Efficient fluorescence collection from trapped ions with an integrated spherical mirror

Efficient collection of fluorescence from trapped ions is crucial for quantum optics and quantum computing applications, specifically for qubit state detection and in generating single photons for ion-photon and remote ion entanglement. In a typical setup, only a few percent of the ion fluorescence is intercepted by the aperture of the imaging optics. We employ a simple metallic spherical mirror integrated with a linear Paul ion trap to achieve a photon collection efficiency of at least $10\%$ from a single Ba${}^{+}$ ion. An aspheric corrector is used to reduce the aberrations caused by the mirror and achieve high image quality.

Figure 1

Cross section of the linear quadrupole trap with the integrated mirror, viewed along the axis of the trap. The photons from the ion are partially blocked by the trap quadrupole rods.

Figure 2

Simulations of corrector performance. (a) The curves generated by the segment-fitting (SF) algorithm and the asymptotic integration (AI), with corrector thickness $H$ plotted as a function of distance from the optical axis, $R$. (b, c) Simulated performance of a spherical mirror with the two corrector elements as compared to a parabolic mirror performance for defocusing (b) and the optical axis tilt (c). Dashed line indicates the diffraction limit. Diffraction was not taken into account while calculating the rms spot sizes; points below the dashed line would result in a diffraction-limited image. (d) Image size degradation due to the deviation of the spherical mirror radius of curvature $r$ from the value used in calculating the corrector shape. Dashed line indicates the diffraction limit.

Figure 3

Single ion images formed by the spherical mirror (a) without and (c) with the corrector. The corresponding zoom-in intensity profile plots ($Y=0$) are shown in (b) and (d).

Figure 4

(a) ${}^{138}\mathrm{Ba}$${}^{+}$ energy level diagram and (b) single photon generation logic sequence. The ion is cooled for 300 ns with both lasers on. Then it is exposed to a 493 nm laser for 300 ns to be pumped to the $5D_{3/2}$ state. A 500 ns exposure to a 650 nm laser excites the ion to the $6P_{1/2}$ state and generates a single 493 nm photon when the ion decays back into ground state. A 500 ns time delay is inserted between the laser pulses to ensure the previous laser pulse is completely extinguished. The PMT is gated to count only during the 650 nm laser pulse. A 100 ns time delay between the start of the 650 nm laser pulse and PMT gate is necessary to compensate for the electronic signal delays.