Two researchers from the University of Namur's Department of Physics, Professor Michaël Lobet and his PhD student Adrien Debacq, are taking a close look at a subject that fascinates the scientific community : superradiance in media with a refractive index close to zero. In an article published this summer in Nature's prestigious journal Light: Science & Applications, in collaboration with Harvard University (USA), Michigan Technological University (MTU) and Sparrow Quantum, they contribute to the development of quantum computing.
For the past twenty years, a physical phenomenon has been attracting the attention of scientists over the world: superradiance in media with refractive indices close to zero. Among them, Michaël Lobet, professor at the Physics' Department of UNamur, FNRS research associate and associate at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). "This is one of the major areas I've been studying for ten years now, and for which I was a post-doctoral fellow in Professor Eric Mazur's team at Harvard", explains Michaël Lobet.
Superradiance, is a phenomenon that has been known for over half a century. It was theorized mathematically as early as 1954 by Robert Dicke, who showed that when elements such as atoms interact, they can synchronize to emit more powerful light. A bit like a choir singing in unison, the sound produced is much louder than each voice taken separately. But for doing this, the emitters must be very close to one another.
An index that changes everything
Scientists have discovered that one element can change everything: when the emitters are immersed in a material with a refractive index close to zero, rather than in vacuum, the position of the emitters is no longer an issue. The refractive index is a quantity that describes the behaviour of light in a material. In an ordinary material, light behaves a little like waves on the sea: it moves forward, forming crests and troughs that shift. But in a near-zero index medium, it's as if the sea becomes perfectly flat, with no waves, and starts moving up and down in a block. Everything moves in unison: the sea becomes uniform, and the wave stretches to infinity.
When the light field becomes more uniform, all the atoms find themselves optically close to each other, even if they are spatially distant. In other words, the "ambient" near-zero refractive index relaxes the strict distance between the atoms' positions, an essential condition for the entanglement of quantum particles. Quantum entanglement corresponds to correlations between particles, essential for the development of information and quantum computers.
From electrodynamics to quantum computing
This is where the promising contribution of a team of researchers from UNamur, Harvard and Michigan Technological University (MTU) comes in, supported by Dr. Larissa Vertchenko, from Danish quantum technology company Sparrow Quantum. Adrien Debacq, FNRS aspirant researcher at the Namur Institute of Structured Matter (NISM) and co-author of the paper, assisted by Harvard PhD student Olivia Mello and Dr Larissa Vertchenko, have together theoretically developed a photonic chip capable of radically improving the range of entanglement between transmitters, up to 17 times greater than in a vacuum. "This is the first time that such a long range has been achieved using a compact system that can be easily implemented in photonic chips", says Professor Michaël Lobet. The emitters were made from nitrogen vacancy (NV) diamonds, structures well known in quantum optics.
"This paper shows how near-zero refractive index photonics can transition from classical electrodynamics to the quantum regime, since superradiance is intrinsically quantum.", summarizes Eric Mazur, Professor at the Harvard School of Engineering and Applied Sciences, who has been at the forefront of these innovative materials for the past decade. Entanglement, another purely quantum property, enables the transfer of quantum information, a concept already raised by Einstein in the 1930s as part of his work on quantum mechanics. The present work is part of this continuation, and more generally of the "second quantum revolution", which aims to build on the fundamental discoveries of Einstein and the other founding fathers of quantum mechanics.
Very concrete applications
This prospect confirms the nascent research in recent years to potentially revolutionary applications: more efficient lasers, more sensitive optical sensors and, above all, faster and ultra-secure telecommunication tools, thanks to quantum computers. Cybersecurity, for example, is about to be revolutionized by these discoveries, guaranteeing message security through physical laws rather than complex calculations.
" Preserving the high degree of entanglement on chip over longer ranges may raise the possibility of multipartite entanglement involving many qubits useful for e.g., the construction of cluster states—important resource for universal one-way quantum computing—as well as large-area distributed quantum computing and quantum communication networks that may offer drastic increase on the computational and channel capacity", explains Durdu Güney, Associate Professor at Michigan Technological University (MTU). Together with Dr. Seth Nelson, Güney has helped to study the dynamic response of the quantum system in the presence of a pump laser beam.
The challenge for future research is to transform this theoretical project, combining analytical models and numerical simulations, into concrete experimental realizations. The aim is to get a little closer to practical quantum systems, which fit into dimensions as small as the thickness of a human hair. Who knows, maybe one day we'll have a quantum computer in our pocket?
Acknowledgements
The researchers would like to thank the Department of Physics and the NISM Institute, the FNRS for funding the research mandates of Michaël Lobet and Adrien Debacq, and the PTCI technology platform, whose supercomputers made this study possible, as well as for funding in part by the United States Army Research Office under MURI grant (W911NF2420195).
