Photonic cluster states from diamond

The main objective of the proposed research project is to build a diamond-based single-photon source that is capable of emitting strings of entangled photons for resource-efficient photonic quantum computing. This addresses the main challenge of measurement-based quantum computing in realising scalable multi-photon quantum information processing. We will follow recent theoretical work to realise the generation of a string of single photons that are entangled as a cluster state the generic resource for measurement-based quantum computing. To achieve this ambitious goal we will push current technology far beyond state-of-the-art by obtaining high light-collection efficiency from and advanced quantum control of single-photon emitters based on nitrogen-vacancy centres in diamond. To date, these light sources have been only used for obtaining individual, un-entangled photons. This will break new ground for the efficient generation of entangled photons, and will make large regions of previously inaccessible quantum state space available for modern quantum technologies. We will develop new theory to perform feasible quantum state tomography of the emitted light by using only passive optical elements. Furthermore, we will use new experimental methods for the implementation of quantum gates; the combination of fast electro-optical switches and high- efficiency superconducting detectors will enable adaptive measurements for error correction and deterministic quantum logic operations. The results of these experiments will be crucial in the development of scalable quantum computing in realistic scenarios, and open alternative perspectives for novel applications using quantum-enhanced information technology.

Devices which provide an interface between light and matter are strong candidates for building blocks of future quantum networks and quantum computers. Several systems currently under examination present promising features, but none of them yet fulfil all requirements for these applications. A particularly attractive protocol for the operation of these devices is the emission of strings photons which are entangled1 with one another. Entanglement between the photons is mediated by sending control pulses to the electronic spin2 of the emitter before and in-between photon emission events. We demonstrate a scalable protocol based on this principle, through the generation of entanglement between the polarisation of a photon and the electronic spin of the emitter. Many of the imperfections of currently available emitters are bypassed using this approach. Our method relies on the conversion of photons correlated in the time domain to photons correlated in polarisation which allows it to be performed with a large variety of emitters. We demonstrate the principle using a single (nitrogen-vacancy) defect centre in diamond. The electronic spin of the emitter has a long coherence lifetime, thereby offering an outlook towards the creation of large entangled states. 1 Entanglement is a special property seen in physics which intrinsically correlates one or more degrees of freedom of quantum particles (such as photons) before a measurement is taken on them. This special property makes entanglement a key ingredient for many quantum information applications. 2 Electronic spin characterises the intrinsic angular momentum that every electron possesses.