# Atom interferometer

Atomic interferometry.

An Atom interferometer is an interferometer based on exploiting the wave character of atoms. Interferometers are often used to make high-precision comparisons of distances. This can be used to constrain fundamental constants like the Gravitational Constant or possibly to detect Gravitational Waves.[1]

## Overview

Interferometry inherently depends on the wave nature of the object. As pointed out by de Broglie in his PhD-thesis, particles, including atoms, can behave like waves (the so-called Wave-particle duality, according to the general framework of quantum mechanics). More and more high precision experiments now employ atom interferometers due to their short de Broglie wavelength. Some experiments are now even using molecules to obtain even shorter de Broglie wavelengths and to search for the limits of quantum mechanics.[2] In many experiments with atoms, the roles of matter and light are reversed compared to the laser based interferometers, i.e. the beam splitter and mirrors are lasers while the source instead emits matter waves (the atoms).

## Interferometer types

While the use of atoms offers easy access to higher frequencies (and thus accuracies) than light, atoms are affected much more strongly by gravity. In some apparatuses, the atoms are ejected upwards and the interferometry takes place while the atoms are in flight, or while falling in free flight. In other experiments gravitational effects by free acceleration are not negated; additional forces are used to compensate for gravity. While these guided systems in principle can provide arbitrary amounts of measurement time, their quantum coherence is still under discussion. Recent theoretical studies indicate that coherence is indeed preserved in the guided systems, but this has yet to be experimentally confirmed.

The early atom interferometers deployed slits or wires for the beam splitters and mirrors. Later systems, especially the guided ones, used light forces for splitting and reflecting of the matter wave.[3]

### Examples

Group Year Atomic Species Method Measured effect(s)
Pritchard 1991 Na, Na2 Nano-fabricated gratings Polarizability, Index of Refraction
Clauser 1994 K Talbot-Lau interferometer
Zeilinger 1995 Ar Standing light wave diffraction gratings
Sterr Ramsey-Bordé Polarizability,
Aharonov–Bohm effect: exp/theo $0.99 \pm 0.022$,
Sagnac $0.3 rad / s \sqrt{Hz}$
Kasevich Doppler on falling atoms Gravimeter: $3 \cdot 10^{-10}$
Rotation: $2\cdot 10^{-8} /s /\sqrt{Hz}$,
fine structure constant: $\alpha\pm 1.5 \cdot 10^{-9}$
Berman Talbot-Lau

## History

The separation of matter wave packets from complete atoms was first observed by Esterman and Stern in 1930, when a Na beam was diffracted off a surface of NaCl.[4] The first modern atom interferometer reported was a Young's-type double slit experiment with metastable helium atoms and a microfabricated double slit by Carnal and Mlynek[5] in 1991, and an interferometer using three microfabricated diffraction gratings and Na atoms in the group around Pritchard at MIT.[6] Shortly afterwards, an optical version of Ramsey spectrometer typically used in atomic clocks was recognized also as an atom interferometer at the PTB in Braunschweig, Germany.[7] The largest physical separation between the partial wave packets of atoms was achieved using laser cooling techniques and stimulated Raman transitions by S. Chu and coworkers in Stanford.[8]

• A. D. Cronin, J. Schmiedmayer, D. E. Pritchard, „Optics and interferometry with atoms and molecules“ Rev. Mod. Phys. 81, (2009).
• Electron interferometer
• C. S. Adams, M. Sigel & J. Mlynek, "Atom Optics", Phys. Rep. 240, 143 (1994). Overview of the atom-light interaction
• P. R. Berman [Editor], Atom Interferometry. Academic Press (1997). Detailed overview of atom interferometers at that time (good introductions and theory).
• Stedman Review of the Sagnac Effect