Pascale Senellart developed a method to efficiently generate single photons using quantum dots. She discussed her career, her quantum tech startup, and her life in this interview.

Pascale Senellart: "I have to do quantum physics"

She brought quantum light to the world: French physicist Pascale Senellart developed a method to efficiently and specifically generate single photons using so-called quantum dots. Today, her startup brings this principle to the market. After completing her doctorate in 2001, the 49-year-old joined the French national research organization CNRS, where she now works at the Centre for Nanoscience and Nanotechnology (C2N). At the same time, she is a professor at the Ecole Polytechnique.

Her first contact with quantum physics? "I was 20 years old then, and I was in the oral entrance exam for the elite university École Normale Supérieure," Senellart recounts. "I had never learned about quantum physics and the examiners asked me what would happen if you sent a photon through two slits." At first, she didn't have a clue, but when the professors gave her a little encouragement and suggested the Heisenberg uncertainty principle, she was able to successfully deduce the phenomenon: The light-typical interference of photons disappears the moment you try to measure which slit the photon had flown through. "After that, I knew: Oh, I have to do quantum physics" recalls Senellart. "That one test determined the rest of my path."

In her studies she was significantly influenced by the lectures of brilliant atomic physicists like physics Nobel laureate Serge Haroche and semiconductor luminaries like Claude Weisbuch. "While I was fascinated by atomic physics, somehow it was a little too perfect and too clean for me," the physicist says. "I found semiconductors just as exciting; there are vibrations, there's the noise of the charges." So Senellart decided on a project that combines the two directions: exploring quantum dots. "It's a combination of the two fields I love most in physics."

Quantum dots are tiny semiconductor nanostructures embedded in another semiconductor. Because charge carriers can move within them only in a very limited way, they act like artificial atoms. If you put energy into the quantum dots, they send out individual photons: extremely weak pulses of light of a specific color. These photons could be useful for various future technologies, such as quantum computers and quantum cryptography (which ensures tap-proof data transmission).

For a long time, however, there was a problem with quantum dots: they emit their photons randomly in all directions, making it difficult to capture them specifically and use them for applications. This is where the work of Pascale Senellart and her team came in: it had been known for some time that the quantum dot could be made to emit photons in a specific direction with the help of a so-called microcavity - an arrangement of two mirrors facing each other. The only problem is that the quantum dots are not produced at predictable positions, but are distributed randomly throughout the semiconductor crystal.

Around 2006, Senellart came up with an idea: "Instead of trying to force the quantum dots into a desired location, we found a way to measure their position and craft microcavities around them," the physicist says. "This has allowed us to efficiently collect the photons emitted by the artificial atoms." The microcavity not only makes the quantum dot emit the light particles in one direction, but also speeds up their emission. For her work, Senellart was awarded the prestigious silver medal of the French research agency CNRS in 2014 and the “Grand Prix Mergier-Bourdeix” of the French Science Academy in 2021.

Over time, the team was able to continuously increase the performance of its "nanolamps", for example by improving the quality of the mirrors. "At the beginning, I was only concerned with the basics; I just wanted to show that we could get artificial atoms to emit in a certain direction," Senellart recalls. "But then more and more people asked us if they could also have a light source like that. So, we started thinking about applications for our technology."

In 2017, Senellart together with two members of her team – Niccolo Somaschi and Valérian Giesz – founded the startup Quandela with the goal of commercializing quantum dot light sources. "We have since sold several systems around the world - to research groups in Australia, Russia, Austria, Italy and the Netherlands," the researcher reports. "The first version was still a bit difficult to use, but now the product is much more user-friendly." Now the startup is working on a much more ambitious goal -an optical quantum computer based on the luminous quantum dots. These are intended to function as qubits, the elementary computational units of a quantum computer.

"Right now, there's a lot of hype around quantum computing, and it's possible that it will be followed by a lean period. But hype like that creates a lot of momentum and creativity"

"In my lab, we have already realized a prototype with six qubits," Senellart reports. "It combines our quantum dot light sources with a chip that integrates components such as waveguides, phase shifters, and beam splitters. This allows simple optical quantum calculations to be performed." More qubits would be needed for greater computing power; over time, the researcher would like to get a system up and running with 20 or even 40 qubits. To obtain even more computing power, however, would require changing the strategy to work with entangled photons that are related to each other after they are created together, almost as if by a spooky action at a distance. "But there is still a long way to go before we get there, similarly to other approaches to quantum computing," the physicist estimates. "We still need many, many more advances."

But isn't there a danger that industry, which has high expectations for the future computing behemoth, will lose patience if development times are too long? "Right now, there's a lot of hype around quantum computing, and it's possible that it will be followed by a lean period," Senellart answers. "But hype like that creates a lot of momentum and creativity. And I'm confident that on the long road to quantum computing, we'll develop many things that will then lead to technological applications we're not even thinking about today."

Further Viewing

Pascale Senellart was an MCQST Distinguished Lecturer in 2020. As part of this program, she gave a public lecture at the Bavarian Academy of Sciences. You can watch the recording below.