single photons
The new single-photon source is based on excitation of a quantum dot (shown as a bulge on the bottom left), which then emits photons. A micro-cavity ensures that the photons are guided into an optical fiber and emerge at its end. Credit: University of Basel, Department of Physics

Scientists 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 addresses another and powerful building-block for quantum technologies.

Quantum cryptography guarantees totally secure communications. A key component here is strings of single photons. Information can be stored in the quantum states of these light particles and transmitted over long distances. Later on, remote quantum processors will communicate with each other via single photons. Also, maybe 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 group led by — Professor Richard Warburton, Natasha Tomm, & 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 made by exciting a single “artificial atom” (a quantum dot) inside a semiconductor. Usually, these photons leave the quantum dot in all possible directions & thus a large fraction is lost. In the photon source now presented, the scientists 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 group: 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%, more than double that of previous single-photon sources. “This is a really special moment,” explains lead author Richard Warburton. “We have known for a year or 2 what is possible in principle. Now we have succeeded in putting our ideas into practice.”

An 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, twenty 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.”

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