Scattered Photons Transfer Quantum Coherence in Free Space

Researchers at the University of Waterloo report the first successful transfer and recovery of quantum coherence using photons scattered in free space. The work points toward research opportunities and applications in fields ranging from quantum communication to imaging and beyond.

Quantum coherence, while fundamental to applications of quantum technology, has been difficult to transfer in free space. It has so far been limited to direct line-of-sight channels as quality degrades severely with atmospheric turbulence and scattering.

“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 of the paper and a postdoctoral fellow at the Institute for Quantum Computing (IQC) and in the department of physics and astronomy at the University of Waterloo.

Each optical pulse from the laser is sent through a phase converter, which creates two coherent pulses, while the multimode analyzer measures the signals scattered off the target surface, implemented with regular bright paper. A single-photon detector array is used as the detection device, with 8 × 8 individual pixels that are each time-tagged separately. Courtesy of Shihan Sajeed and Thomas Jennewein, University of Waterloo.

Sending and receiving photons through the air for quantum communication or other quantum-encoded protocol would normally require a direct line-of-sight between the transmitter and receiver. Any objects in the optical path (even as small as a molecule and as large as a wall) will reflect and scatter some of the photons, depending on the object’s reflective qualities. Any quantum information encoded in the photons is typically lost, therefore interrupting the quantum channel.

Together with Thomas Jennewein, principal investigator of the Quantum Photonics lab at IQC, Sajeed found a way to encode quantum coherence in pairs of photon pulses sent one after the other so that they would maintain coherence even after scattering from a diffuse surface. The researchers sent out a train of pulse pairs with a specific phase coherence that could be measured from the scattered photons using quantum interference. They also used a single-photon detector array sensor that, in addition to solving wavefront distortions caused by atmospheric turbulence, acted as an imager, thereby allowing the observation of single-photon interference and imaging simultaneously. They oriented the detector so that it would only absorb scattered photons from the laser pulses, and they observed a visibility of more than 90%, meaning that the scattered photons retained their quantum coherence, even after colliding with an object.

The method required custom hardware to make use of the coherent light that the researchers generated in the process. The single-photon detector they employed, for example, was capable of detecting a billion photons every second with a precision of 100 picoseconds. The team also had to design its own electronics adapter board to be able 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,” Sajeed said. “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 believes that the work will serve as a catalyst for new research and applications in quantum sensing, communication, and imaging in free-space environments. Although Sajeed and Jennewein were able to demonstrate quantum and communication and imaging, further research is needed to determine how their techniques could be used in practical applications, Sajeed said.

“We believe this could be used in quantum-enhanced lidar, quantum sensing, non-line-of-sight imaging, and many other areas — the possibilities are endless,” Sajeed said.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00565-y).