Enhancing single-atom loading in tightly confined dipole traps with an ancillary dipole beam

Single atoms trapped in tightly focused optical dipole traps provide an excellent experimental platform for quantum computing, precision measurement, and fundamental physics research. In this work, we propose and demonstrate an approach to enhancing the loading of single atoms by introducing a weak ancillary dipole beam. The loading rate of single atoms in a dipole trap can be significantly improved by only a few tens of microwatts of a counterpropagating beam. It was also demonstrated that multiple atoms could be loaded with the assistance of a counterpropagating beam. By reducing the power requirements for trapping single atoms and enabling the trapping of multiple atoms, our method facilitates the extension of single-atom arrays and the investigation of collective light-atom interactions.

Figure 1

The experimental setup for investigating the single-atom loading dynamics. The dipole beam is provided by an $852\mathrm{nm}$ laser. The dipole beam output from fiber coupler 3 is focused by a metalens ($\mathrm{N}.\mathrm{A}.=0.46$), with its polarization controlled by a half-wave plate (HWP) and a quarter-wave plate (QWP). The ancillary dipole beam output from fiber coupler 1 is focused by a commercial objective ($\mathrm{N}.\mathrm{A}.=0.28$), with its polarization controlled by a polarization beam splitter (PBS) and a QWP. All the fibers in this experiment are polarization-maintaining fibers, and the beam's polarization has been matched to the principal axis of the fibers. The waists of two Gaussian dipole laser beams overlap in the magneto-optical trap (MOT). The fluorescence from the trapped atoms in the dipole trap is collected by the objective and detected by a single-photon count module (SPCM) through a dichroic mirror (DM) and fiber coupler 2.

Figure 2

Illustrations of dipole traps with varying ancillary dipole beam power $P_{\mathrm{anc}}$, and the corresponding experimental results of atomic fluorescence signals and histogram statistics of counts. Panels (a1)–(a3), (b1)–(b3), (c1)–(c3), and (d1)–(d3) show the results for $P_{\mathrm{anc}}=11\mu \mathrm{W}, P_{\mathrm{anc}}=92\mu \mathrm{W}, P_{\mathrm{anc}}=238\mu \mathrm{W}$, and $P_{\mathrm{anc}}=476\mu \mathrm{W}$, respectively, while the primary dipole beam power is fixed at $P_{\mathrm{p}}=10\mathrm{mW}$. In panels (a3)–(d3), the lines represent Poisson distribution fitting of photon counts, and the labels are the corresponding fitted mean counts of trapped atoms.

Figure 3

The experimental results for trapping single atoms in a dipole trap affected by different ancillary dipole beam powers $P_{\mathrm{anc}}$. (a) The probabilities of trapping $n$ atoms in the dipole trap affected by $P_{\mathrm{anc}}$, with $n=0,1,2,\cdots$ . (b) The fluorescence signal step of a single trapped atom against $P_{\mathrm{anc}}$. (c) The lifetime of a trapped single atom varying with $P_{\mathrm{anc}}$ when $P_{\mathrm{anc}}<200\mu \mathrm{W}$ in the low-power regime.

Figure 4

The fluorescence signals obtained at different power levels of ancillary dipole beams. Parts (a1) and (a2) show the signals for $P_{\mathrm{p}}=10\mathrm{mW}, P_{\mathrm{anc}}=41\mu \mathrm{W}$ (orange) and $P_{\mathrm{p}}=10\mathrm{mW}, P_{\mathrm{anc}}=0\mu \mathrm{W}$ (green), while (b1) and (b2) show the signals for $P_{\mathrm{p}}=10\mathrm{mW}, P_{\mathrm{anc}}=11\mu \mathrm{W}$ (orange) and $P_{\mathrm{p}}=10\mathrm{mW}, P_{\mathrm{anc}}=0\mu \mathrm{W}$ (green). Parts (c) and (d) show the corresponding histogram statistics for the results presented in (a1), (a2) and (b1), (b2), respectively.

Figure 5

The characterization of a dipole trap. (a) Theoretical results of axial trap depth when the dipole beam power is $10\mathrm{mW}$ and the ancillary dipole beam power is $P_{\mathrm{anc}}=100\mu \mathrm{W}$ (red line) or 0 (blue line). (b) Calculated dipole trap depth enhancement ratio compared to the case without an ancillary dipole beam. (c) Single-atom loading probability against $P_{\mathrm{p}}$ without an ancillary dipole beam ($P_{\mathrm{anc}}=0$), with the dots from experiments and the line from a theoretical model. (d) The single-atom loading probability against the power of an ancillary dipole beam $P_{\mathrm{anc}}$, with $P_{\mathrm{p}}=10\mathrm{mW}$ is fixed. The dots are from experiments, and the black lines show the fitting with the theoretical model.

Figure 6

The performance of a dipole trap under an ancillary dipole beam with varying polarization. Parts (a) and (d1), (d2); (b) and (e1), (e2); and (c) and (f1), (f2) are the results for $P_{\mathrm{anc}}=0.5$, 2.5, and $5\mathrm{mW}$, respectively, while the power of the primary dipole beam is fixed at $10\mathrm{mW}$.