Highly Efficient State-Selective Submicrosecond Photoionization Detection of Single Atoms

We experimentally demonstrate a detection scheme suitable for state analysis of single optically trapped atoms in less than $1\mu \mathrm{s}$ with an overall detection efficiency $\eta$ exceeding 98%. The method is based on hyperfine-state-selective photoionization and subsequent registration of the correlated photoion-electron pairs by coincidence counting via two opposing channel electron multipliers. The scheme enables the calibration of absolute detection efficiencies and might be a key ingredient for future quantum information applications or precision spectroscopy of ultracold atoms.

Figure 1

(a) Setup for single-atom ionization: Single atoms are trapped in an optical dipole trap, prepared into selected hyperfine states and subsequently ionized. (b) Photoionization level scheme: Single ion-$e^{-}$ pairs are generated by hyperfine-state-selective, resonant two-step, two-color photoionization. (c) Joint channel electron multiplier (CEM) detector: two opposing CEMs and compensation electrodes against stray fields are built into a glass cell identical to (a). (d) Detector geometry in section view (laser beam waist not to scale).

Figure 2

Hyperfine-state-selective, single-atom ionization probability for different laser pulse lengths. Atoms in the trap are initially prepared either in the $5{}^{2}S_{1/2}$ $F=2$ (□) or the $F=1$ (○) hyperfine state, respectively. Only atoms in $F=2$ driven by both laser fields (${\lambda }_{12}$, ${\lambda }_{2i}$) are ionized. Inset: Scheme showing the timing of the excitation and ionization pulses.

Figure 3

(a) Time-of-flight difference $\Delta t$ of generated ${}^{87}\mathrm{Rb}$ ions to their corresponding photoelectrons for different acceleration voltages $U_{\mathrm{acc}}$. Inset: Sample histogram of time differences between ${}^{87}\mathrm{Rb}$-ion and electron detections for $U_{\mathrm{acc}}=3.8\mathrm{kV}$. (b) Absolute detection efficiency for ${}^{87}\mathrm{Rb}$-ions (□, black), electrons (○, red) for different acceleration voltages and the calculated, total detection efficiency (▵, blue). Inset: Zoom for acceleration voltages from 3.2 to 3.8 kV.

Figure 4

(a) 2D scans of the sensitive areas at a gain voltage of 2.8 kV for the $e^{-}$ CEM (top) and ion CEM (bottom) for an acceleration voltage of 3.8 kV. Dashed lines indicate respective line scans in (b). (b) Line scans through the center at $y=-0.4\mathrm{mm}$. Electron detection efficiencies (top) at different gain voltages ($\diamond =2.4\mathrm{kV}$ (brown); $◃=2.5\mathrm{kV}$ (green); $\star =2.6\mathrm{kV}$ (blue); $▿=2.8\mathrm{kV}$ (red); $△=3.2\mathrm{kV}$ (black)). ${}^{87}\mathrm{Rb}$-ion detection efficiency (bottom) for a gain voltage of 2.7 kV (□). The acceleration voltage is the same as in (a).