CAMBRIDGE, England — Quantum dots (QDs) can provide the control necessary to "squeeze" individual photons — a feat previously considered impossible to observe.
Squeezing is quantum phenomenon that produces an extremely low-noise signal potentially useful in technology designed to pick up faint signals, such as the detection of gravitational waves, as well as in optical communications.
Laser light was used to excite individual quantum dots to create squeezed single photons in a quantum optics laboratory at the University of Cambridge. Courtesy of Mete Atature.
While other experiments have demonstrated the effect in light pulses, this was the first time it has been observed in single photons, according to researchers at St. John's College at the University of Cambridge.
"It's one of those cases of a fundamental question that theorists came up with, but which, after years of trying, people basically concluded it is impossible to see for real — if it's there at all," said professor Mete Atature.
"We managed to do it because we now have artificial atoms with optical properties that are superior to natural atoms. That meant we were able to reach the necessary conditions to observe this fundamental property of photons and prove that this odd phenomenon of squeezing really exists at the level of a single photon. It's a very bizarre effect that goes completely against our senses and expectations about what photons should do."
The standard approach to squeezing light involves firing an intense laser beam at a material, usually a nonlinear crystal, which produces the desired effect. The Cambridge researchers pursued an alternative technique, proposed in 1981, that called for exciting a single atom with a tiny amount of light to produce squeezed photons through resonance fluorescence.
At the quantum level, vacuum fluctuations always produce some degree of electromagnetic noise. Light is said to be squeezed when this noise is lowered below the level of naturally occurring vacuum fluctuations.
The left diagram represents electromagnetic activity associated with light at its lowest possible level, according to the laws of classical physics. On the right, part of the field has been reduced to lower than is classically possible, at the expense of making another part of the field less measurable. This effect is called squeezing because of the shape it produces.
In the Cambridge experiment, the researchers achieved this by shining a faint laser beam onto a QD. This led to the emission of a stream of individual photons. Although normally the noise associated with this photonic activity would be greater than a vacuum state, when the QD was only excited weakly, the noise associated with the light field actually dropped, becoming less than the supposed baseline of vacuum fluctuations.
The researchers achieved this by exploiting Heisenberg's uncertainty principle: In exchange for reducing the noise to an extremely precise and low level, other attributes of the electromagnetic field became less measurable.
Plotting the uncertainty with which fluctuations in the electromagnetic field could be measured on a graph creates a shape where the uncertainty of one part has been reduced, while the other has been extended. This creates a squashed-looking cross section, hence the term "squeezed light."
The demonstration expands our understanding of light, Atature said.
"It's just the same as wanting to look at Pluto in more detail or establishing that pentaquarks are out there," he said. "Neither of those things has an obvious application right now, but the point is knowing more than we did before. We do this because we are curious and want to discover new things. That's the essence of what science is all about."
The research was published in Nature (doi: 10.1038/nature14868).