Single Ion Coupled to an Optical Fiber Cavity

We present the realization of a combined trapped-ion and optical cavity system, in which a single ${\mathrm{Yb}}^{+}$ ion is confined by a micron-scale ion trap inside a $230\mu \mathrm{m}$-long optical fiber cavity. We characterize the spatial ion-cavity coupling and measure the ion-cavity coupling strength using a cavity-stimulated $\Lambda$ transition. Owing to the small mode volume of the fiber resonator, the coherent coupling strength between the ion and a single photon exceeds the natural decay rate of the dipole moment. This system can be integrated into ion-photon quantum networks and is a step towards cavity quantum electrodynamics based quantum information processing with trapped ions.

Figure 1

Experimental setup and relevant energy levels of ${\mathrm{Yb}}^{+}$. (a) The ion trap is formed by end cap electrodes (gray, vertical), which are surrounded by compensation electrodes (yellow, arranged on a square) and the optical cavity (oriented horizontally). For loading and initial cooling of the ion, the cavity is retracted using a nanopositioning stage. (b) Composite image of the end cap electrodes (shadow image) and the trapped ion (fluorescence image, inset). (c) Composite image with the cavity in position. (d) Cooling and detection of the ion is performed on the $S_{1/2}-P_{1/2}$ transition, the cavity operates on the $D_{3/2}-D[3/2{]}_{1/2}$ transition at 935 nm, and the $\Lambda$ transition is driven by a coupling laser at 297 nm and the vacuum cavity mode. The branching ratio from the $D[3/2{]}_{1/2}$ state is $55:1$ in favor of decay into the $S_{1/2}$ state.

Figure 2

Scanning the cavity mode profile. (a) Laser pulse sequence. An initialization pulse prepares the ion in state $D_{3/2}$ out of which it is repumped using photons on the cavity mode. Subsequently, fluorescence on the 369-nm transition is detected. (b) The repumping rate $1/{\tau }_D$ out of the $D_{3/2}$ state is determined by an exponential fit $A[1-\exp (-T/{\tau }_D)]$ to the fluorescence data. (c) Transverse mode profile showing the intensity profile of the cavity field. Panels (d) and (e) show cuts through the mode profile together with Gaussian fits to determine the mode waist. (f) The optical standing wave inside the resonator is scanned by displacing the cavity assembly relative to the ion trap. The relative error on each data point is 15%, given by the fit error to determine $1/{\tau }_D$.

Figure 3

Cavity-stimulated two-photon transition rate. Round symbols show the population of the $S_{1/2}$ state after applying a resonant laser on the $S_{1/2}-D[3/2{]}_{1/2}$ and the cavity vacuum field on the $D[3/2{]}_{1/2}-D_{3/2}$ transitions as a function of the interaction time. The square symbols show the data with the cavity detuned from the atomic transition by half a free spectral range. The solid lines are exponential fits. The inset shows the laser pulse sequence used. The error bars denote the photon shot noise.