In some embodiments, the chip-scale atomic beam clock further comprises a first in-plane photodetector configured to detect the first photon beam after being retroflected by the first in-plane mirror, and a second in-plane photodetector configured to detect the second photon beam after being retroflected by the second in-plane mirror. In some embodiments, the first in-plane photodetector is present, while the second in-plane photodetector is not present. In some embodiments, neither the first in-plane photodetector nor the second in-plane photodetector are present.

In some embodiments of the chip-scale atomic beam clock, the microchannels are bonded to a contiguous top layer to form a closed environment under vacuum. The microchannels may be bonded to the contiguous top layer via direct wafer bonding, anodic bonding, metal-metal bonding, or a combination thereof, for example.

The contiguous top layer may be a different composition than the silicon-containing material. In certain embodiments, the contiguous top layer is fabricated from alkali-free glass. In certain embodiments, the contiguous top layer is fabricated from borosilicate glass. Other types of glass may be employed for the contiguous top layer.

The contiguous top layer may be fabricated from a material that has a coefficient of thermal expansion (CTE) less than 0.1 ppm/° C., preferably less than 0.05 ppm/° C., between 25° C. and 300° C. In this specification, CTE refers to the volumetric coefficient of thermal expansion. The value of 0.05 ppm/° C. is equivalent to 5×10^?8 per degree Celsius. In various embodiments, the CTE of the contiguous top-layer material, calculated as the average from 25° C. to 300° C., is about, or less than about 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01 ppm/° C.