Duke University, Physics Department and the Fitzpatrick Institute for Photonics · Durham, NC

1. Amplified Spontaneous Emission (ASE) Spontaneous Emission (SE) Superfluorescence (SF) Collectivity 1 SF Thresh 3 cm Vacuum Trapping MOT y Mirror x Cell z Cooling Magnets Probe 3 cm cold atoms Scattering enhances grating Grating enhances scattering Collective Nonlinear Optical Effects in an Ultracold Thermal Vapor Joel A. Greenberg, Daniel J. Gauthier Duke University, Physics Department and the Fitzpatrick Institute for Photonics · Durham, NC Introduction Collective Effects2 Superfluorescence Nonlinear Optics (NLO) with Cold Atoms Collective optical effects occur when the radiative properties of an atom are effected by the presence of additional atoms We observe SF light generated along the trap’s long axis in both the forward and backward directions3 • Few-photon NLO elements are critical for quantum information applications, but large atom-photon interaction strengths are needed • We obtain large nonlinear couplings in cold atoms by controlling the atoms’ internal and external states Experimental Setup • Counter-propagating pump beams • Detect emitted light in forward (F) and backward (B) directions Detector (B) Pump (B) SF light Goal: Single-photon NLO The influence of the radiators on one another can take on a continuum of values (described by a collectivity parameter). On one end, atoms radiate independently (SE) – on the other, all atoms release their energy at the same time (SF) SF light • Collective nonlinear effects allow for a drastic enhancement of the atom-photon coupling strength over single-atom effects, and may lower NLO thresholds to the single-photon limit Pump (F) Cold atoms Detector (F) SF Light Observed on Detectors Collective Emission Characteristics Forward SF Characteristics Backward Ppeak • SF light is nearly degenerate with pump frequency • Light persists until atomic density falls below threshold • F/B SF temporal correlations • ~1 photon emitted/atom • Cooperative emission produces a short, intense pulse of light • Ppeak N2 (N times larger than SE!) • Delay time (tD) before pulse occurs • Threshold density/ pump power Anisotropic MOT1 tSFtSE/N Power (mW) Power Our magneto-optical trap (MOT) uses lasers and magnetic fields to trap and cool atoms tSE t (ms) on MOT Setup: time tD F/B Pump beams MOT beams Laser timing scheme off Self-organization SF Light Trends We find good agreement with the predictions of superfluorescent collective atomic recoil lasing (CARL) theory An atom recoils when it absorbs or a emits a photon Example: Absorption Ppeak (mW) atom atom tD (ms) Trapping laser beam configuration Photo of MOT setup before after MOT Characteristics: The forces exerted on atoms by multiple light beams give rise to a global spatial organization of the atoms PF/B (mW) PF/B (mW) SF Power vs N We may be seeing a nonlinear (N2) scaling of the peak SF power with atom number SF light Ppeak (mW) Atomic density grating • Length: 3 cm, Radius: 150 mm • Optical Depth ~55 (Iout/Iin = e-OD) • Density 7x1010 atoms/cm3 • Temperature ~30 μK • 87Rb trapped on F=2F’=3 The degree of atomic organization affects the radiation field, thus producing a nonlocal atom-atom coupling. The net result is a runaway process that gives rise to the collective emission of light OD  N CCD image of trapped atoms Applications • Citations • J.A. Greenberg, M. Oria, A.M.C. Dawes, D.J. Gauthier, Opt. Express 15, 17699 (2007) • M. Malcuit, Univ. of Rochester, PhD Dissertation (1987) • J.A. Greenberg and D.J. Gauthier, OSA Opt. Photonics Cong. Tech. Digest, ISBN 978-1-55752-873-5 (2009) • New insight into free electron laser dynamics • Possible source of correlated photon pairs • Optical/Quantum memory Funding NSF AMO Grant # PHY-0855399; DARPA Slow Light Contract PO #412785-G-2