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Mete Atature and researchers at the University of Cambridge conducted an experiment in quantum optics to create single photons that are “squeezed.” Credit: Mete Atature

In 1981, physicists predicted a quantum physics effect called the “squeezing of light” in resonance fluorescence that creates a very specific form of low-noise light. As scientists believed the phenomenon was too difficult to measure, evidence of its existence has remained elusive, until now. Physicists in the Cavendish Laboratory at the University of Cambridge (U.K.) report that they have managed to observe the odd effect via semiconductor quantum dots acting as artificial atoms (Nat., doi: 10.1038/nature14868). The artificial atom features a large optical dipole, which enables a 100-fold improvement in photon detection rate over natural atoms.

The resonance fluorescence effect occurs when an atom or molecule excited with a faint light emits light at the same frequency as the absorbed light. Mete Atature and colleagues used a faint laser beam to excite their quantum dot, causing it to emit individual photons. The photons, like all light, have associated electromagnetic fluctuations or noise that increases with the intensity of the light.  Even at very low intensities, like in absolute darkness, some of this noise still exists, called vacuum fluctuations.

Quantum physics explains this vacuum noise from nonexistent photons via Heisenberg’s uncertainty principle. Whenever a particle is associated with two states, only one can be measured. The other must be uncertain. Atature and his team were able to plot this uncertainty of the vacuum fluctuations: in one measurement, the uncertainty is reduced, while in the other, the uncertainty is extended. Plotting this trade off reveals a shape that is “squeezed” down to three percent below the fundamental minimum floor of possible fluctuation.

The existence of squeezed light is non-classical proof that light is both a particle and a wave. The technique may also have use in quantum optical applications such as interferometry with low quantum noise.