Researchers have successfully transferred quantum coherence through photons scattered in free-space for the first time, enabling new research opportunities and applications in fields ranging from quantum communication to imaging and beyond.
“The ability to transfer quantum coherence via scattered photons means that now you can do many things that previously required direct line-of-sight free-space channels,” said Shihan Sajeed, lead author on the paper and a postdoctoral fellow at IQC and in the Department of Physics and Astronomy.
Normally, if you try to send and receive photons through the air (free-space) for quantum communication or any other quantum-encoded protocol, you need a direct line-of-sight between transmitter and receiver. Any objects—from as big as a wall to as small as a molecule—in the optical path will reflect some photons and scatter others, depending on how reflective the surface is.
Any quantum information encoded in the photons is typically lost in the scattered photons, interrupting the quantum channel.
Together with Thomas Jennewein, principal investigator of the Quantum Photonics lab at IQC, they found a way to encode quantum coherence in pairs of photon pulses sent one after the other so that they would maintain their coherence even after scattering from a diffuse surface.
The researchers emitted two laser pulses with a different phase from each other that would interfere when combined—in other words, that were coherent. This quality of coherence is measured by visibility. The team placed detectors where they would only absorb scattered photons from the laser pulses, and observed a visibility of over 90%, meaning that they maintained their quantum coherence even after smashing against an object.
Their novel technique required custom hardware to make use of the coherent light they were generating. The single-photon-detector-array the team used to measure the returning light could detect one billion photons every second with a precision of 100 picoseconds. Only cutting-edge time-tagging electronics could handle the demands of this flow of light, and the team had to design their own electronics adapter board to communicate between the detectors and the computer that would process the data.
“Our technique can help image an object with quantum signals or transmit a quantum message in a noisy environment,” said Sajeed. “Scattered photons returning to our sensor will have a certain coherence, whereas noise in the environment will not, and so we can reject everything except the photons we originally sent.”
Sajeed expects their findings will stimulate new research and applications in quantum sensing, communication, and imaging in free space environments. The duo demonstrated quantum communication and imaging in their paper, but Sajeed said more research is needed to find out how their techniques could be used in various practical applications.
“We believe this could be used in quantum-enhanced Lidar (Light Detection and Ranging), quantum sensing, non-line-of-sight imaging, and many other areas—the possibilities are endless,” said Sajeed.
Observing quantum coherence from photons scattered in free-space was published in Light: Science & Applications on June 7, 2021.
This work was supported by the National Research Council Canada, Defence Research Development Canada, Industry Canada, Canada Fund for Innovation, Ontario MRI, Ontario Research Fund, NSERC through the Discovery, CryptoWorks21, Strategic Partnership Grant programs, and the Canada First Research Excellence Fund through TQT.