Heating in Nanophotonic Traps for Cold Atoms

Abstract

Laser-cooled atoms that are trapped and optically interfaced with light in nanophotonic waveguides are a powerful platform for fundamental research in quantum optics as well as for applications in quantum communication and quantum-information processing. Ever since the first realization of such a hybrid quantum-nanophotonic system about a decade ago, heating rates of the atomic motion observed in various experimental settings have typically been exceeding those in comparable free-space optical microtraps by about three orders of magnitude. This excessive heating is a roadblock for the implementation of certain protocols and devices. Still, its origin has so far remained elusive and, at the typical atom-surface separations of less than an optical wavelength encountered in nanophotonic traps, numerous effects may potentially contribute to atom heating. Here, we theoretically describe the effect of mechanical vibrations of waveguides on guided light fields and provide a general theory of particle-phonon interaction in nanophotonic traps. We test our theory by applying it to the case of laser-cooled cesium atoms in nanofiber-based two-color optical traps. We find excellent quantitative agreement between the predicted heating rates and experimentally measured values. Our theory predicts that, in this setting, the dominant heating process stems from the optomechanical coupling of the optically trapped atoms to the continuum of thermally occupied flexural mechanical modes of the waveguide structure. Surprisingly, the effect of the high-Q mechanical resonances which have previously been observed in this system can be neglected, even if they coincide with the trap frequencies. Beyond unraveling the long-standing riddle of excessive heating in nanofiber-based atom traps, we also study the dependence of the heating rates on the relevant system parameters and find a strong R −5/2 scaling with the inverse waveguide radius. Our findings allow us to propose several strategies for minimizing the heating which also provide guidelines for the design of next-generation nanophotonic cold-atom systems. Finally, given that the predicted heating rate is proportional to the mass of the trapped particle, our findings are also highly relevant for optomechanics experiments with dielectric nanoparticles that are optically trapped close to nanophotonic waveguides.We discuss additional heating mechanisms for the motion of atoms in nanofiber-based optical dipole traps. In particular, we provide estimates for the setup described in [1] and [2], which is the setup the case study in our manuscript refers to.

A Versatile Quantum Simulator for Coupled Oscillators Using a 1D Chain of Atoms Trapped near an Optical Nanofiber

“…Luckily, to a large extent, this problem can be overcome by choosing the fiber radius to be as large as possible and by optimizing the taper at both ends of the nanofiber. In this case, one can choose special positions of the particles along the waveguide to mimimize vibration effects [49].…”

Coupling of light and mechanics in a photonic crystal waveguide

“…The possibilities for band-gap engineering for both photons and phonons ( 50 ) for application to atomic physics (e.g., for coupling mechanics to both Zeeman and hyperfine atomic states) represent an exciting frontier beyond the work reported here. One example to note is that the curvature of phonon bands can strongly enhance heating rates for atom traps ( 65 ), which might offer new possibilities for engineering better atom traps in PCWs for atomic physics.…”

Reduced volume and reflection for bright optical tweezers with radial Laguerre–Gauss beams

“…Necessarily, tight focusing of optical fields is accompanied by a longitudinal polarization component, which requires a description beyond the atomic scalar polarizability and which results in spatially dependent elliptical polarization and dephasing mechanisms for atom trapping ( 38 – 41 ). Given the local polarization vector , one can define the vector , which measures the direction and degree of ellipticity.…”