We demonstrate the trajectory measurement of the single neutral atoms deterministically using a high-finesse optical microcavity. The single atom strongly couples to the high-order transverse vacuum TEM 10 mode, instead of the usual TEM 00 mode, and the parameters of the system are (g 10 ,κ,γ ) = 2π × (20.5,2.6,2.6) MHz. The atoms simply fall down freely from the magneto-optical trap into the cavity modes, and the trajectories of the single atoms are linear. The transmission spectra of atoms passing through the TEM 10 mode are detected by single-photon counting modules and are well fitted. Thanks to the tilted cavity transverse TEM 10 mode, which is inclined to the vertical direction ∼45 • , it helps us to eliminate the degenerate trajectory of the single atom falling through the cavity and to obtain a unique atom trajectory. An atom position with a high precision of 0.1 µm in the off-axis direction (axis y) is obtained, and a spatial resolution of 5.6 µm is achieved in a time interval of 10 µs along the vertical direction (axis x). The average velocity of the atoms is also measured from the atom transits, which determines independently the temperature of the atoms in a magneto-optical trap, 186 ± 19 µK.

Based on the real sense of the time-of-flight , we demonstrate an alternative method of measuring the temperature of cold atoms in magneto-optical traps (MOTs) using a high-finesse optical microcavity, which acts as a pointlike single-atom counter. A cloud of atoms trapped in magneto-optical traps is positioned about 5 mm above the cavity, and the atoms fall freely down through the cavity. The temperature of the cold atoms in the MOT is determined by counting the exact arrival times of the single atoms. A theoretical model based on a ballistic expansion of a cloud of trapped atoms falling in the earth's gravitational field is used to fit the probability distribution of atom arrivals, and the fittings agree very well with the experimental results. This method could be used for systems with little room, where an extra probe beam is hard to involve, or with fewer atoms initially.