Photons must interact to process information. But, these tiny packets of light typically stay independent of each other, passing by without altering each other. Recently, researchers at Stevens Institute of Technology have succeeded in coaxing photons into interacting with one another, that too with unprecedented efficiency. This is a key advance in the direction of realizing long-awaited quantum optics technologies for computing, communication, and remote sensing.
The research led by Yuping Huang, an associate professor of physics and director of the Center for Quantum Science and Engineering, takes a significant step closer to that goal with a nano-scale chip that enables photon interactions with much higher efficiency than any preceding system. The new method was reported as a memorandum in Optica on Sept. 18 and works at very low energy levels. This suggests that it could be optimized to operate at the level of individual photons considered the holy grail for room-temperature quantum computing and secure quantum communication.
Huang said, “We’re pushing the boundaries of physics and optical engineering in order to bring quantum and all-optical signal processing closer to reality”.
To achieve this capability, Huang’s team fired a laser beam into a racetrack-shaped microcavity carved into a sliver of crystal. As the laser light bounces around the racetrack, its confined photons interact with one another, producing a harmonic resonance that causes some of the circulating light to change wavelength.
Although not an entirely new method, Huang and colleagues with graduate student Jiayang Chen and senior research scientist Yong Meng Sua, dramatically boosted its efficiency than before by using a chip made from lithium niobate on insulator which is a material that has a unique way of interacting with light. Unlike silicon, it is difficult to chemically etch lithium niobate with common reactive gases. So, the team employed an ion-milling tool, essentially a nano sandblaster, to define a tiny racetrack about one-hundredth the width of a human hair.
Before etching the racetrack structure, the team had to apply high-voltage electrical pulses to form carefully calibrated regions of alternating polarity, or periodic poling, that alter the way photons move around the racetrack, thereby increasing their probability of interacting with each other.
Chen explained, “To both etch the racetrack on the chip and tailor the way photons move around it, requires dozens of delicate nanofabrication steps, each requiring nanometer precision. To the best of our knowledge, we’re among the first groups to master all of these nanofabrication steps to build this system—that’s the reason we could get this result first.”
In the coming days, Huang and his team aim to boost the crystal racetrack’s ability to confine and recirculate light i.e. its Q-factor. They have already identified ways to increase this Q-factor by a factor of at least 10, but as each level up makes the system even more sensitive to imperceptible temperature fluctuations, a jump of a few thousands of a degree requires very careful fine-tuning.
The team believes they’re closing in on a system capable of generating interactions at the single-photon level reliably, an innovation that would permit the creation of many powerful quantum computing components such as photonics logic gates and entanglement sources, which along a circuit, can canvass multiple solutions to the same problem at the same time, conceivably allowing tedious calculations that take years to be solved in seconds.
Chen, the paper’s lead author, said, “ We could still be a while from that point but for quantum scientists the journey will be thrilling. It’s the holy grail. And on the way to the holy grail, we’re realizing a lot of physics that nobody’s done before.”