Quantum repeaters on the horizon?
Scientists have developed a broadband optical antenna for highly efficient extraction of entangled photons. With a yield of 37% per pulse, it is billed as the brightest source of entangled photons reported so far.
Working at the Leibniz Institute for Solid State and Materials Research Dresden (IFW), and at Leibniz University Hannover (LUH), the scientists note that 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 has great potential for application in future quantum communication, and 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.
However, the transition of photons over long distances is associated with large losses, so that only 100 km could be realised in fibre optic cables so far. 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 achieve long-distance quantum communication.
The work at IFW and LUH sets a new record in this respect. A research team headed by Professor Oliver G. Schmidt and Professor Fei Ding has designed a source of entangled photons with unprecedented brightness. The entangled photon pair efficiency of the new device is 37 %. 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. With regard to other parameters, the new photon source also delivers impressive results: a high single-photon purity (99.8%) and a high entanglement fidelity (90%).
“Optimising 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.
“Our entangled photons are generated by the semiconductor material commonly used in optoelectronics, gallium arsenide,” adds Professor Ding. This makes it possible to produce components based on established semiconductor technologies and which are thus suitable for future industrial production.
“The work represents an important step towards exploring the potential of optical quantum technologies”, emphasises Professor 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. This aims at the realisation of the core component for long-range quantum communication, a so-called quantum repeater, within three years. A quantum repeater represents the quantum mechanical counterpart to the classical signal amplifier and could revolutionise optical communication as we know it. .
Photo caption: Optical setup for experiments with entangled photons at IFW Dresden; photo Jürgen Loesel