Fig. 1. Schematic diagram of an optical atomic clock.^[12] Atomic spectroscopy is measured from the optical lattice trapped atoms or the trap ion. The interrogation laser is precisely locked to the atomic transition. An error signal is derived from atomic spectroscopy that is fed back to the laser for closed-loop locking. An optical frequency comb, as a counter, converts the optical frequency to a microwave that can be used as the standard frequency of time.

Fig. 2. (a) Sketch of the MOT setup.^[12] Three pairs of retro-reflected laser beams cross each other at the center of the trap. A pair of anti-Helmholtz coils provide the necessary quadrupole magnetic field for trapping. The atomic cloud is collected in the center of the trap. (b) The diagram of one linear 4-rod trap (adapted from Ref. [37]).

Fig. 3. Optical Mach–Zehnder interferometer and Raman atom interferometer. (a) An optical Mach–Zehnder interferometer with beam splitters and mirrors. (b) A Raman atom interferometer with a standard π/2–π–π/2 Raman sequence. (c) Momentum transfer of an atom when its internal state is changed by a Raman pulse. (d) Two-photon Raman process.

Fig. 4. Inertial sensors based on a Raman atom interferometer. (a) An atomic gyroscope for angular velocity.^[114] (b) An atomic gravimeter for gravity acceleration.^[133] (c) An atomic gravity gradiometer for the gradient of gravity acceleration.^[109] (d) An atomic gravity gradiometer for the Newton gravitational constant.^[110]

Fig. 5. New variants of Raman atom interferometers. (a) A transportable absolute quantum gravimeter with a long-term stability below 1 × 10⁻⁹g.^[159] (b) An atom-chip fountain gravimeter based on a freely falling Bose–Einstein condensate from an atomic chip.^[186]

Table 1. Measured performance of atomic gyroscopes.

Table 2. Some reported performance of atomic gravimeters.

Table 3. Reported performance of atomic gravity gradiometers.