Quantum ‘stopwatch’ holds promise for better imaging tech

The study focused on an established technology called time-correlated single photon counting (TCSPC), which involves directing a laser at a sample of any size and recording how long it takes for photons to be emitted. This timing contains useful information about the property of the sample, such as metabolism of a cell.

“TCSPC gives you the total number of photons,” explained Dr Bowen Li, who led the Optica study. “It also times when each photon hits your detector. It works like a stopwatch.”

TCPSC is a well established technology first developed in 1960. It has transformed how humans experience the world, being key to lidar (used to create geological maps as well as 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 ultrafast optics tool called a time lens.

Traditional TCSPC tools can only measure timings down to a certain level of precision; if two photons arrive at the device too close together (such as 100 trillionths of a second or less apart) the detector records them as a single photon. According to Li, this makes a big difference when trying to image small molecules: “In a microscope, we use optical lenses to magnify a small object into a big 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 pass both of those photons through their time lens, which is made up of loops of silica fibres. In the process, one of the photons slows down, while the other speeds up. Instead of a close race, there now appears to be a large gap between the runners - one that a detector can record. Effectively, the separation between the photons is magnified.

Li and his colleagues demonstrated that they can record the arrival of photons with a precision 100 times higher than existing tools. This allows them to distinguish between photons arriving at a detector with a gap of just several hundred quadrillionths of a second.

“We can add this modification to almost any TCSPC system to improve its single-photon timing resolution,” explained Professor Shu-Wei Huang, co-author of the study. Even the cheapest such devices work well with their adaptation.

The researchers still have some work to do before time lenses become common in scientific labs. But they hope that their tool will one day lead to significant improvements in a range of imaging technologies: from sensors that map out entire forests and mountain ranges to more detailed devices that can diagnose human diseases.