A photodetector may be mounted on the contiguous top layer to collect photoluminescence from an atom-photon interaction region. When the first in-plane photodetector and the second in-plane photodetector are used, the photodetector mounted on the contiguous top layer is a third photodetector. The third photodetector is labeled PD3 in FIGS. 2A, 2B, and 2C, discussed below, in reference to certain embodiments using three photodetectors.

In some embodiments of the chip-scale atomic beam clock, the vertical-cavity surface-emitting laser is configured to emit circularly polarized light. In certain embodiments, the first and second in-plane mirrors are configured to flip the polarization of the first and second photon beam, respectively, between left and right.

In some embodiments of the chip-scale atomic beam clock, the vertical-cavity surface-emitting laser is configured to emit photons with one or more wavelengths selected from about 500 nm to about 2000 nm. In various embodiments, the vertical-cavity surface-emitting laser is configured to emit photons with a wavelength of about, at least about, or at most about 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, or 2000 mm, including any intervening ranges. Here, intervening ranges, such as 700-1500 nm, means either that a wavelength may be specifically selected with such subrange (700-1500 nm), such as being selected specifically as 789 nm; or, that multiple wavelengths may be used within the subrange, and those wavelengths are not necessarily adjacent. For instance, wavelengths of 700-800 nm and 1200-1300 nm may be selected within the 700-1500 nm subrange.