How to take a quantum photograph

Kai Wang

To the untrained eye it looks like a random arrangement of dots, but to PhD student Kai Wang it's a snapshot of a multidimensional quantum world at work.

His snapshot is taken by a camera that doesn't have a lens. Instead it has metasurface, a flat piece of glass with an intricate pattern of tiny rectangular pillars carved into its silicon surface. The structures are smaller than the wavelength of light and are carefully designed to reveal the quantum image.

"To my knowledge here at ANU we are the first to bring the use of metasurfaces to the imaging of quantum entangled photons," says Kai.

"The surface has been tailored to enable multidimensional quantum states to be measured in a simple and fast manner."

The ability to analyse the quantum information encoded in light beams could be employed in communication between quantum satellites, Kai says.

His experiments have centred on analysing pairs of photons created by a single quantum process - parametric down-conversion in a lithium niobate waveguide.

In this process a photon of wavelength 785 nanometers is split into two photons each of 1570 nanometers, a wavelength relevant to the telecommunications industry.

His metasurface is designed to reveal the links between the two resulting photons.

"I can know in what basis they are entangled and to what extent they are entangled - the multiphoton density matrix that describes all the information about the photons in that basis," Kai says.

Kai was recommended to do a PhD at ANU during his masters research in Germany. Although his background was as an experimental physicist, he chose a theoretical supervisor, Andrey Sukhorukov, and found that their skills have complemented each other nicely.

"I've learned a lot!" he says. "There is a diverse culture in the Nonlinear Physics Centre, so many different backgrounds - but a great strategic plan of research, and opportunities to interact with the international community."

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