Shanxi University: position and momentum of rubidium atoms estimated 20% beyond Heisenberg bound

Researchers led by a team from Shanxi University in China report that they estimated the position and momentum of a quantum system simultaneously with a precision about 20 percent beyond the Heisenberg uncertainty limit, according to a preprint covered by sciencealert.com. sciencealert.comsciencealert.comsciencealert.comOpen source ↗ The paper is available on arXiv and has not yet been peer-reviewed. The experiment used 52 billion rubidium atoms in a small glass chamber.

Measurement with past and future data

The Heisenberg uncertainty principle, introduced by Werner Heisenberg in 1927, says that if one characteristic of a quantum particle is predicted precisely, the paired characteristic becomes less certain. Position and momentum are the best-known pair of such non-commuting variables. This fuzziness complicates measurements of weak signals, from the LIGO gravitational wave detector to the atomic clocks that underpin systems such as GPS.

Instead of predicting the future state, the team took a measurement using data from both before and after the moment of interest. The hypothesis was that this two-way view would remove some of the fuzziness in observations. According to the authors, the approach consistently incorporates past and future measurement information to refine the inference of the system state at a given time.

Rubidium experiment

A laser aligned the spins of the atoms, which are their magnetic properties, so the atoms behaved as one quantum system. A second laser then measured its momentum and position. sciencealert.comsciencealert.comsciencealert.comOpen source ↗ Measurements ran continuously for 84 minutes, and the setup allowed both quantities to be measured simultaneously, easing the usual trade-off between them.

The estimates were around 20 percent more precise than the Heisenberg limit allows. The authors say they verified the results using weak measurement.

Limits of the method

The Heisenberg limit still applies. The study shows that the principle can be circumvented to some extent by working out probabilities in a different way. Because the method uses data from after the moment of interest, it limits real-time monitoring. In many quantum systems, including atomic clocks, retrospective readings are already the norm.

The researchers plan to improve the setup with more precise instrumentation and more accurate readings, and name quantum entanglement as one possible direction. sciencealert.comsciencealert.comsciencealert.comOpen source ↗ They also write that the work opens prospects for quantum-enhanced sensing that combines trajectory and entanglement strategies.