Researchers have developed a new approach that can precisely count from 0 to more than 9,000 individual photons. The advance opens new possibilities for studying quantum light at large scales and making precise optical measurements.
“Precisely counting photons becomes more challenging as their numbers increase,” said Timon Schapeler from Paderborn University in Germany. “Our approach extends photon counting to larger numbers of photons, bridging the gap between single-photon measurements and high-sensitivity optical power meters.”
In Optica, Optica Publishing Group’s journal for high-impact research, the researchers describe how they developed a detection system that combines superconducting nanowire single-photon detectors (SNSPDs) with multiplexing to create 1,024 detection bins for precise photon counting.
They used this approach, together with model-informed characterization of each detection bin, to measure light pulses containing up to 276 photons with an uncertainty smaller than a single photon. A conventional multiplexed system would need about 37,000 detector elements to achieve the same precision, compared with their 1,024 detection bins.
“Large-scale measurement of individual photons is a cornerstone of photonic quantum computing and single-photon-level imaging,” said Tim Bartley from Paderborn University. “With this new macroscopic photon-counting technology, we can learn a lot about how photonic quantum computers and single-photon-sensitive cameras can be scaled.”
Scaling Up Single-Photon Detection
As the size of photonic quantum systems increases, building and analyzing sufficiently large detection systems will become increasingly important. However, making detectors that can distinguish large numbers of individual photons is extremely challenging.
Common single-photon detectors such as avalanche photodiodes typically distinguish only whether photons are present or absent, not their number. On the other hand, photodiodes and other detectors sensitive to large numbers of photons exhibit significant electrical noise, which obscures the ability to resolve photon number.
Photon-number-resolving detectors, including SNSPDs, offer another approach by distinguishing between different numbers of arriving photons. SNSPDs are known for their high efficiency, very high timing resolution and low noise. It was recently shown that these detectors can resolve a limited number of photons that arrive at the detector, which can be measured with high-precision timing measurements.
“We wanted to know how high we can push the number of photons that can be counted with SNSPDs,” said Schapeler. “This led to a detector system that uses multiplexing and the intrinsic photon-number resolution of SNSPDs. Although these technologies have been individually studied, there has not previously been a demonstration of using both approaches together at this scale, which is an order of magnitude larger than previous multiplexed single-photon counting systems.”
Multiplexing provides a way to mimic photon-number resolution by using many individual detector elements. In the new work, the researchers used multiplexing to split one light pulse into 128 smaller pulses separated in time. They then further split these onto eight different detectors, producing 1,024 individual smaller pulses that contain the original number of photons split between them.
“Our approach uses low-loss optical components to distribute light evenly across 1,024 detection bins while preserving photon-number information in each bin,” said Bartley. “This enables precise measurements, while multiplexing expands the range of photon numbers the system can measure.”
Another critical piece to achieving high-precision photon measurements was a detector model that the researchers previously developed with Mariia Sidorova from PTB in Germany. This model accurately describes each individual detector element making up the overall system.
Achieving Sub-Photon Precision
To test the new approach, the researchers sent laser pulses with average photon numbers per pulse between 1 and 15,000 to their device. This allowed them to investigate how the detector responds under different conditions and determine the precision with which it can count the number of photons in a laser pulse.
Thanks to the accuracy of their model, the researchers demonstrated photon counting of up to 276 photons with measurement uncertainties below 1 photon.
While the current study only applied SNSPDs to measuring laser pulses, the researchers are planning to apply them to bright quantum states of light. This would make it possible to uncover quantum effects at large scales. As SNSPD technology continues to improve, this new detector system could enable precise, highly sensitive measurements in a range of sensing applications.