the structure and stability of singular singly quantized vortices in a rotating spin-1 Bose-Einstein condensate. We show that the singular vortex can be energetically stable in both the ferromagnetic and polar phases despite the existence of a lower-energy nonsingular coreless vortex in the ferromagnetic phase. The spin-1 system exhibits energetic hierarchy of length scales resulting from different interaction strengths and we find that the vortex cores deform to a larger size determined by the characteristic length scale of the spin-dependent interaction. We show that in the ferromagnetic phase the resulting stable core structure, despite apparent complexity, can be identified as a single polar core with everywhere nonvanishing axially symmetric density profile. In the polar phase, the energetically favored core deformation leads to a splitting of a singly quantized vortex into a pair of half-quantum vortices that preserves the topology of the vortex outside the extended core region, but breaks the axial symmetry of the core. The resulting half-quantum vortices exhibit nonvanishing ferromagnetic cores.<br/
Radiative Toroidal Dipole and Anapole Excitations in Collectively Responding Arrays of Atoms
A toroidal dipole represents an often overlooked electromagnetic excitation distinct from the standard electric and magnetic multipole expansion. We show how a simple arrangement of strongly radiatively coupled atoms can be used to synthesize a toroidal dipole where the toroidal topology is generated by radiative transitions forming an effective poloidal electric current wound around a torus. We extend the protocol for methods to prepare a delocalized collective excitation mode consisting of a synthetic lattice of such toroidal dipoles and a nonradiating, yet oscillating charge-current configuration, dynamic anapole, for which the far-field radiation of a toroidal dipole is identically canceled by an electric dipole
Spontaneous photon emission stimulated by two Bose condensates
We show that the phase difference of two overlapping ground state
Bose-Einstein condensates can effect the optical spontaneous emission rate of
excited atoms. Depending on the phase difference the atom stimulated
spontaneous emission rate can vary between zero and the rate corresponding to
all the ground state atoms in a single condensate. Besides giving control over
spontaneous emission this provides an optical method for detecting the
condensate phase difference. It differs from previous methods in that no light
fields are applied. Instead the light is spontaneously emitted when excited
atoms make a transition into either condensate.Comment: 14 pages, 2 postscript figures, Revtex. Corrections and significant
additions in revisio
Subradiance-protected excitation spreading in the generation of collimated photon emission from an atomic array
We show how an initial localized radiative excitation in a two-dimensional array of cold atoms can be converted into highly directional coherent emission of light by protecting the spreading of the excitation across the array in a subradiant collective eigenmode with a lifetime orders of magnitude longer than that of an isolated atom. We demonstrate how to reach two such strongly subradiant modes, a uniform one where all the dipoles are oscillating in phase normal to the plane and an antiferromagnetic mode where each dipole is Ο out of phase with its nearest neighbor. The excitation, which can consist of a single photon, is then released from the protected subradiant eigenmode by controlling the Zeeman level shifts of the atoms. Hence, an original localized excitation which emits in all directions is transferred to a delocalized subradiance-protected excitation, with a probabilistic emission of a photon only along the axis perpendicular to the plane of the atoms. This protected spreading and directional emission could potentially be used to link stages in a quantum information or quantum computing architecture
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