Researchers at the University of Colorado-Boulder have developed one of the most accurate timekeeping devices capable of counting individual photons. It could hold the key to significant improvements in a range of imaging technologies.
The study focused on an established technology called time-correlated single photon counting (TCSPC), which focuses a laser on a sample of any size and records how long it takes for photons to be emitted. This timing contains useful information about the property of the sample, such as the metabolism of a cell.
“TCSPC gives you the total number of photons,” explains Dr Bowen Li, who led the Optics study. “These are also moments when each photon hits your detector. It works like a stopwatch.”
TCPSC is an established technology that was first developed in 1960. It has transformed the way people experience the world, as it holds the key to lidar (used to create geological maps and in autonomous vehicles), fluorescence lifetime microscopy and various diagnostic imaging technologies, including those used to identify Alzheimer’s disease and cancer.
This “stopwatch” has been improved by the Colorado researchers using an ultra-fast optical tool called a time lens.
Traditional TCSPC tools can only measure timings to a certain level of precision; if two photons come too close to the device (such as 100 trillionths of a second or less), the detector registers them as a single photon. According to Li, this makes a big difference when we try to image small molecules: “In a microscope, we use optical lenses to magnify a small object into a large image. Our time lens works in a similar way, but for time,” he said.
This can be understood by imagining two photons as two runners racing too close for a timekeeper to tell them apart. Li and his colleagues allow both photons to pass through their time lens, which is made up of loops of silica fibers. In doing so, one of the photons slows down while the other accelerates. Instead of a close race, there now seems to be a big gap between the runners – one that a detector can register. Effectively, the separation between the photons is increased.
Li and his colleagues showed that they can record the arrival of photons with a precision 100 times higher than existing instruments. This allows them to distinguish between photons arriving at a detector spaced only a few hundred quadrillionths of a second.
“We can add this modification to almost any TCSPC system to improve the timing resolution of one photon,” explains Professor Shu-Wei Huang, co-author of the study. Even the cheapest of such devices work well with their customization.
The researchers still have some work to do before time lenses become commonplace in science labs. But they hope their tool will one day lead to significant improvements in a range of imaging technologies: from sensors that map entire forests and mountain ranges to more detailed devices that can diagnose human disease.