Osaka Physicists Wired 10 Atoms Into 10 Optical Fibers at Once. Nobody Had Managed More Than One.

Researchers at the University of Osaka, working with Japan's National Institute of Information and Communications Technology and Hamamatsu Photonics, have coupled photons emitted by 10 individual neutral atoms into 10 parallel optical channels simultaneously. The result, published in Optica in September by Yuya Maeda and colleagues, is the largest such multiplexing demonstrated for a neutral-atom quantum system.

Neutral atoms are one of the leading platforms for quantum computing. Individual atoms held in optical tweezers make excellent qubits — they are identical by construction, they hold their quantum state for a long time, and hundreds can be arranged in a plane. The difficulty is getting information out of one processor and into another. That job falls to photons, and until now the standard approach has been to collect light from atoms one channel at a time, which puts a hard ceiling on how fast two processors can be linked.

The Osaka group's approach was to build the parallelism into the optics. Atoms were spaced at micrometer intervals, and the light each one emitted was coupled into a separate channel of an integrated optical waveguide array with 32 channels total. Ten of those channels were fed through optical fibers and read out simultaneously by a multichannel superconducting nanostrip photon detector system.

Two measurements matter for whether the scheme is actually usable. The first is crosstalk: whether light from one atom leaks into a neighboring channel. The team reported interchannel crosstalk at a negligible level. The second is whether the quantum information survives the trip. The researchers confirmed correlations between the atomic quantum states and the polarization states of the detected photons, which is the signature that entanglement between atom and photon is being preserved through the waveguide and fiber rather than destroyed by it.

"Going forward, we will scale up the degree of multiplexing and work toward connecting neutral-atom quantum computers," said Takashi Yamamoto, a professor and deputy director of the Center for Quantum Information and Quantum Biology at Osaka's Graduate School of Engineering Science.

The group expects the architecture to reach roughly 100 parallel channels, since the waveguide array already has 32 and nothing in the design forbids larger arrays. That number is the point of the exercise. Entanglement distribution between separate quantum processors is probabilistic — most attempts fail — so the rate at which two machines can be linked scales roughly with the number of channels running in parallel. Going from one channel to a hundred turns a link that succeeds occasionally into one that succeeds often enough to be a component in a larger machine.

Fault-tolerant quantum computing almost certainly requires more qubits than any single trap or chip will hold, which means modules that talk to each other over optical fiber. Photonic interfaces like this one are the connective tissue of that architecture, and multiplexing is the difference between a demonstration and a network.