Optical cavity QED provides a platform with which to explore quantum many-body physics in drivendissipative systems. Single-mode cavities provide strong, infinite-range photon-mediated interactions among intracavity atoms. However, these global all-to-all couplings are limiting from the perspective of exploring quantum many-body physics beyond the mean-field approximation. The present work demonstrates that local couplings can be created using multimode cavity QED. This is established through measurements of the threshold of a superradiant, self-organization phase transition versus atomic position. Specifically, we experimentally show that the interference of near-degenerate cavity modes leads to both a strong and tunable-range interaction between Bose-Einstein condensates (BECs) trapped within the cavity. We exploit the symmetry of a confocal cavity to measure the interaction between real BECs and their virtual images without unwanted contributions arising from the merger of real BECs. Atom-atom coupling may be tuned from short range to long range. This capability paves the way toward future explorations of exotic, strongly correlated systems such as quantum liquid crystals and driven-dissipative spin glasses.

Photons can have a half-integer angular momentum.

Previous realizations of synthetic gauge fields for ultracold atoms do not allow the spatial profile of the field to evolve freely. We propose a scheme which overcomes this restriction by using the light in a multimode cavity, in conjunction with Raman coupling, to realize an artificial magnetic field which acts on a Bose-Einstein condensate of neutral atoms. We describe the evolution of such a system, and present the results of numerical simulations which show dynamical coupling between the effective field and the matter on which it acts. Crucially, the freedom of the spatial profile of the field is sufficient to realize a close analogue of the Meissner effect, where the magnetic field is expelled from the superfluid. This back-action of the atoms on the synthetic field distinguishes the Meissner-like effect described here from the Hess-Fairbank suppression of rotation in a neutral superfluid observed elsewhere.The Meissner effect [1] is the sine qua non of superconductivity [2]. As captured by the Ginzburg-Landau equations [3], the superfluid order parameter couples to the electromagnetic fields such that there is perfect diamagnetism. Physically this arises because the normal paramagnetic response of mater is completely suppressed by the phase stiffness of the superfluid, leaving only the diamagnetic current [4]. Magnetic field thus decays exponentially into the bulk [5]. Exponentially decaying fields are symptomatic of a massive field theory, and so can be seen as a direct consequence of the the Anderson-Higgs mechanism [6,7] giving the electromagnetic field a mass gap. Central to all these phenomena is that minimal coupling between the electromagnetic field and the superfluid modifies the equations of motion for both the superfluid and the electromagnetic field.The concept of "synthetic" gauge fields has attracted much attention over the last few years. In the context of ultracold atoms, realizations have included schemes based on dark states [8,9] or Raman driving [10][11][12], or inducing Peierls phases in lattice systems [13][14][15][16][17][18][19][20][21][22][23] (for a review, see [24][25][26][27]). There have also been proposals to realize gauge fields for photons, including "free space" realizations using Rydberg atoms in non-planar ring cavity geometries [28] as well as Peierls phases for photon hopping in coupled cavity arrays [29][30][31]. However, with a few exceptions, all these have involved static gauge fields-there is no feedback of the atoms (or photons) on the synthetic field. Thus, even in the pioneering demonstration of a Meissner phase of chiral currents [23], it is noted that these experiments are closer to the HessFairbank effect [32] (suppression of rotation in a neutral superfluid), and do not show expulsion of the synthetic field. In contrast, a charged superfluid acts back on the magnetic field.The synthetic field cannot be expelled in the above schemes because it is set by a fixed external laser or the system geometry. The exceptions are thus proposals where the strength of synth...

Internal conical diffraction by biaxial crystals with aligned optic axes, known as cascade conical diffraction is investigated. Formulae giving the intensity distributions for a cascade conically diffracted Gaussian beam are shown to compare well with experiment for the cases of two biaxial crystals with the same and different lengths and with the second crystal rotated with respect to the first. The effects of placing half wave-plates between crystals are also investigated.

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. 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. arXiv:1909.01912v2 [cond-mat.quant-gas]