Coherence Properties of Nanofiber-Trapped Cesium Atoms

We experimentally study the ground state coherence properties of cesium atoms in a nanofiber-based two-color dipole trap, localized $\sim 200\mathrm{nm}$ away from the fiber surface. Using microwave radiation to coherently drive the clock transition, we record Ramsey fringes as well as spin echo signals and infer a reversible dephasing time of $T_2^{*}=0.6\mathrm{ms}$ and an irreversible dephasing time of $T_2^{\prime }=3.7\mathrm{ms}$. By modeling the signals, we find that, for our experimental parameters, $T_2^{*}$ and $T_2^{\prime }$ are limited by the finite initial temperature of the atomic ensemble and the heating rate, respectively. Our results represent a fundamental step towards establishing nanofiber-based traps for cold atoms as a building block in an optical fiber quantum network.

Figure 1

(a) Sketch of the experimental setup including the tapered optical fiber, the trapping, probe, and push-out laser fields, the microwave antenna, and the single-photon counter (SPCM). (b) End view of the nanofiber displaying the orientation of the plane of the quasilinear polarizations of the blue- and red-detuned trapping fields, the atoms, and the magnetic offset field.

Figure 2

(a) Rabi oscillations: probability $p_e(t_p)$ for an atom to be in state $|e⟩$ after a resonant MW pulse of length $t_p$. The data points correspond to an average over 80 experimental realizations (scaling factor $\alpha =0.015$). The solid line is the result of a global fitting routine (see the text). (b) Ramsey interferometry: data averaging and fitting procedure like in (a); the probability $p_e^{(\mathrm{Ram})}(\tau )$ is plotted as a function of the time interval $\tau$ between the $\pi /2$ pulses (scaling factor $\alpha =0.018$). (c) Vibrational state-dependent differential light shift ${\overline{\delta }}_{\mathrm{ls}}(n)$ (squares) and histogram of the phonon number probability distribution $P(n,T)$ calculated at $T=71\mu \mathrm{K}$.

Figure 3

Probability $p_e^{(\mathrm{SE})}(\tau )$ for an atom to be in $|e⟩$ after a SE sequence. The data points correspond to an average over 80 experimental realizations (scaling factor $\alpha =0.018$). The solid lines are the result of the global fitting routine. The legend in each panel indicates $t_{\mathrm{echo}}$.

Figure 4

Irreversible decay of the atomic ground state coherence. Black squares: Fitted parameters $C(t_{\mathrm{echo}})$. The error bars are obtained by the standard error propagation method. Red dots: Numerical simulation results obtained with a heating rate $\gamma \sim 3\mathrm{mK}/\mathrm{s}$ and an initial temperature $T_0=71\mu \mathrm{K}$.