Chip-scale atomic clock

In military equipment it is expected to provide improved location and battlespace situational awareness for dismounted soldiers when the global positioning system is not available,[3] but many civilian applications are also envisioned.

[4] The CSAC, the world's smallest atomic clock, is 4 x 3.5 x 1 cm (1.5 x 1.4 x 0.4 inches) in size, weighs 35 grams, consumes only 115 mW of power, and can keep time to within 100 microseconds per day after several years of operation.

A feedback mechanism keeps a quartz crystal oscillator on the chip locked to this frequency, which is divided down by digital counters to give 10 MHz and 1 Hz clock signals provided to output pins.

The light source in conventional atomic clocks is a rubidium atomic-vapor discharge lamp, which was bulky and consumed large amounts of power.

In the CSAC this was replaced by an infrared vertical cavity surface emitting laser (VCSEL) fabricated on the chip, with its beam radiating upward into the caesium capsule above it.

The physics package of the NIST chip-scale atomic clock
The heart of NIST's next-generation miniature atomic clock -- ticking at high "optical" frequencies-- is this vapor cell on a chip, shown next to a coffee bean for scale.