Single atoms coupled to a near-concentric cavity

Concentric cavities can lead to strong photon-atom coupling without a need for high finesse or small physical-cavity volume. In this proof-of-principle experiment we demonstrate coupling of single Rb atoms to an 11-mm-long near-concentric cavity with a finesse $F=138\left(2\right)$. Operating the cavity $1.7\left(1\right)\mu \mathrm{m}$ shorter than the critical length, we observe an atom-cavity coupling constant $g_0=2\pi \times 5.0\left(2\right)$ MHz which exceeds the natural dipole decay rate $\gamma$ by a factor of $g_0/\gamma =1.7\left(1\right)$.

Figure 1

Optical setup. A near-resonant probe field at 780 nm impinges on the cavity to characterize the light-atom interaction. The transmitted and reflected light is coupled into single-mode fibers connected to avalanche photodetectors. The cavity length is stabilized close to the concentric length by a Pound-Drever-Hall lock to a frequency-stabilized 810-nm laser. The intracavity field at 810 nm provides also a far-off-resonant standing-wave dipole trap for the atoms. BS: beam splitter with 70% reflectivity; DM: dichroic mirror; PZT: 3D-piezo actuator stack; PD: photodiode; MOT: magneto-optical trap; $D_{1\left(2\right)}$: avalanche photodetectors.

Figure 2

Tuning the frequency of the probe field with respect to the cavity resonance, we detect (a) the reflection and (b) the transmission spectrum after mode cleaning with the single-mode fiber. Solid lines are Lorentzian fits. (c) Normalized cavity transmission as one mirror is transversally displaced. Throughout the experiment, the cavity length is actively stabilized to be resonant with the probe field.

Figure 3

(a) Typical trace of detection events at detector $D_1$ with an atomic cloud in the MOT inside the cavity. The cooling light is 10 MHz red-detuned from the natural $5S{}_{1/2}, F$=2 to $5P{}_{3/2}, F$=3 transition frequency. The sudden increase of fluorescence indicates the entering of an atom into the FORT. At 1 s, an atom is loaded into a side of the intracavity optical lattice which does not couple strongly to the cavity mode. We choose a high threshold value to select only strongly coupled atoms. (b) Lifetime of single atoms in FORT without cooling light for a time $\tau$. The solid line represents an exponential fit with a $1/e$ lifetime $t_0=230\left(30\right)$ ms.

Figure 4

Onset of the normal-mode splitting in the (a) reflection and (b) transmission spectra when an atom is trapped in the FORT. Error bars are smaller than symbol size (one standard deviation). Red solid lines are fits based on Eqs. (6) and (7). For comparison the empty cavity reflection and transmission spectra [Figs. 2 and 2] are shown in gray.