Researchers have developed a new detector capable of accurately measuring single photons at very high rates. This new device may help make high-speed quantum communication a reality. Quantum communication uses light at the single-photon level to send encoded quantum information, such as the encryption key in quantum key distribution. Data transmitted in this way is guaranteed to be secure because of the laws of physics. Sending information at faster speeds requires a single-photon detector that not only detects photons quickly, but also measures their arrival time precisely.
In Optica, the high-impact research journal of the Optica Publishing Group, researchers led by Matthew D. Shaw of NASA’s Jet Propulsion Laboratory describe and demonstrate their new detector for measuring photon arrival times, which they call the PEACOQ (Performance Enhanced Array for Computing Light Quantum) detector.
“Our new detector is composed of 32 niobium nitride superconducting nanowires on a silicon chip, which allows for high count rates with high precision,” said Ioana Craiciu, a postdoctoral scholar and member of the research team.” The detector was designed with quantum communication in mind, as this is an area of technology that has been limited by the performance of existing detectors.”
Research team leader Matthew Shaw examines the PEACOQ detector installed in the cryostat for testing
The detector was developed as part of a NASA project to enable new technologies for space-to-ground quantum communications that will enable the future sharing of quantum information across intercontinental distances. This work builds on technology developed for NASA’s Deep Space Optical Communications Project, which will demonstrate the first free-space optical communications from interstellar space.
No other detector has been able to compute single photons so fast with the same temporal resolution,” Craiciu said. We know this detector will be useful for quantum communication, but we also hope it will enable other applications that we have not yet considered.”
Faster quantum communication
Faster quantum communication transmission rates require a detector at the receiving end that can make fast measurements and exhibit short dead times so that it can compete with arriving high-rate photons. The detector must also accurately measure the photon’s arrival time.
While there are detectors that can measure the arrival time of photons with high precision, they have a hard time keeping up when photons arrive in rapid succession and may miss some or get their arrival times wrong, we designed the PEACOQ detector to accurately measure the arrival times of individual photons even when they are hitting the detector at very high speeds,” Craiciu said. It is also efficient in that it does not miss many photons.”
The PEACOQ detector is made of nanowires that are only 7.5 nanometers thick, or about 10,000 times thinner than a human hair. Operating it at very cold temperatures – about 1 Kelvin, or -458°F – makes the nanowires superconducting, which means they have no resistance. Under superconducting conditions, any photon that hits a wire has a good chance of being absorbed by that wire. Any absorbed photon creates a hot spot that increases the wire’s resistance in a detectable way. A computer and a time-to-digital converter are used to record the time at which the resistance changes, and thus the time at which a photon reaches the detector.
When the detector measures a photon, it outputs an electrical pulse, and the time-to-digital converter measures the arrival time of this electrical pulse very precisely, with a resolution of less than 100 picoseconds, or 70 million times faster than the snap of a finger. A newly developed new time-to-digital converter can measure up to 128 channels simultaneously with this temporal resolution, which is important because the detector requires 32 channels.
To demonstrate the new detector, the researchers cooled it to 1 kelvin by mounting it in a cryostat. They used a custom test setup to feed light into the cryostat to the detector and used a sequence of electronics to transmit the detector’s output signal from the cryostat, amplify it and record it. With 32 nanowires, the researchers had to use 32 sets of each component, including 32 cables and 32 amplifiers of each type.
Unprecedented counting capability
“We are very pleased with how the detector works,” Craiciu said.” The rate at which it can measure photons is the highest we’ve ever seen. It requires a complicated setup because each of the 32 nanowires has to be read out individually, but for applications where you really need to measure photons at high rates and high precision, it’s worth the trouble.”
Typically, the quantum information being transmitted is set up as a clock, and each message is encoded as a photon and sent on a scale. How accurately one can measure the time it takes for a photon to reach the receiver determines how close the arrival distance can be without error, and therefore it determines how quickly the message can be sent. The new detector makes quantum communication at the state-of-the-art clock frequency of 10 GHz practically possible.
The researchers are still working on improvements to the PEACOQ detector, which currently has an efficiency of about 80 percent – meaning that 20 percent of photons hitting the detector are not measured. They also plan to build a portable receiver unit that could be used for quantum communication experiments. It will contain several PEACOQ detectors as well as optics, readout electronics and a cryostat.
