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03 Dec 2007
Clever use of quantum feedback and single photons has produced the most sensitive interferometer yet devised, with a precision approaching the Heisenberg limit.
Interferometers detect the differences in phase between two separate light beams, and have become a crucial tool for measuring tiny distances with high precision. But their sensitivity is limited by so-called shot noise in the beams, arising from the wave-particle duality of light. These small fluctuations in the phase of the beams affect the accuracy of the measurements being made (Nature 450 393).
The scale of this problem is inversely related to the number of photons in the beam: the more intense the laser beam, the less fluctuation is observed. Increasing the laser power in interferometers is often problematic so in practice the phase uncertainty cannot be improved beyond a value known as the standard quantum limit, related to the square root of the number of photons involved, 1⁄√N. But now a team from three Australian universities has managed to do better.
"It should be possible to achieve a precision limited only by the Heisenberg uncertainty principle, dramatically improving the scaling to 1⁄N," said the team, including Howard Wiseman from Griffith University's Centre for Quantum Dynamics. "This improvement is commonly thought to require the use of exotic quantum entangled states, which are extremely difficult to generate."
Now the team claims to have reached the Heisenberg limit using unentangled single-photon states, dubbed "kitten" states after the famous Schrödinger's cat. Each kitten involves a single photon, generated from a 820 nm source with a bandwidth of 2 nm.
"We replaced entangled input states with multiple applications of unentangled single-photon states, a drastic reduction in the complexity of achieving quantum-enhanced measurement precision," said Wiseman.
Single photons are easier to prepare and more robust against noise and loss than the more complex entangled sets used previously. The lower flux is compensated by cycling the states through the interferometer many times, and using a complex quantum feedback loop to adjust the phase shift of the reference beam.
“A drastic reduction in the complexity of achieving quantum-enhanced precision.”
"We estimate an unknown phase with a variance more than 10 dB below the standard quantum limit when using 378 resources (ie. photons) in our algorithm," said Wiseman's team. "Achieving the same level of uncertainty using standard interferometic techniques would require 4333 resources."
The discovery is too low in intensity to be of use in either common interferometery or more advanced experiments such as the Laser Interferometer Gravitational Wave Observatory (LIGO), but could have more immediate applications in areas such as quantum metrology, quantum imaging and quantum sensing.
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