Quantum photonics

Quantum technologies will be able to implement faster algorithms, allow more secure transmission of information, and perform more accurate measurements. For all its theoretical potential, we need a solid experimental platform with which quantum properties can be addressed with ease and precision, whilst simultaneously preserving those fragile nonclassical features. Moreover, the ability to do so even in a hostile environment will be desirable, if not necessary, to facilitate further technology.

Photons are a viable option to probe and exploit quantum phenomena since they do not suffer from thermal coupling with the external world. They also benefit from a range of degrees of freedom onto which quantum states can be prepared, processed and measured with relative simplicity. Given the vast literature which has shown how to tame photons, the aim of our research is now to foster this novel technology to a point where it will realise the promises of the quantum revolution in information processing.

The fundamental unit of quantum optical computation is phase-sensitive interference of two optical modes at a beam splitter. As with classical computation, increasing complexity requires increasing resources. This is difficult to achieve in bulk optics as the “circuit” of beam splitters and phase shifters becomes increasingly large and unstable. One solution is to shrink the circuit to the microscale. To that end, our laboratory is putting a considerable effort in the direction of demonstrating a network of integrated quantum optical sources, circuits and detectors on optical waveguide chips. Integrating components on chip not only increases the density of those components, permitting far more complex operations to be performed, but also decouples the circuit from the environment, significantly reducing errors.

Quantum communication poses a slightly different problem to computation: how to distribute quantum resources over large distances. One proposal to mitigate loss of entanglement is called entanglement distillation: extracting a smaller group of more strongly entangled states from a large group of weakly entangled states.

The states we use to achieve this are multi-photon states. In contrast to single-photon communication protocols, if photons are lost from a multi-photon state, not all of quantum information is lost too. For many applications, multi-photon states offer interesting features: when many photons are involved, a more compact description is obtained by considering the continuous variables of the field. For these to be appealing, though, their quantum state has to be prepared in quite exotic non-Gaussian states, which show quantum signatures such as negative probabilities! We are investigating how well we can prepare and control non-Gaussianity, and how to use it to reduce the effects of loss on quantum entanglement.