Scientists at Cambridge University have squeezed light in a manner thought impossible to enact, or at least to observe. In the process they reduced the electromagnetic noise associated with light to less than that measured in the complete absence of light.
To non-physicists the idea of squeezing light at all sounds crazy, but we have been squeezing light, or at least squeezing the random variation (noise) of light, for decades. We've even used the technique for optical communication and in quantum entanglement projects. Now Cambridge University scientists have found a new way to do the same thing, and while they don't yet know if it will have any applications, they're excited simply to have performed an experiment that was previously thought to be impossible to measure properly.
To a physicist, squeezing light means to remove the noise that is normally an inherent property of electromagnetic radiation. Noise is essentially tiny fluctuations at a quantum level, depending on how intense the light is. Under Heisenberg’s Uncertainty Principle we can’t eliminate the noise, but we can shift it around, increasing the precision of the amplitude (size of the wave) at the expense of its phase consistency (knowing when it peaks), or vice versa.
The established method uses non-linear crystals and powerful lasers and operates on a large scale, rather than with single atoms and small numbers of photons.
In 1981 Daniel Walls and Peter Zoller proposed a different technique, which was dubbed "squeezing of resonance fluorescence". This would involve using very small amounts of light and a single atom. While sound in theory, many physicists concluded it couldn't be measured. Without being measurable, it might as well not occur.
However, Mete Atature, of St John's College at the University of Cambridge, has announced in Nature a demonstration of squeezed resonance fluorescence. The key that has made it possible is the creation of artificial atoms known as semiconductor quantum dots.
"We managed to do it because we now have artificial atoms with optical properties that are superior to natural atoms,” said Atature in a statement. “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 universe is filled with electromagnetic noise, which can be defined as a measure of uncertainty in the strength of the electromagnetic field. One of the consequences of quantum mechanics is that this noise exists even in a total vacuum. When we emit light, such as by shining a laser on an object, the noise normally increases. However, just as the Uncertainty Principle allows us to be very accurate in measuring a particle’s location, as long as its momentum is poorly known, it allows us to turn down the fluctuations in an electrical field as long as we have very little information about the radiation's phase.
Atature and the team put this theory to use, creating a trade-off between what could and could not be measured. They squeezed the uncertainty in the amplitude of the electromagnetic field to 3% below vacuum fluctuations, in other words, less than what is normally the lowest level in the universe. In the paper, Atature and his colleagues said this "appears counter-intuitive.” This sort of squeezing is possible, they note, only through the behavior of a photon as both a wave and a particle, something with "no classical analogies" from the macroscopic world where quantum mechanics effects are not seen.
The squeeze could be detected because the quantum dots enable a “100-fold improvement of the photon detection over the natural atom counterpart,” the authors write, but the same effect would be expected to occur with natural atoms too.
