An artistic representation of separate quantum dots emitting identical photons. Image: University of Basel, Department of Physics.
An artistic representation of separate quantum dots emitting identical photons. Image: University of Basel, Department of Physics.

Identical light particles (photons) are important for many technologies based on quantum physics. A team of researchers from the universities of Basel in Switzerland and Bochum in Germany has now produced identical photons from separate quantum dots – an important step towards applications such as tap-proof communications and the quantum internet.

Many technologies that make use of quantum effects are based on identical photons. Producing such photons, however, is extremely difficult. Not only do they need to have precisely the same wavelength (color), but their shape and polarization also have to match.

A team of researchers led by Richard Warburton at the University of Basel, in collaboration with colleagues at the University of Bochum, has now succeeded in creating identical photons from different and widely separated sources.

In their experiments, the physicists used so-called quantum dots, which are tiny crystal semiconductors only a few nanometres in size. In quantum dots, electrons are trapped such that they can only take on very specific energy levels. Light is emitted by a quantum dot when an electron transitions from one energy level to another. With the help of a laser pulse that triggers such a transition, single photons can thus be created at the push of a button.

“In recent years, other researchers have already created identical photons with different quantum dots,” explains Lian Zhai, a postdoctoral researcher and first author of a paper on this work in Nature Nanotechnology. “To do so, however, from a huge number of photons they had to pick and choose those that were most similar using optical filters.” This meant they were only able to obtain a few usable photons.

Warburton and his collaborators adopted a different, more ambitious approach. First, the specialists in Bochum produced extremely pure gallium arsenide, from which the quantum dots were made, allowing the natural variations between separate quantum dots to be kept to a minimum. The physicists in Basel then used electrodes to expose two quantum dots to precisely tuned electric fields. Those fields modified the energy levels of the quantum dots in such a way that the photons emitted by the quantum dots had precisely the same wavelength.

To demonstrate that the photons were actually indistinguishable, the researchers directed them to a half-silvered mirror. They observed that, almost every time, the light particles either passed through the mirror as a pair or else were reflected as a pair. From that observation they could conclude that the photons were 93% identical. In other words, the photons formed twins even though they were ‘born’ completely independently of one another.

Moreover, the researchers were able use the identical photons to produce an important building block of quantum computers, a so-called controlled NOT gate (or CNOT gate). Such gates can be used to implement quantum algorithms that can solve certain problems much faster than classical computers.

“Right now, our yield of identical photons is still around 1%,” said PhD student Gian Nguyen. Together with his colleague Clemens Spindler, Nguyen was involved in running the experiment. “We already have a rather good idea, however, how to increase that yield in the future.” That would make the twin-photon method ready for potential applications in different quantum technologies.

This story is adapted from material from the University of Basel, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.