Researchers have created an all-optical transistor capable of being switched on and off by a single photon, or light particle - an impressive demonstration of physics that arguably places scientists one step closer to the complete harnessing of optical computing, the act of using light rather than electricity to perform calculations.
Much research has been done in the pursuit of similar devices based on perceived benefits for both quantum and conventional computing. However, again and again scientists have run into the problem of getting photons to modify each other's behavior, something that they're naturally averse to doing. For example, when two photons collide in a vacuum, they simply pass through each other.
In order to overcome this obstacle, the international team of scientists used a pair of highly reflective mirrors that, when switched on, allowed for the passage of an optical signal, or a beam of light. When switched off, only about 20 percent of the light in the signal squeezed its way through.
Together, these mirrors constitute what's known as an optical resonator.
"If you had just one mirror, all the light would come back," Vladan Vuletić, the Lester Wolfe Professor of Physics at Massachusetts Institute of Technology and lead researcher, said in a press release. "When you have two mirrors, something very strange happens."
Light can be thought of as particles but it can also be thought of as a wave - an electromagnetic field, Vuletić explains. Even though photons are stopped by the first mirror, the electromagnetic field laps into the space between the mirrors. When the distance between the mirrors is precisely calibrated to the wavelength of the light, according to the scientist, "a very large field builds up inside the cavity that cancels the field coming back and goes in the forward direction."
In other words, the mirrors become transparent to light of the right wavelength.
The researchers filled this cavity with a gas of supercooled cesium atoms, and while ordinarily these atoms don't interfere with the light passing through the mirrors, if a single "gate photon" is fired into their midst at a different angle, kicking just one electron of one atom into a higher energy state, it changes the physics of the cavity enough that light can no longer pass through it.
Ultimately, however, clouds of supercooled atoms do not represent a practical design for the transistors in, for example, a Web server - something the researchers readily admit.
"For the classical implementation, this is more of a proof-of-principle experiment showing how it could be done," Vuletić said. "One could imagine implementing a similar device in solid state - for example, using impurity atoms inside an optical fiber or piece of solid."