New Technique to Generate Single Photons for Quantum Communication

An innovative method for producing single photons that involves moving single electrons in a specially developed light-emitting diode (LED) has been devised by scientists from the University of Cambridge.

The method has been described in the Nature Communications journal and could be useful for the advancement of the emerging fields of quantum computation and quantum communication.

A single photon, which is the elementary particle of light, has the ability to carry a quantum bit of information for hundreds of kilometers. Consequently, a source with the ability to produce single photons is a crucial building block in several quantum technologies. To date, single-photon sources have been produced in research labs from structural defects in diamonds or self-assembled quantum dots in semiconductors.

These defects and dots tend to form in a random process; thus, it is difficult to estimate the photon energy (or wavelength) and the location of these single-photon sources. The randomness could be a challenge in combining a source with a large quantum network.

In this study, the scientists demonstrate that it is possible to produce a single photon in a different, controlled, manner, without requiring a defect or a quantum dot, by only moving one electron at an instant to recombine with a “hole” (a missing electron in a filled “band” of electrons).

Imagine trying to send a digital message by firing a stream of blue or red balls over a wall in the following way. A conveyor belt with ball-sized indentations drags a series of white balls up a slope and drops the balls off a cliff at the end. Each ball picks up speed as it falls, is then sprayed blue or red (depending on the message) as it bounces off to the side and over the wall.

Dr Tzu-Kan Hsiao, Researcher, University of Cambridge

Dr. Hsiao performed the experiment while pursuing his Ph.D. at Cambridge.

Dr. Hsiao added, “The indentations in the conveyor belt can only carry one ball each. Only one ball gets sprayed at a time, and there’s no chance some of the balls are intercepted by an eavesdropper without the person on the receiving end noticing a missing ball, whereas if sometimes two or more balls come at a time, the eavesdropper can catch odd balls and the receiver is none the wiser. In that way, some of the message may be unintentionally disclosed.”

In the experiment, we made a device near the surface of Gallium Arsenide (GaAs) by using only industry-compatible fabrication processes. This device consists of a region of electrons close to a region of holes, and a narrow channel in between.

Christopher Ford, Professor and Research Team Leader, University of Cambridge

To move only a single electron at a time, a sound wave is launched along the surface. In GaAs, such a ‘surface acoustic wave’ also produces an associated electrical potential wave, where each potential minimum transports just one electron.

Similar to a conveyor belt, the potential wave carries individual electrons to the region of holes one by one. When each electron swiftly recombines with a hole before the arrival of the next electron, a series of single photons is produced.

One of two polarizations can be given to each single photon to carry a message such that it is not possible for an intruder to intercept the message without being detected.

Apart from being an innovative single-photon source, more significantly, the new method could be used to transform an electron spin’s state to the polarization state of a photon. The challenging goal of developing large-scale distributed quantum-computing networks may be realized by bridging semiconductor-based quantum computers by making use of single photons as “flying” qubits.



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