Illinois Team Stores Multiple Photons on a Chip for Over a Microsecond Using Erbium Atoms
The Nano Letters demonstration on thin-film lithium niobate is a step toward chip-scale quantum memories that could be manufactured at scale.
Quantum information travels in particles of light called photons, and those photons often have to wait. Slower quantum operations need time to catch up, which means the light must be paused, or stored, somewhere. Physicists at the University of Illinois Urbana-Champaign have now shown a way to do it on a chip.
The team, led by physics professor Elizabeth Goldschmidt at The Grainger College of Engineering, reports in Nano Letters an integrated nanophotonic platform that holds photons for more than a microsecond and can store several at once. The paper is titled "Telecom Quantum Memory over 1 μs in Nanophotonic Lithium Niobate." It combines a technique called spectral hole burning with thin-film lithium niobate, a material suited to large-scale manufacturing.
The scale of the problem is stark. Quantum chips are often only about 1 centimeter across, a distance light crosses in a few trillionths of a second. Storing a photon for a microsecond is therefore a massive leap. The usual alternative, delaying light by lengthening its path with fibers and mirrors, fails because photons are easily absorbed by matter, and a photon is unlikely to survive the distances needed for quantum-relevant delays. Coupling photons to highly coherent atoms works for storage, but integrating such atoms with scalable nanophotonic platforms has been largely out of reach.
The Illinois device uses erbium atoms embedded in the chip. A tunable laser sorts the atoms by frequency into an evenly spaced, comb-like pattern known as an atomic frequency comb. That design lets the atoms temporarily catch an incoming photon and hold it for a predetermined time. The result, the team reports, preserves quantum information with high fidelity while storing more than one photon at a time.
"No one else has stored light on a chip in a platform like this, with this potential for scalability," said Priyash Barya, an electrical engineering graduate student and co-first author. Co-first author Daren Chen, a physics graduate student, said long delays are "an outstanding problem in quantum information processing." The researchers say the platform is easy to replicate and has implications for both classical and quantum photonics.
Goldschmidt sees it as one piece of a larger effort. "This project is one component in the set of things we're working on," she said, adding that the group is building capabilities in this integrated photonics platform and has "lots of other plans for using this technique of spectrally tailoring the ensemble to build other devices for quantum photonics." The long-term goal is to fold quantum memories into chip-based quantum computers and communication networks, where timing mismatches between components are a basic obstacle.
