Technique Enables Better Quantum Detectors

An array of single-photon detectors on a silicon chip could aid the development of quantum computating.

Researchers at MIT, IBM and NASA’s Jet Propulsion Laboratory describe the technique for fabricating and testing individual low-jitter detectors, then assembling them into a microscale array of 10, in Nature Communications (doi: 10.1038/ncomms6873 [open access]).

A new photon detector, deposited athwart a waveguide (horizontal black band) on a silicon optical chip. Courtesy of Nature Communications.

“You make both parts — the detectors and the photonic chip — through their best fabrication process, which is dedicated, and then bring them together,” said MIT graduate student Faraz Najafi.

Single-photon detectors are notoriously temperamental: Of 100 deposited on a chip using standard manufacturing techniques, only a handful will generally work. In previous arrays, the detectors registered only 0.2 to 2 percent of the single photons directed at them.

In addition to yielding much denser and larger arrays, the new approach also increases the detectors’ sensitivity. The detectors accurately registered the arrival of more than 10 percent of the single photons directed at them. For four detectors operating simultaneously, the estimated on-chip detection efficiency is 14 to 52 percent.

That’s still a long way from the 90 percent or more required for a practical quantum circuit, the researchers said.

An illustration of superconducting detectors on arrayed waveguides on a photonic integrated circuit for detection of single photons. Courtesy of Faraz Najafi/MIT.

The fabrication process involves growing a thin, flexible film of silicon nitride on a silicon chip, then depositing the superconductor niobium nitride in a pattern useful for photon detection. A small droplet of polydimethylsiloxane is then applied to one end of the SiN film. A tungsten probe is used to peel the combined film off its substrate and attach it to a silicon optical chip.

“It’s almost like Silly Putty,” said MIT professor Dr. Dirk Englund. “You put it down, it spreads out and makes high surface-contact area, and when you pick it up quickly, it will maintain that large surface area. And then it relaxes back so that it comes back to one point. It’s like if you try to pick up a coin with your finger. You press on it and pick it up quickly, and shortly after, it will fall off.”

Future quantum computers that use entangled photons to move information will require even smaller, more sensitive equipment.

“Because ultimately one will want to make such optical processors with maybe tens or hundreds of photonic qubits, it becomes unwieldy to do this using traditional optical components,” Englund said. “It’s not only unwieldy but probably impossible, because if you tried to build it on a large optical table, simply the random motion of the table would cause noise on these optical states. So there’s been an effort to miniaturize these optical circuits onto photonic integrated circuits.”

For more information, visit www.mit.edu.