Researchers at the University of Basel and Ruhr University Bochum have
developed a source of single photons that can produce billions of these
quantum particles per second. With its record-breaking efficiency, the
photon source represents a new and powerful building-block for quantum
technologies.

Quantum cryptography promises absolutely secure communications. A key
component here are strings of single photons. Information can be stored in
the quantum states of these light particles and transmitted over long
distances. In the future, remote quantum processors will communicate with
each other via single photons. And perhaps the processor itself will use
photons as quantum bits for computing.

A basic prerequisite for such applications, however, is an efficient source
of single photons. A research team led by Professor Richard Warburton,
Natasha Tomm and Dr. Alisa Javadi from the University of Basel, together
with colleagues from Bochum, now reports in the journal Nature
Nanotechnology on the development of a single-photon source that
significantly surpasses previously known systems in terms of efficiency.

### "Funnel" guides light particles

Each photon is created by exciting a single "artificial atom" (a quantum
dot) inside a semiconductor. Usually, these photons leave the quantum dot in
all possible directions and thus a large fraction is lost. In the photon
source now presented, the researchers have solved this problem by
positioning the quantum dot inside a "funnel" to send all photons in a
specific direction.

The funnel is a novel micro-cavity that represents the real innovation of
the research team: The micro-cavity captures almost all of the photons and
then directs them into an optical fiber. The photons, each about two
centimeters long, emerge at the end of an optical fiber.

The efficiency of the entire system—that is, the probability that excitation
of the quantum dot actually results in a usable photon—is 57 percent, more
than double that of previous single-photon sources. "This is a really
special moment," explains lead author Richard Warburton. "We've known for a
year or two what's possible in principle. Now we've succeeded in putting our
ideas into practice."

### Enormous increase in computing power

The increase in efficiency has significant consequences, Warburton adds:
"increasing the efficiency of single photon creation by a factor of two adds
up to an overall improvement of a factor of one million for a string of,
say, 20 photons. In the future, we'd like to make our single-photon source
even better: We'd like to simplify it and pursue some of its myriad
applications in quantum cryptography, quantum computing and other
technologies."

## Reference:

A bright and fast source of coherent single photons, Nature Nanotechnology
(2021).
DOI: 10.1038/s41565-020-00831-x

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