Toward photonic circuits for quantum computers

What recipe enables the integration of crucial components to emit, manipulate and detect individual particles of light on a single chip? During her PhD research at Eindhoven University of Technology, Giulia Digeronimo managed to fabricate several working chips that are able to process individual photons, thus paving the way toward photonic transistors for light-based quantum computing. Digeronimo defended her thesis on October 16th 2018.

The promise of quantum computers is that they will be able to perform a multitude of calculations simultaneously, enabling fast solutions for problems that would literally take ages for a classical computer. Photons – light particles – are seen as promising candidates to carry the quantum information that is processed in a quantum computer. In analogy to classical computers, quantum photonic circuits are needed for such a photon-based quantum computer.

‘The main aim of my research was to integrate a single photon source, a single photon detector and the connecting structure guiding photons onto a single chip. Furthermore, I wanted to demonstrate that it is indeed possible to make a fully functional quantum photonic circuit,’ Digeronimo explains.

Connecting the dots
When she started working on her project, there were different groups working on the development and optimization of highly sensitive detectors that are able to detect the quantum state of a single photon. And there were groups working on light sources that are able to emit one photon at a time with a distinct quantum state. But there were not that many people trying to connect the two and fabricate them from the same material, on the same chip.

Digeronimo explicitly chose to produce the chip in gallium arsenide, though many of the current high-efficiency detectors are based on silicon. ‘Since silicon is not able to emit light, and therefore not suitable to make photon sources, we had to look for another material to integrate all components,’ she explains. ‘Though in gallium arsenide it is difficult to achieve the same high detector efficiencies as in silicon, that is not a big problem,’ she states. ‘Since we are using gallium arsenide for all components, we suffer less optical loss on the chip. That way, a detector efficiency of thirty percent, like we achieved, can be enough, at least for first-generation circuits’

Solving technological problems
Digeronimo struggled her way through a multitude of technological problems to find the optimal balance between the often clashing restrictions set on the production process by the different components. ‘For example, a single-photon detector consists of a five nanometer thick superconducting film, which easily degrades when heated. But to fabricate a high-quality single photon source, you need high process temperatures.’ One of the biggest problems the PhD student had to face was that she had to come up with new ways to realize the photonic crystal in between the source and the detector, which is needed to produce single photons. ‘Usually this crystal is made through an etching step. But that turned out to destroy the detector, so I had to find a workaround.’

Working quantum photonic circuit
In the end, Digeronimo managed to fabricate several working chips containing a single photon source consisting of quantum dots, a waveguide structure to guide the light, a photonic crystal to filter out the desired photons, and several detectors. ‘With these chips we were able to perform state-of-the-art exciton lifetime measurements, thus proving that we actually managed to make a working quantum photonic circuit.’

The PhD student also worked on a more complex circuit design, which incorporates tunable sources and filters, increasing the yield of the single-photon source. This turned out to be much more complicated though, since the tuning required the introduction of probes, which all introduced disturbances that negatively affected the efficiency of the detector.

European quantum information network
During her PhD research, the Italian born physicist was part of PICQUE, the Marie Curie initial training network in photonic integrated compound quantum encoding. The goal of this network is to establish a world-class training platform spreading around the highly interdisciplinary/inter-sectorial European-led area of integrated quantum photonics. ‘That was very nice,’ she says. ‘We got technical training, but also training in starting a new business, how to communicate science and so on. Since I am really interested in this quantum information field, I am now using this network to look for an interesting new position in this field.’

Giulia Enrica Digeronimo defended her thesis entitled ‘Single-photon detectors integrated in quantum photonic circuits’ on Tuesday October 16th at Eindhoven University of Technology. Promotor prof. dr. A. Fiore, co-promotor dr. R. Leoni (National Research Council).