Observation of Ultrastrong Spin-Motion Coupling for Cold Atoms in Optical Microtraps

Abstract

We realize a mechanical analogue of the Dicke model, achieved by coupling the spin of individual neutral atoms to their quantized motion in an optical trapping potential. The atomic spin states play the role of the electronic states of the atomic ensemble considered in the Dicke model, and the in-trap motional states of the atoms correspond to the states of the electromagnetic field mode. The coupling between spin and motion is induced by an inherent polarization gradient of the trapping light fields, which leads to a spatially varying vector light shift. We experimentally show that our system reaches the ultra-strong coupling regime, i.e., we obtain a coupling strength which is a significant fraction of the trap frequency. Moreover, with the help of an additional light field, we demonstrate the insitu tuning of the coupling strength. Beyond its fundamental interest, the demonstrated one-to-one mapping between the physics of optically trapped cold atoms and the Dicke model paves the way for implementing protocols and applications that exploit extreme coupling strengths.The quantum Rabi model (QRM) describes the interaction of a two-level emitter with a single quantized mode of the electromagnetic field or, more generally, of a two-level system (TLS) with a bosonic mode. Together with its extension for an ensemble of emitters, i.e., the Dicke model (DM), it constitutes a cornerstone of quantum optics [1]. The physics predicted by the QRM and the DM strongly depends on the relative values of the mode frequency, ω, and the coupling strength between the TLS and the bosonic mode, g. For weak coupling, i.e., g/ω 1, the rotating wave approximation (RWA) applies. In this case, the QRM and the DM reduce to the Jaynes-Cummings and the Tavis-Cummings models, respectively. The RWA breaks down in the ultra-strong coupling regime (USC), i.e., for g/ω 0.1. When increasing the coupling strength further, one enters the deep-strong coupling regime (DSC) [2]. For such high values of g/ω, new phenomena are expected [3][4][5][6][7]. The existence of a quantum phase transition in the thermodynamic limit adds to the richness of the DM [8-10]. Furthermore, USC and DSC may enable novel protocols for quantum communication and quantum information processing [11][12][13].

Collectively Enhanced Chiral Photon Emission from an Atomic Array near a Nanofiber

“…If the polarization of a nearby emitter is aligned with that of a guided mode, the emission will occur predominantly into this mode, travelling in either the forwards or backwards direction along the fiber. This so-called chiral coupling has been observed experimentally for circularly polarized atoms near an optical fiber of sub-wavelength thickness (herein referred to as a nanofiber) [28][29][30][31], as well as for a variety of other emitter types coupled with guided structures [32,33].…”

Heating in Nanophotonic Traps for Cold Atoms

“…Both optical and surface potentials depend on the internal state |λ of the atom because the electric polarizability of the atom is state dependent [56,92]. Moreover, the atom-light interaction can couple internal and motional states [24,28]. We focus on scenarios without coupling of internal and motional states, such that the potential operatorsV opt and V ad are block diagonal in the dressed hyperfine-structure levels:V opt +V ad = λ [V opt,λ (r) + V ad,λ (r)] |λ λ|.…”

“…A key challenge in this context is the heating of the atomic motion observed in these systems [25,26] which can reach rates of several hundred motional quanta per second -about three orders of magnitude larger than in comparable free-space optical traps. Large cooling rates realized, for example, by ultrastrong spin-motion coupling [27,28], are required to overcome the heating and prepare atoms close to their motional ground state. In essence, the observed storage times of atoms in nanophotonic traps have fallen short of expectations, both for trapped cesium [2,[5][6][7]9] and rubidium [8] atoms, ever since the first implementation of a nanofiber-based trap for laser-cooled atoms [4].…”

Non-linear optical effects in cold and hot rubidium gases

“…The size of the cavity and consequently of the complete device has since then become smaller and smaller. As examples of such atomnanophotonic systems, ultracold atoms were trapped in the evanescent field of light in a bottleneck resonator of a stretched fibre [28][29][30] and in the evanescent field of a nanocavity resonance of a 1D photonic crystal waveguide attached to the tip of a tapered fibre [26,27]. In an alternative approach, a fibre was buttcoupled to a waveguide bridge in vacuum.…”

“…Locally, the intensity of light in these nanophotonic structures can be very high and have extremely high gradients, which can be exploited to form efficient atom traps. Atoms have been trapped in the evanescent field of a nanocavity in a 1D photonic crystal on the tip of a tapered fibre [26,27], in the evanescent field of a bottleneck resonator in a stretched fibre [28][29][30] and in an alligator resonator trap in a waveguide bridge in vacuum [31][32][33]. In addition, several other traps have been proposed, including nanoplasmonic traps [34][35][36] and a single atom trap around a nanocavity in a 1D photonic crystal in a waveguide bridge, that requires only a fraction of a photon to trap an atom [37].…”