Finally, Single-Photon Cameras Can View Your Brain

 

Researchers at NIST Boulder create the first-ever large-scale chip that detects single photons with superconducting nanowires.

Since they can only be scaled up to a few pixels, superconductor-based cameras that can detect a single photon—the tiniest particle of light—have only been used in laboratories. A 0.4-megapixel single-photon camera has now been developed by a team at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. This camera is 400 times bigger than the largest of its kind previously. They published a preprint on arXiv on June 15 in which they described their findings.

       Superconducting nanowire-based single-photon cameras provide unparalleled sensitivity, speed, and frequency range for measuring light. The single-photon camera is prepared to make the leap from being a lab curiosity to an industrial device. Such cameras could find use in photonic quantum computers and communications, picturing the cosmos on the next James Webb-type observatory, measuring light, and using non-invasive light-based approaches to look inside the brain.

"From a scientific perspective, this is definitely opening a new avenue in optical brain imaging," says Stefan Carp, an associate professor of radiology at the Harvard Medical School who was not involved in the research. Other methods for optically imaging cortical brain flow may be less expensive, but they also have signal quality issues that frequently necessitate complicated signal processing. From a performance standpoint, nanowires offer no compromise.

    Superconducting nanowire detectors can capture almost every photon, work at visible, ultraviolet, and infrared frequencies, and spit out results in mere picoseconds for high frame-rate detection. The detectors’ sensitivity stems from the fact that a high enough electrical current running through a superconductor will destroy its superconducting properties. Each pixel of the camera is a superconducting wire with a current set just below the threshold, so that a single photon colliding with the wire will break its superconductivity. The break results in increased resistance across the wire, which can be detected almost immediately.

The Secret to Scaling Up Single-Photon Cameras

Although a single pixel has excellent performance, it has long been difficult to fit numerous of them near to one another on a single chip. It is prohibitive to wire up many pixels into the cooling system since superconductivity requires cooling the device to cryogenic temperatures. Adam McCaughan, the project's principal investigator and a staff physicist at the NIST, adds, "I certainly can't put a million wires into my cryostat." It would require absurdly complex tech to make it happen, let alone read it out. 

            The researchers looked to other detecting technologies for inspiration in order to get over these obstacles. In order to collect detector information from a complete row or column of pixels at once, they adapted the concept of a shared read-out bus. But a simple implementation of the bus caused cross talk between the pixels, which rendered the device insensitive. Because readout buses are often designed symmetrically, anything that can go out can also come in, according to McCaughan. So we asked ourselves, "How can we asymmetrically couple the detector to the bus?"

    Understanding the asymmetrical system by which the signal from a detector would be sent to the bus but not the other way around was crucial. To do this, the researchers created an intermediary step with a heating element wired in parallel with the superconducting nanowire right next to each detecting pixel. When a photon strikes a nanowire, superconductivity is broken, and the current is diverted into the heating element. The bus, which is also constructed of superconducting wire, would then experience localized superconductivity breakdown when the heating element heated up naturally. As a result, the desired asymmetric coupling would be produced without disturbing nearby heating elements.

 Bigger cameras may have more useful applications

        This strategy was really successful. According to McCaughan, who is referring to main author Bakhrom Oripov, "after the fabrication process was optimised, I remember Bakhrom kept coming and telling me, 'Hey, Adam, I think I got the 2000-pixel camera working.'" "And after a week, he returns and tells me, 'I got the 8000-pixel one operating.' And after that, "I got the 40,000-pixel one." It simply kept climbing higher and higher.

  Numerous applications are made possible by the enormous size improvement, particularly in biological imaging. Roarke Horstmeyer, an assistant professor of biomedical imaging at Duke University, and others are developing methods to picture the brain by putting light into it and detecting the minute amounts of light that scatter back out. Making a portable MRI is the "grand vision," according to Horstmeyer.

          Near-infrared frequencies are good for illuminating human tissue. Higher intensities are possible since they are less damaging and can pierce deeper into the tissue. At these frequencies, silicon-based detectors that are readily available do not work effectively. According to Horstmeyer, "This nanowire technology is really well suited for the light that is preferred to be used in bio-optical devices." Possibilities such as real-time imaging of the entire brain are made possible by having a huge equipment of this kind.

          The Boulder team is currently collaborating closely with a number of bioimaging organisations to modify the system to meet their unique requirements, such as enhancing timing sensitivity. These advancements, according to the experts, are easily attainable. The sky is the limit in terms of this technology's potential applications, claims Carp.