Pioneering researchers in Europe have done what no one has been able to do: surpass the limit on the sensitivity of a quantum measurement. The result could play a key role in interferometry and quantum limits of measurement. Presented in the journal Nature, the research was funded in part the EMALI ('Engineering, manipulation and characterization of quantum states of matter and light') project, which clinched a Marie Curie Research Training Networks grant worth more than EUR 439 000 under the EU's Sixth Framework Programme (FP6), to develop general theoretical and experimental techniques for engineering, manipulating and characterising quantum states of matter and light.

Interferometry uses the quantum superposition principle which enables quantum particles to take a number of paths simultaneously. Doing so allows them to detect tiny path differences. Thanks to this latest study, interactions among the particles can be used to generate more sensitive interferometers.

Lead author Mario Napolitano, a doctorate student at the Institut de Ciències Fotòniques (ICFO) in Barcelona, Spain and his colleagues used interacting photons to investigate an atomic ensemble, effectively achieving sensitivity to surpass a fundamental limit, the so—called Heisenberg limit — what experts describe as the ultimate limit for various measurements including magnetic imaging and wave detection.

'The most precise instruments are interferometric in nature, and operate according to the laws of quantum mechanics,' the authors write in Nature. 'A collection of particles, for example photons or atoms, is prepared in a superposition state, allowed to evolve under the action of a Hamiltonian containing an unknown parameter, X, and measured in agreement with quantum measurement theory. The complementarity of quantum measurements determines the ultimate sensitivity of these instruments.'

Quantum physicists say that under the Heisenberg uncertainty principle, certain pairs of physical properties, such as position and momentum, cannot both be known to arbitrary precision — the more precisely one property is known, the less precisely the other can be known. Based on their finding, interparticle interactions could be used in quantum metrology.

Because certain restrictions exist on the act of measurement in quantum physics, achieving sensitivity becomes a complex process. According to the experts, in a measurement involving quantum interference among N probe particles, the sensitivity improves as N increases, scaling as 1/N1/2 if the particles are independent, and as 1/N (the Heisenberg limit) if they are quantum—mechanically 'entangled'.

Scientists recently suggested that improving sensitivity is possible if particles interact with one another. Basically, how one particle behaves is contingent on the presence of others.

In this study, the researchers developed a system to obtain this 'super—Heisenberg' scaling. The team used nonlinear optical effects in an ensemble of cold atoms to produce interactions among photons used to investigate the ensemble's magnetisation. The measurement demonstrated improved scaling beyond the Heisenberg limit, topping the conventional interferometer by a factor of 10.

'Our work shows that interparticle interactions can improve sensitivity in a quantum—limited measurement, and experimentally demonstrates a new resource for quantum metrology,' the authors conclude.