Light caught among crystals on macroscopic scale

Integrated chips emit photons on demand at room temperature

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Researchers from KTH Royal Institute of Technology in Stockholm have developed light emitters for quantum circuits which operate without the need for extreme refrigeration, in a potential step forwards for quantum information.

The promise of a quantum internet - a network in which quantum devices exchange information using the laws of quantum physics - depends on harnessing light to transmit quantum information reliably over fibre-optic networks. A potential step towards making this a physical reality has been marked with the development of integrated chips which can generate photons on demand at room temperature.

Quantum computing, which remains at a nascent stage of technological development, relies on maintaining fragile states of matter and preventing interference from the environment.

Integrating quantum computing seamlessly with fibre-optic networks to create a quantum internet could be approached using optical photons (quanta of visible light). Professor Val Zwiller of KTH, co-author of the Advanced Quantum Technologies study, explained: “The photonic approach offers a natural link between communication and computation. That’s important, since the end goal is to transmit the processed quantum information using light.”

In order for photons to deliver qubits for quantum systems, they need to be emitted in a deterministic rather than probabilistic fashion; this can be simply described as emitting photons on demand rather than leaving it to chance. This had been accomplished at extremely low temperatures in artificial atoms.

Now, the KTH research group has found a method for producing photons on demand at room temperature, using optical integrated circuits. It enables photon emitters to be positioned precisely within integrated optical circuits which resemble copper wires but carry photons, rather than electrons.

The researchers used hexagonal boron nitride, a layered material with the useful property of emitting individual photons and often used in ceramics, alloys, resins, plastics and rubbers to give them self-lubricating properties. They integrated this material with silicon nitride waveguides to direct the photons.

According to Professor Ali Elshaari, another co-author of the study, quantum circuits which use light are either operated at temperatures near absolute zero with single photon sources or at room temperature using random single photon sources. By contrast, this new technique allows for on-demand emission of photons at room temperature, pairing improved control and convenience.

“In existing optical circuits operating at room temperature, you never know when the single photon is generated unless you do a heralding measurement,” said Elshaari. “We realised a deterministic process that precisely positions light-particle emitters operating at room temperature in an integrated photonic circuit.”

In the first stage of their work, they coupled a single photon emitter to the silicon nitride waveguides and developed a method to image the quantum emitters. Next, they built the photonic circuits with respect to the locations of the quantum sources, via electron beam lithography and etching.

Their achievement opens a path to hybrid integration - incorporating atom-like single photon emitters into photonic devices which cannot emit light efficiently on demand.

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