Brightest Source of Entangled Photons

Optical setup for experiments with entangled photons at IFW Dresden. (Source: J. Loesel)

The rules of quantum physics state, that two photons can interact in such a way that they become deeply linked and remain connected even when separated by great distances. Any change in the quantum state of one photon results in a corre­sponding change in the remote partner. This promises a great potential for application in future quantum commu­nication, in particular for secure quantum crypto­graphy. The efficient generation of entangled pairs of photons is an important prere­quisite for the imple­mentation of such a tech­nology. The transition of photons over long distances is asso­ciated with large losses, so that only 100 kilometers could be realized in fiber optic cables so far. One year ago, Chinese scientists achieved a distance of 7,600 kilo­meters by satellite in empty space. The better the brightness of the photon source, the better the losses over long distances can be tolerated. The develop­ment of bright entangled photon sources is therefore an important approach to realize the long-distance quantum communi­cation.

Scientists from Leibniz Institute for Solid State and Materials Research Dresden IFW and at Leibniz Uni­versity Hannover LUH have set a new record in this respect: A research team headed by Oliver Schmidt and Fei Ding has designed a source of entangled photons with unprece­dented bright­ness. The entangled photon pair effi­ciency of the new device is 37 percent. It consists of a broadband optical antenna that emits entangled pairs of photons very effi­ciently from semi­conductor quantum dots. The antenna operates in a broad wave­length range and is able to emit energeti­cally different photons simul­taneously. Also with regard to other parameters, the new photon source attains top marks: a high single-photon purity of 99.8 percent and a high entanglement fidelity of 90 percent.

“Opti­mizing such a photon source for a variety of proper­ties is a parti­cular challenge to our work,” says Robert Keil, who is currently completing his PhD at the IFW, thus addressing a key problem in quantum tech­nologies. “Our entangled photons are generated by the semiconductor material commonly used in opto­electronics, gallium arsenide,” adds Ding. This makes it possible to produce components based on established semi­conductor tech­nologies and thus suitable for future industrial pro­duction. “The work represents an important step towards exploring the potential of optical quantum tech­nologies”, emphasizes Schmidt, who, with his team, was able to demon­strate the fastest source of entangled photons three years ago.

The research work of IFW and t LUH is funded by the Federal Ministry of Edu­cation and Research BMBF as part of the joint project Q.Link.X aiming at the reali­zation of the core component for long-range quantum communi­cation, a quantum repeater, within three years. A quantum repeater represents the quantum mechanical counter­part to the classical signal amplifier and could revolu­tionize the optical communi­cation known so far. (Source: IFW)

Reference: Y. Chen et al.: Highly-efficient extraction of entangled photons from quantum dots using a broadband optical antenna, Nat. Commun. 9, 2994 (2018); DOI: 10.1038/s41467-018-05456-2

Links: Federal Research Project for a Quantum Repeater Q.Link.X, Berlin, Germany • Institute for Integrative Nanosciences, IFW Dresden, Dresden, Germany

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