The team, led by Chao-Yang Lu and Jian-Wei Pan of the University of Science and Technology of China in Hefei, and Sven Höfling of Würzburg University in Germany, has succeeded in producing single photons on demand – that is, for every laser pulse shot at a quantum dot, one and only one photon is generated. The photons produced also possess the important property of “near-unity indistinguishability”, or put another way, they are nearly identical to each other. “Our work nicely combines these two features: deterministic generation of highly identical single photons,” said Lu.

Practical quantum computers may require up to hundreds of single photons to work. The photons act as quantum bits (fundamental computational units) that go through numerous controlled quantum operations. To scale up such systems, the single photons need to be supplied deterministically – that is, one by one. If this is not the case, the overall probability of success rapidly falls to zero, explains Lu.

The single photons also need to be quantum mechanically identical so that the computer can take advantage of quantum interference effects – to make, for example, devices such as controlled-NOT gates between quantum bits, he adds.

The team, which also includes researchers from the Cavendish Laboratory in Cambridge in the UK, began by shining a pulsed laser on a single quantum dot. A quantum dot can be thought of as an artificial atom permanently positioned on a semiconductor chip. The laser pulse lasts just 3 picoseconds and has a central wavelength that is tuned to exactly match the quantum dot optical transition. The laser excites the quantum dot from the ground, vacuum, state to an excited state made up of a bound electron-hole pair within a very short time. The electron and hole pair then recombines, emitting just one photon.

Record-breaking quality

“Ours is the highest quality single-photon source so far achieved in a semiconductor quantum dot system, reaching for the first time a level comparable to those produced by more conventional, natural, sources like trapped atoms and ions,” Lu told nanotechweb.org. “Moreover, our work on so-called s-shell pulsed excitation in quantum dots is the first to go beyond the previous record holding p-shell pulsed excitation technique developed 10 years ago by Yoshihisa Yamamoto’s team at Stanford University.”

And that is not all: the single-photon source developed by the researchers can be scaled up to more photons, which can then be used in multi-photon entanglement and simple quantum algorithms – for example, those employed in the famous “Grover’s database”. “Perhaps more importantly still, we expect that the high-quality single-photon source we have developed will be helpful in quantum interference between single photons from separate quantum dots – a necessary step when it comes to entangling and interacting distant quantum dot spins,” said Lu.

However, all is not perfect, he adds. The main problem is that only a few percent of the photons generated can be collected in the planar microcavity employed in these experiments. Ideally, this figure should be nearer 100%.

“We are busy working to overcome this challenge by improving photon extraction efficiencies in better-designed microcavities and other novel optical structures,” revealed Höfling. “We are also looking at applying pulsed resonance fluorescence techniques to deterministically generate high-quality entangled photon pairs from the semiconductor quantum dots.”

The current research is detailed in Nature Nanotechnology.