PHYSICISTS have made the most accurate quantum measurement yet, breaking a theoretical limit named for Werner Heisenberg.

The most accurate quantum measurements possible are made using an interferometer, which exploits the wave nature of matter and light. In this method, two identical beams of particles are sent along different paths to a detector, with one interacting with an object of interest along the way. Recombining the beams afterwards creates an interference pattern that reflects how much the interacting beam was disturbed – providing details about the object’s properties.

Assuming that the particles interact with the object, but not with one another, the accuracy of such measurements grows in proportion to the number of particles in the beams, N. By allowing such particle interactions, Mario Napolitano of the Institute of Photonic Sciences in Barcelona, Spain, and colleagues have now demonstrated a way to break this so-called Heisenberg limit.

As well as interacting with the object they are fired at, photons in the beam also interact with each other


They used a beam of photons to measure the small magnetic field produced by a gas made up of a million ultra-cold rubidium atoms. Normally, the spin of each photon would rotate by a certain amount, thanks to its interactions with the magnetic field of the atoms. But the frequency of the photons was chosen so that the photons also interacted with each other when they were in the gas, so that the presence of one photon altered the way a second behaved. These interactions led to a measurement accuracy that grew in proportion to N3/2 – greater than Heisenberg’s limit (Nature, DOI: 10.1038/nature09778).

The technique could pave the way for more sensitive searches for gravitational waves – ripples in space triggered by moving objects. The waves should cause the distance between two objects to change, and the study suggests that the laser interferometers used to look for such changes could be made more precise.