Photonic quantum circuits with only synchronous directional couplers in ion-exchanged glass

Abstract

We show that photonic quantum circuits for implementing arbitrary SU(N) transformations can be made using only synchronous directional couplers. These simple elements provide both beam splitting and phase shifting so that, under a proper concatenation of such elements, unitary transformations of arbitrary dimension can be implemented. The circuits made in this way are robust and controllable, because only one type of element is used and the technology involved in their fabrication is very mature. We present several examples of original passive photonic circuits intended for quantum projective measurements and quantum state generation due to their interest in photonic quantum technology for quantum key distribution systems. Likewise, we have designed, fabricated, and characterized, on an ion-exchanged glass platform, different couplers, which form the basic elements or building blocks of these passive circuits highly compatible with optical fibres.