No abstract

The evanescent field surrounding nanoscale optical waveguides offers an efficient interface between light and mesoscopic ensembles of neutral atoms. However, the thermal motion of trapped atoms, combined with the strong radial gradients of the guided light, leads to a time-modulated coupling between atoms and the light mode, thus giving rise to additional noise and motional dephasing of collective states. Here, we present a dipole force free scheme for coupling of the radial motional states, utilizing the strong intensity gradient of the guided mode and demonstrate all-optical coupling of the cesium hyperfine ground states and motional sideband transitions. We utilize this to prolong the trap lifetime of an atomic ensemble by Raman sideband cooling of the radial motion which, to the best of our knowledge, has not been demonstrated in nano-optical structures previously. This Letter points towards full and independent control of internal and external atomic degrees of freedom using guided light modes only. Light guided by nano-optical waveguide and resonator structures propagates partly as an evanescent wave; its tight subwavelength confinement allows for strong interactions between guided light and single atoms [1][2][3][4][5] or atomic ensembles [6][7][8][9][10][11][12][13] trapped within the confined field.The inherent intensity gradients of evanescent modes are a necessity for the realization of dipole traps close to the surface of the structure. However, if the atoms are probed or manipulated also by evanescent modes, the gradients lead to detrimental effects, such as time-dependent coupling for moving atoms, additional quantum partition noise in probing atomic ensembles, and motional dephasing of collective internal quantum states. As strong gradients imply strong dipole forces for any Stark shift induced by guided light [14], a scheme for optical manipulation of the internal degrees of freedom without perturbation of the motional state is desirable.Additionally, any non-zero temperature above the motional quantum ground state potentially decreases the average interaction of atoms with the guided light mode, reducing the single atom optical depth.Previous results for addressing these challenges in the nanofiber platform [15] include microwave cooling of the azimuthal degree of freedom [16] by exploiting the state dependency of the trapping potentials for different Zeeman sub-states [17], as well as polarization gradient cooling [7].In this Letter, we present a Raman coupling scheme that allows us to drive coherent transfers on the hyperfine transition in cesium (Cs), as well as radial motional sideband transitions, while canceling all quadratic ac-Stark shifts and, thus, dipole forces. By driving Raman transitions with a single beam propagating through the waveguide, we implement a cooling protocol that relies on the gradient of the coupling strength, rather than its phase [18].A key ingredient in our experimental implementation is the Stark shift canceling the Raman coupling scheme presented in Fig. 1(a): w...

Atoms trapped in the evanescent field around a nanofiber experience strong coupling to the light guided in the fiber mode. However, due to the intrinsically strong positional dependence of the coupling, thermal motion of the ensemble limits the use of nanofiber trapped atoms for some quantum tasks. We investigate the thermal dynamics of such an ensemble by using short light pulses to make a spatially inhomogeneous population transfer between atomic states. As we monitor the wave packet of atoms created by this scheme, we find a damped oscillatory behavior which we attribute to sloshing and dispersion of the atoms. Oscillation frequencies range around 100 kHz, and motional dephasing between atoms happens on a timescale of 10 µs. Comparison to Monte Carlo simulations of an ensemble of 1000 classical particles yields reasonable agreement for simulated ensemble temperatures between 25 µK and 40 µK.

Calibrating the strength of the light-matter interaction is an important experimental task in quantum information and quantum state engineering protocols. The strength of the off-resonant light-matter interaction in multi-atom spin oscillators can be characterized by the readout rate ΓS. Here we introduce the method named Coherently Induced FAraday Rotation (CIFAR) for determining the readout rate. The method is suited for both continuous and pulsed readout of the spin oscillator, relying only on applying a known polarization modulation to the probe laser beam and detecting a known optical polarization component. Importantly, the method does not require changes to the optical and magnetic fields performing the state preparation and probing. The CIFAR signal is also independent of the probe beam photo-detection quantum efficiency, and allows direct extraction of other parameters of the interaction, such as the tensor coupling ζS, and the damping rate γS. We verify this method in the continuous wave regime, probing a strongly coupled spin oscillator prepared in a warm cesium atomic vapour.