Identical light particles (photons) are important for many technologies that
are based on quantum physics. A team of researchers from Basel and Bochum
has now produced identical photons with different quantum dots—an important
step toward applications such as tap-proof communications and the quantum
internet.
Many technologies that make use of quantum effects are based on exactly
equal 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 originating from different and
widely-separated sources.
Single photons from quantum dots
In their experiments, the physicists used so-called quantum dots, structures
in semiconductors only a few nanometers in size. In the quantum dots,
electrons are trapped such that they can only take on very specific energy
levels. Light is emitted on making a transition from one 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 the study that was recently published 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." In
that way, only very few usable photons remained.
Warburton and his collaborators chose a different, more ambitious approach.
First, the specialists in Bochum produced extremely pure gallium arsenide
from which the quantum dots were made. The natural variations between
different quantum dots could thus 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, and they were adjusted in such a way that the photons emitted by the
quantum dots had precisely the same wavelength.
93% identical
To demonstrate that the photons were actually indistinguishable, the
researchers sent them onto 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 to realize 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 one percent,"
Ph.D. student Gian Nguyen concedes. Together with his colleague Clemens
Spindler he 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.
Reference:
Liang Zhai et al, Quantum interference of identical photons from remote GaAs
quantum dots, Nature Nanotechnology (2022).
DOI: 10.1038/s41565-022-01131-2
Tags:
Physics