A remarkably similar phenomenon occurs in atom interferometers, one of the most promising technologies for future quantum sensors. Here, measurements rely on observing tiny changes in periodic quantum signals, which reveal small physical effects. As different parts of the quantum system gradually fall out of sync, the periodic interference signal fades before later reappearing. Until now, this temporary collapse of the signal has largely been regarded as a limitation for precision measurements.

Researchers at TU Darmstadt have now turned this apparent weakness into an advantage. Their work, published in Science Advances Volume 12 | Issue 35, shows that the collapse itself contains valuable information. Rather than discarding these fading measurements, the team developed a new analysis method that uses them to extract physical quantities more accurately and with reduced systematic bias.

The new approach, called Parameter Estimation from Amplitude Collapse (PEAC), does not rely solely on the interference pattern. It makes additional use of how the signal strength changes as the quantum system dephases and later rephases, revealing information that conventional methods largely overlook.

The accompanying illustration captures this process at a glance. At first, the measurements form two clearly separated bands, indicating a strong quantum signal. As the experiment progresses, the bands move closer together until they merge, as if the signal has almost disappeared. Later, they separate again as the signal returns.

This collapse and revival arises because the atoms are prepared in two different magnetic states. Since the two states respond slightly differently to a magnetic-field gradient, they gradually fall out of sync before naturally coming back into alignment. Alas, the quantum oscillations persist during all times and the signal only apparently disappears temporarily. The compass needles shown alongside the data provide a simple illustration of this changing relationship.

The work is the result of a close collaboration between experiment and theory at TU Darmstadt, bringing together researchers from the Atoms–Photons–Quanta group | Prof. G. Birkl and the Theoretical Quantum Optics group | Prof. E. Giese. By combining precision atom-interferometry experiments with new statistical methods, the team demonstrates that sometimes the most informative part of a quantum measurement is when the signal appears to disappear.