A key element for future quantum technology is a step closer with the unveiling of a tiny quantum light source, less than one hundredth the diameter of a hair thick.

The device’s practical qualities will make it a valuable component for future micro-quantum technology, said TMOS Chief Investigator Professor Andrey Sukhorukov who led the project at the Australian National University’s Electronic Materials Engineering (EME) Department.

“The device is small enough to fit on a silicon chip, and operates at room temperature with a tunable wavelength, which are big advantages over existing technology,” he said.

The quantum light source takes energy from an input laser and creates two entangled photons.

Quantum entanglement means that the photons’ properties remain linked, even when far apart, a phenomenon that Einstein thought implausible, dubbing it “spooky action at a distance.”

Nonetheless, a number of methods for creating photons entangled in this way have been developed, and are spurring new quantum technologies: the link between such separate entities can be used to create unbreakable quantum encryption, or to reveal incredibly faint or inaccessible data using quantum imaging.

However, existing quantum light sources each have their own limitations, in frequency of operation, direction of emission or temperature requirements.

The new source boasts not only small size, room temperature operation and wavelength tunability, but also emits two entangled photons that leave the device in different directions – distinguishing them from existing quantum light sources in waveguides, whose photon pairs travel efficiently inside a waveguide circuit but are not easily extracted into free space.

Team member Dr Jinyong Ma performed the quantum characterisation, showing that the photons are spookily linked no matter how far they are separated.

“In fact, it is impossible to find a pair of photons in the same place!” he said.

“This type of light source will underpin free-space quantum communications and quantum imaging,”

The team report in Science Advances that they used a thin, 300 nanometre thick film of the nonlinear material lithium niobate – a commonly used material in photonic devices.

However, they were able to supercharge the performance of the thin film by fabricating a specially designed silica grating with a thickness of 200 nanometres on top of the film.

This grating, instead of splitting light into a rainbow as conventional diffraction gratings do, was designed to couple the input light to the thin film through an optical resonance effect.

The result was a thousand-fold enhancement of the light intensity, leading to a boost in the generation of spooky pairs by a factor of 450. This boost is the highest ever reported, said team member Dr Jihua Zhang, who designed and fabricated the nano-grating.

“The light intensity gets enhanced in an analogous way to how a small periodic force can induce a large motion of a swing,” he said.

A second unique feature of the nano-grating is its ability to support optical resonances at different wavelengths depending on the emission angle of the light.

This allows efficient control of wavelength and quantum behaviour of the generated photons by simply tuning the wavelength of the input laser – a major advantage over crystal sources which require careful temperature control to operate at different wavelengths.

“Controlling the temperature is inconvenient in everyday devices – mobile phones for example need to operate in a range of environments,” said Dr Zhang.

As well as operating at different frequencies, the phenomenon can be reversed in the device, said team member Dr Rocio Camacho Morales.

“The device can combine two photons into one. This is useful in generating new colours of light, which would be useful for night vision cameras,” said Dr Rocio, who confirmed this effect in an experiment.

The Director of TMOS, Professor Dragomir Neshev, was thrilled with the results.

“This exciting work is a firm step towards the goal of TMOS in realising miniaturised optical systems for end-user applications.”

Spatially entangled photon pairs from lithium niobate nonlocal metasurfaces

Jihua Zhang, Jinyong Ma, Matthew Parry, Marcus Cai, Rocio Camacho-Morales, Lei Xu, Dragomir Neshev, and Andrey Sukhorukov.

Science Advances, 29th July 2022

Metasurfaces consisting of nanoscale structures are underpinning new physical principles for the creation and shaping of quantum states of light. Multiphoton states that are entangled in spatial or angular domains are an essential resource for many quantum applications; however, their production traditionally relies on bulky nonlinear crystals. We predict and demonstrate experimentally the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate covered by a silica meta-grating. We measure the correlations of photon pairs and identify their spatial antibunching through violation of the classical Cauchy-Schwarz inequality, witnessing the presence of multimode entanglement. Simultaneously, the photon-pair rate is strongly enhanced by 450 times as compared to unpatterned films because of high-quality-factor resonances. These results pave the way to miniaturization of various quantum devices by incorporating ultrathin metasurfaces functioning as room temperature sources of quantum-entangled photons.