Best-ever quantum measurement breaks Heisenberg limit - physics-math - 23 March 2011 - New Scientist

Best-ever quantum measurement breaks Heisenberg limit: 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...