# Atom interferometer

An atom interferometer uses the wave-like nature of atoms in order to produce interference. In atom interferometers, 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 emits matter waves (the atoms) rather than light. Atom interferometers measure the difference in phase between atomic matter waves along different paths. Matter waves are controlled and manipulated using systems of lasers.$^{:420–1}$ Atom interferometers have been used in tests of fundamental physics, including measurements of the gravitational constant, the fine-structure constant, and universality of free fall. Applied uses of atom interferometers include accelerometers, rotation sensors, and gravity gradiometers.

## Overview

Interferometry splits a wave into a superposition along two different paths. A spatially dependent potential or a local interaction differentiates the paths, introducing a phase difference between waves. Atom interferometers use center of mass matter waves with short de Broglie wavelength. Experiments using molecules have been proposed to search for the limits of quantum mechanics by leveraging the molecules' shorter De Broglie wavelengths.

## 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.

### Examples

| Group | Year | Atomic species | Method | Measured effect(s) |
| Pritchard | 1991 | Na, Na$_2$ | Nano-fabricated gratings | Polarizability, index of refraction |
| Clauser | 1994 | K | Talbot–Lau interferometer | |
| Zeilinger | 1995 | Ar | Standing light wave diffraction gratings | |
| Helmke Bordé | 1991 | | Ramsey–Bordé | Polarizability, Aharonov–Bohm effect: exp/theo $0.99\pm 0.022$, Sagnac effect 0.3 rad/s/$\surd$Hz |
| Chu | 1991 1998 | Na Cs | Kasevich–Chu interferometer Light pulses Raman diffraction | Gravimeter: $3\cdot 10^{-10}$ Fine-structure constant: $\alpha \pm 1.5\cdot 10^{-9}$ |
| Kasevich | 1997 1998 | Cs | Light pulses Raman diffraction | Gyroscope: $2\cdot 10^{-8}$rad/s/$\surd$Hz, Gradiometer: |
| Berman | | | Talbot-Lau | |
| Mueller | 2018 | Cs | Ramsey-Bordé interferometer | Fine-structure constant: $\alpha \pm 2\cdot 10^{-10}$ |

## History

Interference of atom matter waves was first observed by Immanuel Estermann and Otto Stern in 1930, when a sodium (Na) beam was diffracted off a surface of sodium chloride (NaCl). The first modern atom interferometer reported was a double-slit experiment with metastable helium atoms and a microfabricated double slit by O. Carnal and Jürgen Mlynek in 1991, and an interferometer using three microfabricated diffraction gratings and Na atoms in the group around David E. Pritchard at the Massachusetts Institute of Technology (MIT). Shortly afterwards, an optical version of a Ramsey spectrometer typically used in atomic clocks was recognized also as an atom interferometer at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. The largest physical separation between the partial wave packets of atoms was achieved using laser cooling techniques and stimulated Raman transitions by Steven Chu and his coworkers in Stanford University.
In 1999, the diffraction of C$_{60}$ fullerenes by researchers from the University of Vienna was reported. Fullerenes are comparatively large and massive objects, having an atomic mass of about 720 Da. The de Broglie wavelength of the incident beam was about 2.5 pm, whereas the diameter of the molecule is about 1 nm, about 400 times larger. In 2012, these far-field diffraction experiments could be extended to phthalocyanine molecules and their heavier derivatives, which are composed of 58 and 114 atoms respectively. In these experiments the build-up of such interference patterns could be recorded in real time and with single molecule sensitivity.
In 2003, the Vienna group also demonstrated the wave nature of tetraphenylporphyrin—a flat biodye with an extension of about 2 nm and a mass of 614 Da. For this demonstration they employed a near-field Talbot–Lau interferometer. In the same interferometer they also found interference fringes for C$_{60}$F$_{48}$, a fluorinated buckyball with a mass of about 1600 Da, composed of 108 atoms. Large molecules are already so complex that they give experimental access to some aspects of the quantum-classical interface, i.e., to certain decoherence mechanisms. In 2011, the interference of molecules as heavy as 6910 Da could be demonstrated in a Kapitza–Dirac–Talbot–Lau interferometer. In 2013, the interference of molecules beyond 10,000 Da has been demonstrated.
The 2008 comprehensive review by Alexander D. Cronin, Jörg Schmiedmayer, and David E. Pritchard documents many new experimental approaches to atom interferometry. More recently atom interferometers have begun moving out of laboratory conditions and have begun to address a variety of applications in real world environments.

## Applications
### Gravitational physics

A precise measurement of gravitational redshift was made in 2009 by Holger Muller, Achim Peters, and Steven Chu. No violations of general relativity were found to 7×10$^{−9}$.
In 2020, Peter Asenbaum, Chris Overstreet, Minjeong Kim, Joseph Curti, and Mark A. Kasevich used atom interferometry to test the principle of equivalence in general relativity. They found no violations to about 10$^{−12}$.

### Inertial navigation

The first team to make a working model, Pritchard's, was propelled by David Keith. Atomic interferometer gyroscopes (AIG) and atomic spin gyroscopes (ASG) use atomic interferometer to sense rotation or in the latter case, uses atomic spin to sense rotation with both having compact size, high precision, and the possibility of being made on a chip-scale. "AI gyros" may compete, along with ASGs, with the established ring laser gyroscope, fiber optic gyroscope and hemispherical resonator gyroscope in future inertial guidance applications.