New ultra-pure photon sources for quantum applications

Weather forecasts, medical diagnostics, pharmaceutical engineering: in these and other areas, novel processing methods are in high demand to develop innovative, accurate and efficient solutions.
These are examples of complex systems with millions of variables, underpinning a nonlinear dynamics that classic supercomputers are unable to solve efficiently.
The human brain can recognize a face within milliseconds, while a computer takes minutes. The huge gap is in the way in which information is represented and processed. Quantum computers can revolutionize this state of things: they can transform the paradigm of "classic" computing and, with quantum mechanics, be able to solve problems intractable on even the most powerful current supercomputers.
The new device that is capable of generating new power for quantum computers fits into this context.  It is a source of photonic qubits (i.e. quantum bits, the basic unit of quantum information) on a silicon integrated circuit, which was developed within a collaboration between the universities of Trento and Bristol. The results of this collaborative research work were published today in "Nature Communications".
"We demonstrated that our innovative integrated chip generates high-quality photons" explained Massimo Borghi, Stefano Signorini and Lorenzo Pavesi at UniTrento, authors of the paper with the colleagues from the Bristol team.
The research focused on single photon sources for quantum computers, a subject of great interest for basic research and technological applications.
"We have written the article and we have also filed a patent application to protect the innovative device jointly as universities of Trento and Bristol" said Lorenzo Pavesi, head of the Nanoscience Laboratory at the Physics Department of the University of Trento.
But behind this success there is a tale of international student mobility. The youngest co-authors, Massimo Borghi and Stefano Signorini, who currently are postdoc researchers at the Physics Department of UniTrento, spent some time at the University of Bristol for, respectively, their postgraduate and doctoral studies.
"We put together the expertise in nonlinear optics and integrated photonics of Trento and the knowledge in quantum optics of Bristol" commented professor Pavesi. "As a doctoral student in Trento – said Stefano Signorini – I studied a new physical process to generate correlated photon pairs in an integrated optical circuit"; "as a postdoc in Bristol – added Massimo Borghi - I came up with the idea to use this process to generate ultra-pure photons. I presented this idea to Stefano and that is how this project begun".
But what are these ultra-pure photons and why are they so important?
"They are like very small bricks but of great quality. This is exactly what you need if you want to create a solid building, an efficient substrate for quantum computation. Based on our simulations, with 150 highly pure and indistinguishable single-photon sources we will be able to make the most of the processing capacity of quantum computing, surpassing the abilities of traditional computers, entering the realm of quantum supremacy", explained Massimo Borghi.
"Purity is important, but there is more. To generate all these photons, we need multiple identical sources that work simultaneously. Our sources are massively scalable. We can build hundreds of them on the same integrated, miniaturized device. The bricks must be perfect but we also need them in great quantity and in a limited amount of space, and they must all look the same" added Stefano Signorini.
To give an idea of the business potential of the device, specialized journals point to PsiQuantum, a start-up that was established in Palo Alto, California, with the aim to develop quantum computers based on silicon photonics, starting from the studies carried out at the University of Bristol. In a matter of months, the start-up raised funds amounting to more than 200 million dollars, which means that investors are very interested in quantum computing.
"We would like to thank our colleagues of the University of Bristol. Their work was crucial. We hope that this is just the beginning of a long and fruitful collaboration", concluded the researchers of UniTrento.
About the article
The article "Near-ideal spontaneous photon sources in silicon quantum photonics" was written by Massimo Borghi, Stefano Signorini and Lorenzo Pavesi of the University of Trento with Stefano Paesani, Alexandre Mainos and Anthony Laing of the University of Bristol.
It appeared in "Nature Communications" on 19 May 2020.