Identical light particles (photons) are essential for several quantum-physics-based technologies. A team of Basel and Bochum researchers has created identical photons with distinct quantum dots, which is a crucial step toward applications such as tap-proof communications and the quantum internet.
Many quantum-based devices rely on photons that are all precisely the same. However, creating such photons is incredibly challenging. Not only must they have the same wavelength (color), but they must also be the same form and polarity.
In partnership with colleagues at the University of Bochum, a team of researchers led by Richard Warburton at the University of Basel has now succeeded in producing identical photons originating from distinct and widely dispersed sources.
Single photons from quantum dots
The researchers employed quantum dots, which are structures in semiconductors that are only a few nanometers in size, in their tests. Electrons are confined in quantum dots and can only take on highly particular energy levels. When moving from one level to another, light is emitted. Single photons can therefore be manufactured at the push of a button using a laser pulse that causes such a transition.
“Other researchers have already made similar photons with various quantum dots in recent years,” Lian Zhai, a postdoctoral researcher and first author of the work published in Nature Nanotechnology, says. “However, they had to choose and choose the most comparable photons from a large number of photons using optical filters.” As a result, there were just a few useful photons left.
Warburton and his colleagues took a different, more daring approach. First, the Bochum specialists created very pure gallium arsenide from which the quantum dots were created. Natural differences between distinct quantum dots might therefore be minimized. The Basel researchers then used electrodes to subject two quantum dots to carefully adjusted electric fields. These fields changed the quantum dots’ energy levels, and they were adjusted so that the photons released by the quantum dots all had the same wavelength.
The researchers used a half-silvered mirror to demonstrate that the photons were indeed indistinguishable. They noticed that the light particles nearly always traveled through the mirror as a pair or were reflected as a pair. They may deduce that the photons were 93 percent similar based on this finding. In other words, even though the photons were “born” entirely independently of one another, they created twins.
Furthermore, the researchers were able to actualize a key component of quantum computers, the controlled NOT gate (or CNOT gate). Quantum algorithms that handle specific problems significantly quicker than conventional computers can be implemented using such gates.
“Right now, our yield of identical photons is only about 1%,” admits Ph.D. student Gian Nguyen. He assisted his colleague Clemens Spindler in carrying out the experiment. “However, we already have a pretty good understanding of how to boost that yield in the future.” This would prepare the twin-photon technique for prospective applications in other quantum technologies.
(Image: University of Basel, Department of Physics)