Introduction

Atom interferometry makes use of the wave-like properties of matter that become evident at very low energy scales (temperatures). The concept is analogous to an optical interferometer where a coherent source of light is split into separate beams following different paths.  After recombination, the light beams undergo constructive interference and the phase of the interference signal encodes any optical path length difference. Similarly, the wave-function of an atom can be split into a superposition of two states evolving along different paths. This is accomplished through absorption and stimulated emission of photons and the associated momentum transfer (recoil) between light and atom. Such light pulses that split, redirect, and recombine matter-waves are called atom optics.

It is desirable that the two states used in an atom interferometer are long-lived to minimize spontaneous decay which diminishes the output signal. Traditionally, a set of counter-propagating laser beams is used to couple either two electronic ground states (Raman atom optics) or two momentum states of the same electronic ground state (Bragg atom optics) via a far-detuned excited state. However, one can also resonantly drive transitions between an electronic ground state and a metastable state using a single laser beam. Such a clock atom interferometer makes use of the narrow transition commonly used in atomic clocks.

Figure 1 illustrates the trajectories of the wave-packets in a clock atom interferometer sequence. An atom is launched upwards and is freely falling under the influence of gravity. It is manipulated by a series of three laser pulses (wavy lines), emitted at times 0, T, and 2T. The first pulse, with pulse area π/2, puts the atom in an equal superposition of states |1> and |2>. The second pulse, with pulse area π, flips each state and redirects the two parts of the wavefunction. A third pulse with pulse area π/2 recombines the two paths, leading to a population ratio that depends on the relative phase accumulated between the two arms. Note that there is a possibility for imperfect overlap between the two paths at the time of the last pulse.