Physicists have developed an ultra-compact quantum imager, based on an extremely sensitive high-resolution technique known as quantum ghost imaging.

The new system uses a metasurface only 300 nanometres thick to generate entangled pairs of photons and makes use of their quantum properties to create images with extremely low noise and a wide field of view.

Based on their working prototype, the team predict they can improve on the number of resolvable pixels by four orders of magnitude, said Dr Jinyong Ma.

“It’s the first demonstration of a practical application for entangled photon pairs created by a metasurface,” said Dr Ma, a member of the Electronic Materials Engineering Department (EME) and TMOS, the ARC Centre of Excellence for Transformative Meta-Optical Systems. The paper was published in eLight.

Harnessing the properties of quantum-entangled photon pairs enables extraordinary sensitivity, compared with classical devices.

The sensitivity extends beyond imaging, said the leader of the research group, Professor Andrey Sukhorukov.

“Secure communication networks, quantum LiDAR, and advanced sensing systems, could also benefit from the compact, highly efficient photon-pair sources enabled by nonlinear metasurfaces.

“This combination of optical tunability, nanoscale integration, and high-resolution imaging provides a versatile platform for a wide range of quantum applications.”

To date, methods for generating quantum-entangled photons have been limited by their reliance on crystals of centimetre scale. The new device, made from lithium niobate, is thousands of times smaller, and can be incorporated into miniature devices – its properties are created by surface arrays of geometric shapes that are smaller than the wavelength of light (in this experiment around 900 nanometres).

Another advantage of the metasurface-based device is flexibility in the emission direction of the photon pairs.

The pairs are created by converting a visible pump laser photon into two entangled infrared photons, which, in the conventional crystal method, are constrained to have a small angle between them to satisfy the conservation of longitudinal momentum. In contrast, the metasurface is not constrained in this way and can be engineered to create pairs with a wide range of angles between them. This unique feature allows an ultra-wide field view for imaging.

Better yet, this angle can by dynamically changed, said co-lead author Jinliang Ren, PhD student at TMOS and EME.

“A key innovation of the study lies in the ability to manipulate photon emission angles all optically by simply tuning the wavelength of the pump beam. This unique property eliminates the need for mechanical scanning, allowing seamless and precise optical scanning,” he said.

The beam can be scanned in angle across a target with picosecond control, thereby generating a much-enhanced field of view, compared with existing technology. Such high-speed scanning would allow ultrafast performance in monitoring of mechanical stability, LiDAR applications and object tracking.

The image is created by combining quantum ghost imaging and all-optical scanning imaging. In this technique detectors are set up for both of the entangled photons, with the object to be imaged in the path of one of the pair of photons, while the other photon path is used as a reference. By correlating photons in both arms, the shape of the object is outlined in reverse – where the object blocks photons in one path, the reference photons have no partner. These uncorrelated photons are then rejected, creating a shadow in the reference arm.

The advantage of this pair method is that most of the noise - uncorrelated photons – can be immediately filtered out. The technique is also useful for objects in dirty environments with a lot of scattering, as long as the reference arm is clean.

As yet the efficiency is low, but the team believe they can boost it by further optimising the metasurface. For example, existing approaches rely on the optical resonance at the photon pair wavelength only, but it is possible to create an additional optical resonance at the pump laser photon wavelength.

This could be achieved by improvements to geometry or with materials that boost efficiency, said co-author Dr Jihua Zhang.

“This is really the first proof of principle of quantum imaging with spatially entangled photons from metasurfaces. We can now improve it so it can be used for practical purposes,” he said.

“We believe it will be possible to achieve picosecond control. Such high-speed scanning would allow ultrafast performance in  LiDAR applications and object tracking."