Counting Photons for SI-Standards

As part of a research project to help establish standards for photon-counting detectors, NIST physicist Thomas Gerrits adjusts the laser beam hitting a detector. (Source: C. Burrus, NIST)

Since the National Institute of Standards and Tech­nology NIST built its first super­conducting devices for counting photons in the 1990s, these once-rare detectors have become popular research tools all over the world. Now, NIST has taken a step toward enabling universal standards for these devices, which are becoming increa­singly important in science and industry.

Single-photon detectors (SPDs) are now key to research areas ranging from optical communi­cations and astrophysics to cutting-edge information technologies based on quantum physics, such as quantum crypto­graphy and quantum tele­portation. To ensure their accuracy and reliability, SPDs need to be evaluated and compared to some benchmark, ideally a formal standard. The researchers are developing methods to do that and have already started to perform custom cali­brations for the handful of companies that make SPDs.

The team has just developed methods for measuring the efficiency of five SPDs, including one made at NIST, as a prelude to offering an official cali­bration service. “This is a first step towards imple­mentation of a quantum standard – we produced a tool to verify a future single-photon detection standard,” Thomas Gerrits said. “There is no standard right now, but many national metrology institutes are working on this.”

“There have been journal papers on this topic before, but we did in-depth unce­rtainty analyses and described in great detail how we did the tests,” Gerrits said. “The aim is to serve as a reference for our planned cali­bration service.” NIST is uniquely qualified to develop these evaluation methods because the institute makes the most efficient SPDs in the world and is constantly improving their performance. NIST specializes in two super­conducting designs – one based on nanowires or nanostrips, evaluated in the new study, and transition-edge sensors, to be studied in the near future. Future work may also address standards for detectors that measure very low light levels but can’t count the number of photons.

In the modern metric system, the basic unit of measure­ment that’s most closely related to photon detection is the candela, which is relevant to light detected by the human eye. Future SI redefinitions might include photon-counting standards, which could offer a more accurate way of measuring light in terms of the candela. Single-photon light levels are less than one-billionth of the amounts in current standards.

Now,  the researchers use conven­tional tech­nologies to measure SPD detection effi­ciency, defined as the probability of detecting a photon hitting the detector and producing a measurable outcome. The team ensured the measurements are traceable to a primary standard for optical power meters. The meters maintain accuracy as measure­ments are scaled down to low light levels, with the overall measurement uncertainty mostly due to the power meter cali­bration.

The researchers measured the efficiencies of five detectors, including three silicon photon-counting photo­diodes and a nanowire detector. Photons were sent by optical fiber for some measure­ments and through the air in other cases. Measurements were made for two different wave­lengths of light commonly used in fiber optics and communications. Uncer­tainties ranged from a low of 0.70% for measurements in fiber at a wave­length of 1533.6 nanometers to 1.78% for over-the-air readings at 851.7 nm. (Source: NIST)

Reference: T. Gerrits et al.: Calibration of free-space and fiber-coupled single-photon detectors, Metrologia 57, 015002 (2020); DOI: 10.1088/1681-7575/ab4533

Link: Faint Photonics Group, National Institute of Standards and Technology, Boulder, USA

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