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The University of Colorado Boulder has created one of the most accurate stopwatches ever. It is not for timing Olympic swimmers or sprinters, but rather for counting single photons (or the tiny bits of energy that make light).
This team's invention could make a significant impact on a variety of imaging technologies, from sensors that map entire forests and mountain ranges to more precise devices that diagnose human diseases such as Alzheimer's and breast cancer. This week, the group published their results in Optica.
Bowen Li, the lead author of this new study, stated that it focuses on a broadly applied technology known as time-correlated singlephoton counting (TCSPC). It works in a similar way to the Olympic timers: Scientists shine a laser at a sample they choose, including proteins and geologic formations. Then, they record the photons that bounce back at them. Researchers can learn more about an object the more photons they collect.
"TCSPC shows you the total number photons. Li, a postdoctoral research fellow in the Department of Electrical, Computer and Energy Engineering at CU Boulder, said that it also counts how many photons hit your detector. It works as a stopwatch.
This stopwatch is now more accurate than ever. Li and his coworkers demonstrate that they can measure photon arrivals using an ultrafast optics tool called "time lenses". This is more than 100 times faster than any other tools.
Shu-Wei Huang (corresponding author of this new study) stated that the quantum time lens used by the group works with all TCSPC devices, even the most expensive.
Huang, assistant professor in ECEE, stated that "we can add this modification almost any TCSPC systems to improve its single photon timing resolution."
This research is part the new $25 million Quantum Systems through Entangled Science and Engineering center, which is led by CU Boulder.
Photo Finish
Huang stated that TCSPC might not be well-known. The technology was developed in 1960 and has changed the way humans view the world. These photon counters are an important component of lidar (or light detector and ranging) sensors that researchers use to create geologic mapping. These photon counters are also visible in fluorescence lifetime microscopy, a smaller-scale imaging method. The technique is used by doctors to diagnose certain illnesses such as macular degeneration, Alzheimer’s disease, and cancer.
Li explained that people shine a pulse light on their samples and then measure the time it takes for a photon to emit. "That timing can tell you the property of the material such as its metabolism."
However, traditional TCSPC tools can only measure timing to a certain degree of precision. If two photons arrive too close together,say 100 trillionths or less,the detector records them both as one photon. This is a lot like two sprinters running to the finish line during a 100-meter race.
Although these small inconsistencies might seem like a minor issue, Li said that they can be a significant help when trying to understand the incredibly small molecules.
Time lenses
He and his colleagues then decided to use a "time lens" to solve the problem.
Li explained that optical lenses are used to magnify small objects into large images in a microscope. Our time lens works in the same way, but for time.
Picture two photons with two runners racing neck and neck so close that the Olympics timekeeper cannot tell them apart. Li and his coworkers pass both photons through their time lenses, which are made of loops of silica fibres. One of the photons speeds up, while the other slows down. Instead of a close race there is now a wide gap between runners that can be recorded by detectors.
Li stated that the separation between two photons would be magnified.
The team found that the strategy worked: TCSPC devices equipped with time lenses can detect photons arriving at detectors with gaps of hundreds of quadrillionths to a second order of magnitude greater than normal devices.
Researchers still have much work to do before time-lens technology becomes common in scientific laboratories. They hope their tool will allow humans to see objects from very small to very large with clarity previously unimaginable.
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Additional information: Bowen Li and al, Time-magnified Photon Counting with 550-fs Resolution, Optica (2021). Information from Optica Bowen Li et. al., Time-magnified Photon Counting with 550-fs Resolution, (2021). DOI: 10.1364/OPTICA.420816
