Photon twins of unequal origin

The quantum dots of the Basel researchers are different, but send out exactly identical light particles.
Credit: University of Basel, Department of Physics

Researchers have created identical light particles with different quantum dots - an important step for applications such as tap-proof communication.

Many technologies that take advantage of quantum effects are based on exactly the same photons. However, it is extremely difficult to manufacture them. Not only must the wavelength (color) of the photons exactly match, but also their shape and polarization.

A team of researchers from the University of Basel around Richard Warburton, in collaboration with colleagues from the Ruhr University in Bochum, has now succeeded in producing identical photons that come from different, widely separated sources.

Individual photons from quantum dots

In their experiments, physicists use so-called quantum dots, i.e. structures a few nanometers in semiconductor materials. Electrons are trapped in these quantum dots, which only assume very specific energy levels and can emit light when moving from one level to another. With the help of a laser pulse that triggers such a transition, individual photons can be produced at the push of a button.

"In recent years, other researchers with different quantum dots have already produced identical photons," explains Liang Zhai, postdoctoral student and first author of the study published in Nature Nanotechnology. “However, they had to pick those who were most similar from a huge number of photons with optical filters.“By picking out, only a few usable photons remain with this method.

Warburton and his employees chose a different, more demanding path. First, the Bochum specialists produced extremely pure gallium arsenide, from which the quantum dots were formed. In this way, the natural variations between different specimens could be kept as small as possible. In Basel, physicists then exposed two quantum dots using electrodes to precisely dose electric fields. These fields changed the energy levels of the two quantum dots and were set so that the photons emitted by the quantum dots had exactly the same wavelength.

93 percent identical

In order to prove that the photons were actually indistinguishable, the researchers dropped them on a semi-permeable mirror. They observed that the light particles almost always passed the mirror either as a pair or were reflected as a pair. From this, in turn, they were able to deduce that the photons were 93 percent identical. The photons thus formed twin pairs, although they were born completely independently of one another.

In addition, the researchers were able to use the photons to realize an important component of quantum computers, a so-called controlled non-gatter (English controlled-NOT or CNOT gate). Quantum algorithms can be implemented with such gates, which can solve certain problems much faster than classic computers.

"At the moment, our yield of identical photons is still around one percent," admits doctoral student Giang Nguyen, who was involved in the experiment together with his colleague Clemens Spinnler. “But we already know exactly how we can increase them in the future. "This would make the twin photon method ready for possible applications in various quantum technologies.

Source/Credit: Ruhr University Bochum

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