Entangled photon pairs from semiconductor waveguides


bboall_c_smallEntanglement is a fascinating concept in quantum physics and a precious resource for quantum communication and computing. It is usually created in particle pairs, frequently by means of a conserved quantity in pair creation processes. Very little is known about the classification and quantification of entanglement in more complex systems, therefore entanglement itself is a phenomenon that is studied intensively worldwide.

Quantum technologies are under intense research and development worldwide. One of them, quantum key distribution has matured far enough that it is a commercial reality, even though it can only establish secure keys over distances up to 50 km. This distance limitation can be overcome by the use of quantum repeaters, another quantum communication concept whose realization faces many obstacles.

Among these obstacles is the limited performance and physical size of current sources of entangled photon pairs. Traditionally these sources employ the phenomenon of spontaneous parametric down-conversion in nonlinear dielectric crystals. While a lot of progress has been made to implement waveguide sources in these nonlinear crystals, by their very nature they defy the idea of integrating the down-conversion with the required pump laser source.

BRW Pair GenerationIn our project we are exploit the same process of parametric down-conversion, but implement it in semiconductors, for example using Bragg-reflection waveguides (BRW). We have shown that BRW are capapble of producing photon pairs and it has been shown independently that lasers can be build from the same structures. These two demonstrations together open a perspective towards an all-integrated source of entanglement.

The possible impact of our research is tremendous. An on-chip source of entanglement will not only allow the development of improved entanglement-based quantum key distribution systems with tiny components, but also enable previously impossible quantum optics experiments by allowing much more complex combinations of sources and detectors. Finally we can envision a complete quantum optics lab on a chip, where lasers, linear and nonlinear elements as well as detectors can all be built in one package.

R. Horn, P. Abolghasem, B. J. Bijlani, D. Kang, A. S. Helmy, and G. Weihs, Monolithic Source of Photon Pairs, Phys. Rev. Lett. 108, 153605 (2012), DOI: 10.1103/PhysRevLett.108.153605.