A photonic nanostructure for constructing quantum photonic circuits for quantum networks has been developed, which could impact the optics and photonics that underlie quantum information processing. Photons (unlike electrons) display weak interaction with their environment, so they do not lose a lot of energy during transmission. This makes them well suited to carrying and distributing information over long distances. A quantum network based on photons would be able to encode much more information than is possible with existing technology — and the information could not be intercepted while in transit. Professor Peter Lodahl in the Quantum Optics Laboratory at the Niels Bohr Institute in Copenhagen. Courtesy of Ola Jakup Joensen, NBI. However, since information for quantum communication based on photonics is encoded in a single photon, it is necessary to emit and send the photons one at a time. One prerequisite for quantum networks would be the ability to create a stream of single photons on demand. Researchers at the Niels Bohr Institute recognized that photonic nanostructures, because of their strong light confinement, could be used to lock the local polarization of light to its propagation direction, allowing propagation-direction-dependent emission, scattering, and absorption of photons by quantum emitters. "We have developed a photonic chip, which acts as a photon gun,” said professor Peter Lodahl. The photonic chip consists of a crystal that is 10 μm wide and 160 nm thick. Embedded in the middle of the chip is a quantum dot that serves as a light source. “Illuminating the quantum dot with laser light excites an electron, which can then jump from one orbit to another and thereby emit a single photon at a time. Photons are usually emitted in all directions, but the photonic chip is designed so that all the photons are sent out through a photonic waveguide," said Lodahl. This is an illustration of a photon gun. A quantum dot (the yellow symbol) emits one photon (red wave packet) at a time. The quantum dot is embedded in a photonic crystal structure, which is obtained by etching holes (black circles) in a semiconductor material. Due to the holes, the photons cannot be emitted in all directions, but only along the waveguide, which is formed by omitting a number of holes. Courtesy of Søren Stobbe, NBI. The research team developed and tested the photonic chip until they succeeded in getting the photon emission to occur in a novel way. Normally, photons are transmitted in both directions in the photonic waveguide. The team’s custom-made photonic chip allowed this symmetry to be broken, enabling the quantum dot to differentiate between emitting a photon to the right or to the left — in short, to emit directional photons. By customizing the photonic chip, the team was able to achieve full control of the photons. The team is now beginning to explore ways to construct a complete quantum network system based on their discovery. This is a directional emission of photons. The figure shows the calculations of the photon emission in the new directional single-photon source. If the spin of the quantum dot's electron points up, the photon will be emitted in the one direction (blue). If the spin of the quantum dot's electron points down, the photon will be emitted in the opposite direction (red). Courtesy of Sahand Mahmoodian and Søren Stobbe, NBI. According to the research team, the possibility of such a propagation-direction-dependent, or chiral, light-matter interaction could lead to novel applications in the field of quantum optics. Specifically, it could enable the assembly of nonreciprocal single-photon devices that could be operated in a quantum superposition of two or more of their operational states — and to the realization of deterministic spin-photon interfaces. Moreover, the researchers believe that engineered directional photonic reservoirs could lead to the development of complex quantum networks. “The photons can be sent over long distances via optical fibers, where they whiz through the fibers with very little loss. You could potentially build a network where the photons connect small quantum systems, which are then linked together into a quantum network — a quantum internet,” said Lodahl, adding that the challenge is now to expand the basic functionality of chiral quantum optics to large, complex quantum networks. The research was published in Nature (doi:10.1038/nature21037).