From ancient signal fires to modern fibre optics, our ability to harness light at ever-finer scales has led to revolutions in communication, computing and sensing. Today, quantum technologies promise new breakthroughs in these fields by using individual particles of light, or photons, to carry quantum information. However, it remains challenging to produce and control quantum light on devices as compact and scalable as everyday electronics.
To researchers at the A*STAR Quantum Innovation Centre (A*STAR Q.InC), photonic integrated circuits might be the answer. Similar to their electronic counterparts, these circuits consist of densely-packed ‘light conduits’ or waveguides: each one about a hundred times narrower than a human hair, and collectively laid down on fingernail-sized chips.
Normally, light passes through such waveguides without leaving a trace. However, A*STAR Q.InC Senior Scientist Di Zhu, Scientist Xiaodong Shi, Research Engineer Sakthi Sanjeev Mohanraj and A*STAR Q.InC colleagues worked with the National University of Singapore to engineer waveguide structures that would instead generate quantum light, with the help of an emerging material platform called thin-film lithium niobate (TFLN).
The team’s TFLN waveguides would cause a high-energy photon in a laser beam to spontaneously split into a pair of low energy ones through a process called spontaneous parametric down-conversion (SPDC). Like the sometimes-bizarre links between human twins, photon pairs have a correlation over large distances that can be used to realise secure communication systems and quantum networks.
“To have efficient SPDC, the photons’ optical fields need to interact strongly and sync up: a condition called phase matching,” said Zhu. “To achieve this, we developed a trick called ‘layer poling,’ which flips the crystal orientation in the bottom layers of lithium niobate within our waveguides.”
Zhu added that the flipping was created by permanently altering the material’s atomic arrangement using a high voltage applied through microfabricated electrodes on the devices. This method turned out to be more reliable and repeatable than traditional methods based on conventional modal phase matching or periodic poling.
When the team made and tested their layer-poled lithium niobate (LPLN) waveguides, they found that this approach led to a performance breakthrough: not only did their devices achieve a high second-harmonic generation (SHG) conversion efficiency of 4,615% W⁻¹cm⁻², they also significantly improved photon-pair generation rate, surpassing all previously reported results from conventional methods or configurations.
“Through a cascaded SHG and SPDC process, our LPLN waveguides proved capable of efficient, broadband correlated photon-pair generation,” said Shi. “The fabrication process was also simpler and produced more stable devices compared to existing solutions using LN waveguides.”
Moving forward, the team plans to further optimise the performance and scalability of LPLN devices, aiming for their use in quantum communications.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Quantum Innovation Centre (A*STAR Q.InC) and the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
