Laser-cooled cesium atoms confined with a magic-wavelength dipole trap inside a hollow-core photonic-bandgap fiber

We report loading of laser-cooled cesium atoms into a hollow-core photonic-band gap fiber and confining the atoms in the fiber's $7\text{−}\mu \mathrm{m}$-diameter core with a magic-wavelength dipole trap at $\sim 935$ nm. The use of the magic wavelength removes the ac Stark shift of the 852 nm optical transition in cesium caused by the dipole trap in the fiber core and suppresses the inhomogeneous broadening of the atomic ensemble that arises from the radial distribution of the atoms. This opens the possibility to continuously probe the atoms over timescales of a millisecond—approximately 1000 times longer than what was reported in previous works, because the dipole trap does not have to be modulated. We describe our atom-loading setup and its unique features and present spectroscopy measurements of the cesium's $D_2$ line in the continuous-wave dipole trap with up to $1.7\times {10}^4$ loaded inside the hollow-core fiber.

Figure 1

(a) The glass cell containing the MOT and the hollow-core fiber and a detail of the fiber mounting structure. (b) Schematic of the overall experimental setup. (c) Microscope image of the cross section of the hollow-core fiber. (d) Absorption image of the atomic cloud right after polarization gradient cooling. The fiber tip can be seen at the bottom. (e) Time sequence of our experimental procedure.

Figure 2

(a) Diagram of the hyperfine energy levels involved in the “bleaching” measurement of the atom number. (b) Histogram of photon counts with and without atoms inside the fiber showing counts accumulated from 30 cooling and loading cycles. (c) Measured atom numbers with the cooling laser shut off at 0 ms and 40 mW (measured right above the cell) of 935 nm fiber-coupled dipole beam on continuously. (d) Measured atom numbers (black circles) and calculated optical dipole trap depth (blue line) versus the dipole trap power.

Figure 3

(a) Transmission of weak probe light as a function of its frequency for different dipole trap wavelengths. 0 MHz marks the resonant frequency of the $|F=4\rangle \to |F^{\prime }=5\rangle$ transition. (b) Resonant transition frequencies extracted from panel (a) as a function of the dipole-trap wavelength compared with a theoretical prediction (black dashed line). (c) Observed transmission (blue circles) of the probe frequency scanned over transitions to different hyperfine levels in the excited state $5^2{P}_{3/2}$ from the ground $|F=4\rangle$ state with a fit of Eq. (2) (black solid line). (d) The time sequence for the temperature measurement inside the fiber (inset) and the atom numbers in the recaptured cloud inside the fiber as a function of the dipole-trap off time. A fit based on the model (3), shown by the black line, gives a transverse temperature of $\sim 2.3$ mK.