Photons interact on a chip-based system with unprecedented efficiency.
In order to process information, photons must interact. However, these tiny light packets want nothing to do with each other, each passing without changing the other. Now, researchers at the Stevens Institute of Technology have coaxed photons into interacting with unprecedented efficiency – a key step toward realizing the long-awaited quantum optics technology for computing, communications and remote sensors.
The team led by Juping Huang, associate professor of physics and director of the Center for Quantum Science and Engineering, brings us closer to this goal with a nanoscale chip that facilitates the interaction of photons with much higher efficiency than any previous system. The new method reported as a memorandum in the September 18 issue of Optica operates at very low energy levels, suggesting that it can be optimized to operate at the level of individual photons – the holy grail of room-temperature quantum calculations and secure quantum communication.
"We are pushing the boundaries of physics and optical engineering to bring quantum and all optical signal processing to reality," says Huang.  To make this progress, Huang's team shoots a laser beam at a microwave in the shape of a racing track carved in crystal. As the laser light bounces around the track, its limited photons interact with each other, producing a harmonic resonance that causes some of the circulating light to change the wavelength.
It's not a whole new trick, but Huang and his colleagues, including graduate student Jiang Chen and senior scientist Yong Meng Sua, dramatically increased their efficiency by using a chip made from a lithium niobate insulator, a material that has a unique way of interaction with light. Unlike silicon, lithium niobate is difficult to chemically etch with ordinary reactive gases. So Stevens's team uses an ion milling tool, essentially a nanoscale, to cut a miniature runway about one hundredth of a human hair long.
Before determining the structure of the racetrack, the team had to apply high voltage electrical impulses to create carefully calibrated areas with variable polarity or periodic polishing that adjust the way photons move around the racetrack, increasing their likelihood of moving interact with each other. Chen explained that both etching of the chip track and tailoring the way photons move around it require dozens of delicate nano fabrication steps, each of which requires nanometer precision. "To the best of our knowledge, we are among the first groups to master all these steps of nanofactories to build this system – this is the reason for achieving this result first."
Moving forward, Huang and his team strive to strengthen the ability of the crystalline raceway to limit and recycle light known as the Q-factor. The team has already identified ways to increase their Q factor by a factor of at least 10, but each level up makes the system more sensitive to imperceptible temperature fluctuations – several thousand degrees – and requires careful refinement.  Still, the Stevens team claims that they are closing in on a system capable of generating single-photon-level interactions reliably, a breakthrough that would allow the creation of very powerful quantum computing components, such as photonic logic gates and entanglement sources, which along a chain, multiple solutions to the same problem can be found simultaneously, it may be possible to allow calculations that can take years to solve in seconds.
From this point we could still be a while, but for quantum scientists the journey will be exciting. "It's the holy grail," says Chen, the paper's lead author. "And on the way to the holy grail, we realize a lot of physics that no one has done before."
Reference: "Ultra-efficient frequency conversion into quasi-phase-coupled lithium-niobate microviruses" by Jia-Yang Chen, Zhao-Hui Ma , Yong Meng Sua, Zhang Li, Chao Tang and Yu-Ping Huang, September 18, 2019, Optics .
doi: 10.1364 / OPTICA.6.001244