Astronomy is ready for the next generation of detectors, and superconductors are at the heart of the coming revolution.
Back in my former life, I was an X-ray astronomer. While optical astronomy charged ahead with camera technology that benefitted from commercial investment (hello, smartphones), the X-ray detectors I worked with were of a more “homebrew” variety (really good homebrew).
If I point an X-ray telescope at, say, a distant quasar for a few hours, I might get a few hundred photons if I’m lucky. Compare that with an optical image, where the same quasar might emit millions of photons. As a professor of mine once joked, X-rays are so few and far between, they should have names: “Look, there go Peter, Jill, and Harry.”
But, paradoxically, there's a benefit to that. Using detectors aboard telescopes such as ESA’s XMM-Newton or NASA’s Chandra, you really can get to know each individual photon — if not its name, then at least its energy and arrival time. In more scientific jargon, take an X-ray image, and you get both a low-resolution spectrum and a light curve for free.
Typical optical telescopes can’t do that. They use charge-coupled devices (CCDs), like the digital camera in your smartphone, to capture photons. But a CCD image is just an image — to put together a light curve, you would need to take multiple images, and to split the light by wavelength would require a spectrometer.
Now that may be changing. A new technique is emerging called integral field spectroscopy, in which optical detectors use various methods to grab a spectrum at the same time that they take a picture. Among the newest instruments is the Array Camera for Optical to Near-infrared Spectrophotometry (ARCONS), developed by Ben Mazin (University of California, Santa Barbara) and colleagues.
ARCONS transforms visible-light and near-infrared telescopes into multitaskers, measuring the energy (to a few percent or better) and arrival time (to within a microsecond) of each photon. And though the technology is still in its infancy, a 2,024-pixel array is already taking images on the Palomar 200-inch telescope.
Semi vs. Super
The difference between ARCONS and CCDs is due to the difference between superconductors and semiconductors. When a single photon enters a semiconductor-based camera, it unleashes a single electron, which is shuffled along until it reaches the end of the row. The detector counts up the electrons in each pixel to create an image.
But in ARCONS, a single photon unleashes not a single electron, but a cascade of thousands of them. And when the detector counts electrons, it sees not just how many there are, but also when they arrived and with what energy. This is possible because at the heart of ARCONS is a superconductor, a material that lives in the weird world of quantum physics. Normally, electrons repel each other, but in certain materials cooled to a fraction of a degree above absolute zero, electrons can form incredibly weak bonds called Cooper pairs. The material has to stay cold enough to calm the electrons’ thermal wiggles; any disturbance, such as a single intruding photon, can break the pairs and unleash a cascade of electrons — and it’s this flood of electrons that ARCONS picks up.
ARCONS is revolutionary, too, in that its electrons don’t march out of the detector in single file to be counted. Instead, each pixel tallies its own electrons, then the detector collects all the pixels’ information by sweeping them from the outside with carefully tuned microwaves.
The external sweep allows the superconductor to stay barely above absolute zero while the surrounding electronics remain at room temperature. And a supercold detector is a huge plus because it means that it has virtually no “dark noise,” the false photons CCD cameras see even when the shutter is closed. (If you’ve ever closed your eyes in a dark room, you’ll notice it’s never completely dark — your eyes have dark noise, too.)
Detector of the Future
“It is really a complete switch of technologies,” Mazin says. “The switch really opens up a whole new world of possibilities, just like the switch 30 years ago from film to semiconducting detectors.”
One of those possibilities is the direct detection of exoplanets, a task made extraordinarily difficult by the huge contrast between the faint planet and its much brighter star. Mazin’s team already has the funding to build a new ARCONS-like detector designed to work with Palomar’s Project 1640. The detector’s ability to capture images and spectra simultaneously, plus its low noise level, will help catch the “firefly near the searchlight.”
Here are links to two other studies using ARCONS: