International team of scientists finds new and simpler way to generate 'quantum light'

An international team, including University of Calgary scientists, has discovered a simpler and more efficient way to generate entangled states of light, or “quantum light.”

In a new study, co-led by physicists from UCalgary and the French National Centre for Scientific Research (CNRS), the team theoretically proposed and experimentally demonstrated a novel and simple way to control the fundamental process of spontaneous emission from an atom to generate new states of quantum light.

“These states of light could be used for quantum sensing applications or as building blocks to construct resource states for quantum information processing,” says Dr. Christoph Simon, PhD, professor of physics in the Department of Physics and Astronomy in the Faculty of Science.

Entanglement and spontaneous emission are fundamental quantum phenomena that drive many applications of quantum physics.

Cornerstone of quantum computing and secure networks

Quantum light, which can be used to encode and transport information, is the cornerstone of applications ranging from quantum computing to highly secure quantum networks.

“This is the first time, in theory and in experiment, of generating an entangled state of light in this way,” says Dr. Stephen Wein, a PhD student in Simon’s research group when the study was done.

“Not only did we propose this scheme for the entangled state, we were able to prove that this state of light is entangled,” says Wein, the lead author on the team’s published study.

The experiments were performed in France by the research group of Dr. Pascale Senellart, PhD, CNRS senior researcher at the Centre for Nanosciences and Nanotechnologies at the Université Paris-Saclay.

Dr. Carlos Antón-Solanas, PhD, then a post-doctoral researcher with Senellart’s group, implemented the experiments and played a major role in both the experimental and theoretical development of the project.

In their experiments, the researchers used semi-conductor “quantum dots” – artificial atoms that can efficiently generate single photons at a high rate.

They applied two short pulses of laser light to the atom while it is emitting a single photon (a photon is the fundamental particle of light). Applying the laser pulses in sequence at precise times prompted the atom to emit a maximally entangled state of light.

Wein says it is unexpected to find a new entanglement generation scheme using only the manipulation of a single atomic transition in a very simple “two-level” system (which describes the energy level the atom is at) – first theorized in 1930.

“It’s so amazing to see that such simple things can still come from physics that has been understood for so long,” Wein says.

The team’s study, “Photon-Number Entanglement Generated by Sequential Excitation of a Two-Level Atom,” is published in the journal Nature Photonics.

International collaboration was 'absolutely crucial'

Previous approaches used by other research groups to generate entanglement typically have been complex and dynamic, making such schemes difficult to control and measure.

“This is what is beautiful about our scheme: it’s very easy to entangle the light,” says Senellart. “We show that this is applicable to all kinds of systems.”

The team also theoretically showed that applying more sequential laser pulses to an atom would generate ever-larger, readily scalable entangled states of quantum light.

“This opens the door to using more laser pulses and trying to characterize those complex states that will be generated and understanding what can be done with them,” Antón-Solanas says.

“This experimental result achieves with unprecedented simplicity and elegance the sequential generation of photonic signal, which is of paramount importance for quantum information and quantum communication,” says Prof. Enrique Solano, a leading expert on entanglement generation protocols and who wasn’t involved in the study.

“The scaling up towards distributed quantum computing with several nodes and qubits (the basic unit of quantum information) is an open and challenging possibility,” says Solano, Ikerbasque Professor at University of the Basque Country and CEO of two quantum start-up companies.

The research team’s success wouldn’t have been possible without the “absolutely crucial”  international collaboration by quantum theorists and experimentalists, Simon notes.

Not only was the experimental work solidly supported by Simon’s theorist research group at UCalgary, but also by another theorist group led by Dr. Alexia Auffèves, PhD, at the Institut Néel in Grenoble, France.

Wein is now a post-doctoral researcher at Institut Néel. Antón-Solanas is now an assistant professor at the Universidad Autónoma de Madrid in Spain.

Some serendipity also sparked the research.

As it happened, Wein was visiting Senellart’s laboratory in Paris when Antón-Solanas was performing experiments with the double laser pulses but was having difficulty making sense of the data.

“I remember the day Stephen drew on the blackboard what he believed that we were generating, this entangled state of light,” Antón-Solanas says.

Wein developed a theoretical model that explained the results of the experiments and also confirmed that new entangled states of light were being produced.

Another thing that made the research possible, team members say, was the bright single photon sources based on quantum dot technology in which Senellart and her laboratory are international leaders.

“Being able to come up with something simple and clean is opening many doors for other researchers to explore,” Wein says. “We produced a fairly simple type of entangled state that we know is going to be useful.”