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University of Cambridge devises new routes to quantum light

07 Feb 2023

Controlling quantum properties could assist microscopy and computation.

A project involving the University of Cambridge, the Technion–Israel Institute of Technology and the University of Vienna has proposed new mechanisms for the creation of "quantum light" at high energies.

Published in Nature Physics, the findings could help engineers to more easily create and utilize light whose quantum properties are controlled over a broad range of frequencies.

Previous research in this topic has included a project at Tampere University which showed that limiting the number of photons in particular quantum states can produce a light beam that behaves differently to standard focused laser beams, potentially valuable for sensitive distance measurement.

Elsewhere a team at Macquarie University has used thin semiconductor films to convert laser light into single photons with particular quantum signatures, which could be useful in single photon emitters for encryption and computing.

The new project aimed in particular to create quantum light over a broader spectral range than has usually been the case, addressing the limitation whereby sources producing light over broad ranges are usually capable of only classical emission. It did so by treating the emission as a form of many-body problem, made up of multiple individual contributing systems.

According to the team, applying a laser to a collection of emitters can turn low-frequency input into high-frequency output thanks to a phenomenon whereby electrons ripped away from the emitters then recombine with them and release excess energy as light. This usually involves those emitters being independent from one another, creating output with no major quantum fluctuations.

"We wanted to study a system where the emitters are not independent, but correlated, so that the state of one particle tells you something about the state of another," said Andrea Pizzi of the research carried out at Cambridge's Cavendish Laboratory. "In this case, the output light starts behaving very differently, and its quantum fluctuations become highly structured and potentially more useful."

Quantum fluctuations as a practical resource

According to the team's Nature Physics paper, correlations among the emitters then create non-classical many-photon states of light, in particular through the non-linear phenomenon of high-harmonic generation, which can cause photons to be emitted at multiples of the driving field frequency.

Such non-classical effects in the resulting light can include "doubly peaked photon statistics, ring-shaped Wigner functions and correlations between harmonics," commented the project, which also devised some suggested experimental schemes to induce and control these effects.

"We worked for months to get the equations cleaner and cleaner, until we got to the point where we could describe the connection between the output light and the input correlations with just one compact equation," said Pizzi, who is now based at Harvard University.

"Quantum fluctuations make quantum light harder to study but also more interesting, and if correctly engineered, quantum fluctuations can be a resource. Controlling the state of quantum light could enable new techniques in microscopy and quantum computation."

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