Researchers are using a superconducting nanowire design to quickly and accurately count photons.
Researchers have developed a new detector that can precisely measure individual photons at very high speeds. A new device could help make high-speed quantum communication practical.
Quantum communication uses light at the single-photon level to send encoded quantum information, such as encryption keys in quantum key distribution. Due to the laws of physics, data transmitted in this way is guaranteed to remain secure. Sending information at higher speeds requires a single photon detector that can not only quickly detect photons, but also accurately measure their arrival time.
IN Optica Optica Publishing Group’s high-impact research journal, by researchers led by Matthew D. Shaw from NASA’s Jet Propulsion Laboratory describe and demonstrate their new detector for measuring photon arrival times, which they call PEACOQ (Performance Enhanced Counting Array). optical quanta) detector.
“Our new detector consists of 32 superconducting niobium nitride nanowires on a silicon chip, which enables high counting rates with high precision,” said research team member Ioana Kraichiu, a Ph.D. “The detector was designed with quantum communication in mind, as this is a technology area that has been limited by the performance of available detectors.”
Image credit: Ryan Lannom, JPL-Caltech/NASA
The detector was developed as part of the NASA program to create a new space-earth quantum communication technology, which in the future will allow the exchange of quantum information over intercontinental distances. This work is based on technology developed for NASA’s Deep Space Optical Communication project, which will be the first demonstration of optical communication in free space from interplanetary space.
“Currently, there is no other detector that can count individual photons so quickly with the same time resolution,” Krajciu said. “We know that this detector will be useful for quantum communication, but we also hope that it can have other applications that we haven’t considered.”
Faster quantum communication
Increasing the transmission speed of quantum communication requires a detector at the receiving end that can perform fast measurements and has a short delay time to cope with the high rate of incoming photons. The detector must also accurately measure the arrival time of the photons.
“While there are detectors that can time photon arrivals with high precision, they struggle to keep up when photons arrive in rapid succession and may miss some of the photons or mistime them,” Kraichiu said. “We designed the PEACOQ detector to accurately measure the arrival times of individual photons, even when they hit the detector at high speeds. It’s also efficient – it doesn’t let many photons through.”
The PEACOQ detector is made of nanowires only 7.5 nm thick, or about 10,000 times thinner than a human hair. Operating at very low temperatures — about 1 Kelvin, or -458 °F — makes the nanowires superconducting, meaning they have no electrical resistance. In superconductivity, any photon that hits a wire has a good chance of being absorbed by that wire. Any absorbed photons create a hot spot that noticeably increases the electrical resistance of the wire. A computer, along with a time-to-digital converter, is used to record when the resistance changed, and thus when a photon arrived at the detector.
“When the detector measures a photon, it emits an electrical pulse, and the time-to-digital converter measures the arrival time of that electrical pulse very precisely, with a resolution of less than 100 picoseconds, or 70 million times faster than snapping fingers. “, Krychu said. “We have developed a new time-to-digital converter that can measure up to 128 channels simultaneously with this temporal resolution, which is important because our detector requires 32 channels.”
To demonstrate the new detector, the researchers cooled it down to 1 Kelvin in a cryostat. They used a special test rig to send light into the cryostat to the detector and an electronics circuit to transmit the detector output from the cryostat, amplify it, and record it. Since there are 32 nanowires, the researchers had to use 32 sets of each component, including 32 cables and 32 of each type of amplifier.
Unprecedented calculation speeds
“We were very pleased with how well the detector worked,” Krajchu said. “The speed at which it can measure photons was the fastest we’ve seen. It requires a complex setup because each of the 32 nanowires is read individually, but for applications where you really need to measure photons at high speed and high accuracy, it’s worth the effort.”
Typically, the quantum information transmitted is clock-tuned, with each piece of information encoded in a single photon and sent instantaneously. How accurately you can measure the time the photons arrive at the receiver depends on how close the marks can be to each other without error, and therefore determines how fast you can send information. The new detector makes quantum communication practical with a modern clock frequency of 10 GHz.
The researchers are still working on improving the PEACOQ detector, which is currently around 80% efficient, meaning 20% of the photons that hit the detector are not measured. They also plan to build a portable receiver that can be used for quantum communication experiments. It will contain several PEACOQ detectors as well as optics, readout electronics and a cryostat.
