A quantum technique developed by Russian and French researchers allows large distances to be measured with atomic accuracy.
Described in an article in the latest issue of the journal Nature Communications, the ‘quantum ruler’, as the researchers call the technology, takes advantage of a special entangled state of photons known as the N00N state.
"This technique will enable us to use quantum effects to increase the accuracy of measuring the distance between observers that are separated from one another by a medium with losses,” said Alexander Lvovsky from the Lebedev Physical Institute in Moscow, one of the paper’s authors.
A medium with losses can be anything from water to the atmosphere or glass.
The N00N photon states use superposition of spatial positions of several photons. The technique, the team believes, could be used for ultra-high precision applications – for example in systems for detecting gravitational waves, such as the LIGO experiment.
In optical interferometers, laser beams from two mirrors mix with each other and interference occurs - the light waves either strengthen or cancel each other - depending on the exact position of the mirrors. This allows their microscopic displacements to be measured, because the distance between the interferometric fringes is the same as the wavelength - approximately 0.5-1μm. However, many experiments require even greater precision.
"N00N states could be useful to increase the accuracy even further, because the interference fringes they create are much narrower than the wavelength," explained Philippe Grangier, from France’s L'Institut d'Optique.
Previously, it was impossible to transfer entangled photons over large distances as they would disentangle along the way, but the French-Russian team has now managed to overcome the problem.
"There is a phenomenon called entanglement swapping,” said Lvovsky. “Suppose that Alice and Bob have an entangled state. If I then take one part of Alice's entangled state, and another part from Bob, and I do a joint measurement on them, the remaining parts of Alice's and Bob's states will also become entangled even though they never interacted."