Experiments coupling light and sound reveal the surprising effect when 'nothing' is measured the vibrations of objects are cooled.
Researchers at Imperial's Department of Physics shone light into a glass bead - only four times wider than a human hair - that traps both light and high-frequency sound waves by continually reflecting them around its circumference.
In newly published research, they describe how when the absence of scattered light was detected, the object's temperature decreases.
Kyle Major from the Quantum Measurement Lab in Imperial's Department of Physics, who was co-first author of the research paper, said: "Using zero-photon detection to help cool quantum systems into their ground state will help with the development of quantum computers and quantum networks, as well as testing the fundamental laws of physics."
Alongside their experimental demonstration, the researchers from Imperial College London, the University of Oxford, the University of Waterloo, the University of Leeds and the University of Copenhagen developed a mathematical model to describe the effect, and their results are published today as a pair of contributions in the journals Physical Review Letters and Physical Review A.
Professor Michael R. Vanner, Principal Investigator of the Quantum Measurement Lab presented these results at a Nobel symposium in Sweden, and the research was featured on the cover of popular magazine New Scientist.
"These results are an important milestone for our team and provide a powerful new technique to control quantum systems," said Professor Vanner.
"It was a thrill to present these results at a Nobel symposium in Sweden, and more recently at Photonics West in San Francisco! We're very excited to see how zero-photon detection will help the work of our lab and the wider scientific community!"
Whispering Gallery effect
Light (shown in red) is injected into an optical microsphere via a tapered optical fibre where it then circulates many thousands of times. Through its interaction with sound waves in the glass sphere, the light cools the sound waves, which is enhanced via detecting no light scattered by the interaction.
Named after the Whispering Gallery in St Paul's Cathedral where this effect was first explained, these "whispering-gallery-mode resonators" confine the light and sound long enough for them to interact with one another, correlating the two waves. Subsequent measurements of the light leaving the glass bead then provide information about the sound wave.
Using single-photon detectors, the researchers were able to determine whether one or no photons—individual quanta of light—had been scattered by the sound wave at each moment in time. Then, by considering only the times where no photons were detected, the team observed with an independent measurement that the sound waves circling the bead were quieter than usual; when a single photon was detected instead, they were louder.
"This result was certainly surprising at first," said co-first author Evan Cryer-Jenkins from the Quantum Measurement Lab at Imperial College London. "However, it makes sense, as the light and sound are correlated in our experiment so the information gained from the measurement enables the state of the sound wave to be further cooled."
Counterintuitive results
When the light is not measured, represented by the closed eye symbol (top row), the sound wave undergoes laser cooling. Surprisingly, when a single photon is detected, the sound wave heats (middle), and when no photons are detected, the sound wave is colder than that achievable with laser cooling alone (bottom row).
Co-first author Jack Clarke, also from the Quantum Measurement Lab, said: "While it seems counterintuitive at first, updating our knowledge about the world after noticing something isn't there is actually something we do every day. Whether it's checking for rain or realising you've misplaced your keys, noticing absence is often as telling as presence.
Utilising laser light to cool objects is a powerful technique that is applicable to many systems including trapped atoms and ions. Here, the team build on these techniques and show how to go beyond conventional laser cooling limits by utilising measurement. "Quantum measurement is a fascinating subject and I'm sure there are further discoveries to be made ahead," said co-first author Arjun Gupta from the Quantum Measurement Lab.
Kyle Major said: "It was great to see the arXiv preprint of our research be picked up by New Scientist and make it as a subtitle on the cover of the print magazine."
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