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Technology: Time Lenses Creates Quantum Stopwatches

By Dick Weisinger

Time-Correlated Single Photon Counting (TCSPC) is a technique for recording light signals with picosecond time resolution. It is useful in applications like ultra-fast optical recording, precise measurements, DNA sequencing, and imaging at the molecular level. Using TCSPC, it is possible to monitor the mechanical movements of single molecules and metabolisms within cells.

The technique works by shining a laser light at the object studied and then record the the return times of photons collected when the photon bounces back.

Bowen Li, a postdoctoral student at Colorado University Boulder, said that “TCSPC gives you the total number of photons. It also times when each photon hits your detector. It works like a stopwatch. People shine a pulse of light on their sample then measure how long it takes to emit a photon. That timing tells you the property of the material, such as the metabolism of a cell.”

TCSPC was first developed in the 1960s and has enabled very accurate sensing. It is used in lidar sensors and also for geologic mapping. In Biology, it is often used to diagnose diseases like macular degeneration, Alzheimer’s disease, and cancer.

The precision of TCSPC has been limited by the ability to resolve arrival times of photons at the sensor. Two photons, for example, arriving at the sensor trillionths of second apart will be considered too close to resolve and will be recorded as a single photon returning. But better accuracy is required to resolve the workings of extremely tiny molecules.

To improve on the precision of time measurement, a team of researchers at the University of Colorado used the analogy of a magnifying glass to create a “time lens”.

Li said that “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. The separation between the two photons will be magnified.” The techniques allows measurement of photon arrival times orders of magnitude finer than existing techniques — several hundred quadrillionths of a second.

It is still in development, but the technique has the potential to enable much higher imaging resolution clarity than has been possible with current techniques.

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