We have constructed a quantum amplifier that decreases the uncertainty about the state’s phase.
Counter-intuitively, the concept involves the addition of thermal noise.
Phase insensitive optical amplification of an unknown quantum state is known to be a fundamentally noisy operation that inevitably adds noise to the amplified state. However, this fundamental noise penalty in amplification can be circumvented by resorting to a probabilistic scheme as recently proposed and demonstrated. These amplifiers are based on
highly non-classical
resources in a complex interferometer. Here we demonstrate a probabilistic quantum amplifier beating the fundamental quantum limit utilizing a
thermal noise
source and a photon number subtraction scheme. The experiment shows, surprisingly, that the addition of noise to the amplifying process leads to a noiselessly amplified output state with a phase uncertainty below the uncertainty of the pre-amplified state. This amplifier might become a valuable quantum tool in future quantum metrological schemes and quantum communication protocols.
The article has been published in Nature Physics and can be found here (or arXiv: 1005.3706)