According to the laws of quantum mechanics, atoms move like waves with a characteristic de Broglie wavelength *λ = h/p*. Hereby, *h* is the Planck constant and *p* is the momentum of the atom. For a velocity of, for example, 700 m/s, *λ* is on the order of 10 pm. Due to these extremely short wavelengths, very accurate measurements can be made using atomic wave interference.

By using the wave character of calcium atoms, Dr. habil. Fritz Riehle, Dr. Harald Schnatz, Dipl.-Phys. Tilmann Trebst and Dr. Jürgen Helmcke from PTB Braunschweig achieved an optical frequency standard of a performance unattained so far. Thus, they succeeded – with a relative uncertainty of 10^{-13 }– in keeping the frequency and the wavelength of a laser constant and in measuring it. For their work on “Atom interferometry in the time interval for precision measurements” (“Atominterferometrie im Zeitbereich für Präzisionsmessungen”), the four researchers were awarded the 1999 Helmholtz Prize in the field of “Precision Measurement”, which was endowed with 12,000 German marks.

The atomic interferometer developed by Dr. Jürgen Helmcke and his colleagues used single calcium atoms which were cooled with laser light and then trapped in a magneto-optical trap. After the trap had been switched off, such an atom was irradiated with a laser pulse. Thereby, the atom absorbed – with a probability of 50 % – a photon and in doing so, it changed from a ground state into a long-lived excited state. Since the momentum of the atom changed due to the absorption of the photon, the atomic wave packet was split into an excited and a non-excited partial wave, which moved into different directions.

After waiting times of about 100 µs each, the atom was hit by two further laser pulses, which led to the fact that the two partial waves moved towards each other again after the second pulse and interfered with each other at the third pulse. The interference signal was given by the probability that the atom was detected in the excited state, which was determined by its fluorescent light. From the interference signal, the phase difference between the two partial waves could be determined. This difference depended, among other things, on how well the laser was tuned to the excitation frequency of the atom. Due to the fact that the researchers kept the phase difference constant, they were able to stabilize the laser frequency with very high precision.

They estimated the relative uncertainty of the frequency at 2.5 x 10^{-13}. In the course of two and a half years, the measurements of the frequency of the dye laser, which had been stabilized by means of the atomic interferometer, deviated by less than 90 Hz from each other, which corresponded to a relative frequency difference of

10^{-13}. The high frequency stability of such lasers was hoped to be used for ever more precise atomic clocks, while their – correspondingly high – wavelength stability made these lasers the most precise length standards at that time. Since the phase difference between the two interfering partial atomic waves also depended on the acceleration acting on the atoms, the atomic interferometer could also be turned into a highly sensitive sensor for acceleration forces and gravitational forces. Such sensors could be used, for example, to detect crude oil or ore deposits based on the local fluctuations in the Earth's gravitational force.