PROMETEO · Processing of Mesoscopic Time-pulsed Entangled Optical fields

Processing of Mesoscopic Time-pulsed Entangled Optical fields

Miniaturisation techniques have created powerful and compact computing devices, which have changed our everyday life, and have led to a continuous stream of innovation. This process is however close to attain its physical limits, due to quantum laws at the atomic scale. A possible solution is the quest for a quantum computer, a formidable effort that demands an unprecedented level of control over the proposed platforms, such as ions, neutral atoms, superconducting circuits or photons. In this project, we aim to explore the possibility of a recently proposed photonics architecture based on coherent states of light. Beyond the intrinsic scientific interest in the manipulation of such quantum mesoscopic objects, this platform might offers the best performance in terms of resources needed for fault-tolerance, with respect complementary photonics approaches. In detail, we aimed at developing new experimental tools by which we can improve the quality of the resource states adopted in such an architecture. These are generally produced by three-wave mixing in a non-linear crystal in a configuration known as Optical parametric amplifier (OPA). However, the operation of such devices faces competing requirements: on the one side, we need a high power pump beam to obtain enough nonlinear gain; on the other, the mode of the pump - both in its spatial and spectral features - needs to show high quality in order to preserve at best the quantum character of the resources. The solution we investigated is the amplification of the main laser by means of a femtosecond amplifier; it is crucial to demonstrate how it can perform with a considerable gain, preserving the required high mode quality. Our results give quite a positive return to these instances: the level of amplification achieved and the quality of the beam are greatly sufficient for scale the number the resources towards more ambitious goals. In addition to that, we have now developed a well-assessed experimental procedure for the diagnostics of the amplification system, making it a fully reliable tool for further investigations. A different issue that emerged during this project was the lack of characterisation techniques for the coherent-state architecture. In fact, there exist peculiarities in this approach that prevent using these standard techniques developed for quantum gates in other architectures. We found an interesting approach inspired by the security analysis of quantum key distributions. While it demands a physical model of the gate, it provides a powerful tool as it can assess the quality of the gate with no limitation from the quality and the intensity of the inputs, differently from ordinary process analysis. In addition, we wanted to address the problem of quantifying the non-Gaussian character of the states we can produce. We have conducted an experimental test of the non-Gaussian nature of single-photon added coherent states in collaboration with the University of Milan. A fraction of my activities have also been devoted to the study of a novel conception of quantum-enforced amplification of small signals. This device opens up possibilities for quantum state manipulation, pertinent to our approach to quantum computing, and extending into quantum communications. Within this framework, we have made the first steps towards the understanding of its performance as a quantum relay in a key distribution system; first results have appeared in a paper presenting an extensive analysis of the device. In conclusion, this project has made significant steps in the investigation of coherent-state quantum computing: the first is the development and the integration of a device able to extend the capabilities of resource-state generation. The second is the development of a conceptual framework for the analysis of the hardware in this architecture.

Data: CORDIS, © European Union