Single-Photon Detector Counts to 4

Three photons passing through a superconducting nanowire, causing the nanowire to heat up and disrupting the super-current. (Source: Duke U.)

Engineers have shown that a widely used method of detecting single photons can also count the presence of at least four photons at a time. The researchers say this discovery will unlock new capa­bilities in physics labs working in quantum infor­mation science around the world, while providing easier paths to developing quantum-based techno­logies. The study was a colla­boration between Duke Univer­sity, the Ohio State Uni­versity and industry partner Quantum Opus.

“Experts in the field were trying to do this more than a decade ago, but their back-of-the-envelope calcu­lations concluded it would be impos­sible,” said Daniel Gauthier, a professor of physics at Ohio State. “They went on to do different things and never revisited it. They had it locked in their mind that it wasn’t possible and that it wasn’t worth spending time on.” “When we presented our data, world experts were just blown away,” continued Jungsang Kim, professor of elec­trical and computer engi­neering at Duke. “It’s neat having a group like ours that got started a bit later decide to try something because we didn’t have any blinders on.”

The discovery deals with a new method for using a super­conducting nanowire single-photon detector (SNSPD). At the heart of the detector is a super­conducting filament. A SNSPD works by charging a looped segment of super­conducting wire with an electric current close to its maximum limit. When a photon passes by, it causes that maximum limit in a small portion of the wire to drop, creating a brief loss of super­conductivity. That loss, in turn, causes an electrical signal to mark the presence of the photon.

In the new setup, the researchers pay special attention to the specific shape of the initial spike in the elec­trical signal, and show that they can get enough detail to correctly count at least four photons traveling together in a packet. “Photon-number-reso­lution is very useful for a lot of quantum infor­mation/­communi­cation and quantum optics experi­ments, but it’s not an easy task,” said Clinton Cahall, an electrical engi­neering doctoral student at Duke. “None of the commer­cial options are based on super­conductors, which provide the best perfor­mance. And while other labora­tories have built super­conducting detectors with this ability, they’re rare and lack the ease of our setup as well as its sensi­tivity in important areas such as counting speed or timing reso­lution.”

For other labs to make use of the disco­very, all they would need is a specific type of amplifier for boosting the SNSPD’s tiny elec­trical signal. The amplifier must work at the same low tempera­tures as the SNSPD to reduce background noise. It also must have wide bandwidth to avoid distorting the signal. Such ampli­fiers are already commer­cially available and many labs have them. The results will allow researchers around the world working in quantum mechanics to imme­diately gain new abilities with their existing equipment. As one example, the Duke-Ohio State group also recently reported how using the timing of incoming photons in addition to their quantum states could greatly increase the speed of quantum encryp­tion techniques.

The team is now working to optimize their setup to see just how far they can stretch its abi­lities. They believe with the right elec­tronics and a bit of practice, they could count 10 or even 20 photons at a time. The group has also filed for a patent to create off-the-shelf devices based on their method. (Source: Duke U.)

Reference: C. Cahall et al.: Multi-photon detection using a conventional superconducting nanowire single-photon detector, Optica 4, 1534 (2017); DOI: 10.1364/OPTICA.4.001534

Link: Dept. of Electrical and Computer Engineering, Duke Univ., Durham, USA

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