Collecting more than half the fluorescence photons from a single ion

We demonstrate the trapping of a single ion in the focus of a deep parabolic mirror that covers 81$\%$ of the solid angle surrounding the ion. Accounting for the reflectivity of the mirror we infer a photon collection efficiency of 54.8$\%$ for our setup. The underlying experimentally detected maximum fluorescence rate is $1.91\times {10}^6$ s${}^{-1}$ from a single ${}^{174}$Yb${}^{+}$ ion, mainly limited by the quantum efficiency of our photon detector. Besides the high collection efficiency, the integration of an ion trap into a parabolic mirror is a key ingredient for efficient coupling of light to a single ion in free space.

Figure 1

Experimental setup and ion energy levels. (a) Close-up view of the trap geometry, showing the location of a trapped ion in the mirror's focus and the bores in both mirror and trap mounting plate. These bores allow lasers and the neutral atom beam to enter the parabolic mirror from the back. (b) Partial level diagram of ${}^{174}$Yb${}^{+}$ (not to scale). (c) Sketch of the detection setup used: A telescope consisting of lenses T${}_1$ and T${}_2$ reduces the size of the beam leaving the parabolic mirror (PM) and is used in combination with a pinhole (PH) for spatial filtering of stray light. The telescope and a lens L${}_A$ image either the front plane of the mirror or the ion onto an electron-multipliying CCD camera (EMCCD). Optionally either with mirror M${}_{\mathrm{PMT}}$ or mirror M${}_{\mathrm{HBT}}$ the fluorescence light can be directed onto a photomultiplier tube (PMT) or onto a beamsplitter and two avalanche photodiodes (APD${}_1$ and APD${}_2$) connected to a time-to-digital converter (TDC) for a time correlation measurement.

Figure 2

Fluorescence from a single, saturated ${}^{174}$Yb${}^{+}$ ion located in the focus of the mirror. (a) Background corrected intensity distribution at the mirror's front plane. (b) Intensity distribution of Fig. 2a averaged over the azimuth. The vertical dashed line marks the aperture radius of the parabolic mirror. The dashed curve displays the result of a fit with Eq. (1). Data in the interval $r/f=\left[1.12,4.76\right]$ have been used for fitting. (c) Focused intensity with a FWHM spot size of 280 nm.

Figure 3

Saturation of the ion's fluorescence for various powers of the cooling laser. The dashed line is a fit of the theoretical saturation curve to the data, and the solid line is the same fit corrected for the finite pulse width of the PMT counts. Inset: Linewidth of the cooling transition ${}^2{S}_{1/2}$ $\leftrightarrow$ ${}^2{P}_{1/2}$ after micromotion compensation and Doppler cooling. The fit (dashed line) to a Lorentzian results in a FWHM of 20.9 MHz. The spectrum is taken at a laser power corresponding to a saturation parameter on resonance of $S_0\approx 0.14$ and thus consistent with the residual broadening.

Figure 4

Photon correlation measurement distributing $\approx$780 000 events from a total acquisition time of 60 s. The histogram is normalized using average count rates and coincidences.