Astronomers have long sought to develop larger and larger telescopes capable of capturing more light and revealing dimmer, more distant objects. However, the day may be coming when quantity matters more than quality. A new line of investigation could be on the verge of bearing quantum fruit, according to Scientific American. Researchers are now discussing the possibility of designing a globe-spanning quantum telescope modeled on the successful Event Horizon Telescope (EHT) that imaged a black hole in 2019.
The EHT was able to produce the first-ever image of a black hole using a technique called interferometry. By combining radio telescopes from all over the world, the team created a “virtual telescope” the size of Earth itself. You can’t just flip a switch and do the same thing with optical wavelengths, though. Currently, optical interferometry only works over distances of a few hundred meters, but a group of Australian astronomers says we might be able to boost that with a little help from quantum mechanics.
Interferometry with radio sources is easier for several reasons, and the first is simply a product of our level of technology. Radio telescopes are cheaper and easier to build, making it feasible to pull together a large network of them. Astronomical objects also tend to be very bright in radio frequencies, so the telescopes have a lot of data to build interferograms. Visible light is also more likely to be distorted in the atmosphere than radio waves, complicating matters further. Therefore, optical interferometry essentially requires collecting photons one at a time, making interference a quantum phenomenon.
Currently, optical interferometry requires sending photons over fiber optics to a place where they can be combined to produce an interferogram. The problem: they have to travel the same distance to get there, which creates a mess of timing, delay lines, and lost data. The four telescopes of the ESO’s Very Large Telescope (above) are the best we have at optical interferometry, and they’re just a few hundred meters apart.
The M87 supermassive black hole imaged in 2019 with radio interferometry. Astronomers are trying to do something similar with photons.
The Aussie teams have proposed a workaround that is simultaneously high-tech and pedestrian. The key to optical interferometry might be a Quantum Hard Drive (QHD). The purpose of this device would be to store the amplitude and phase of light as a function of time for each photon picked up by the network of telescopes. Instead of trying to transmit them, the hard drives would be physically moved by car, airplane, train, or even by foot to a place where the photons can be combined into an interferogram.
The main barrier right now is building a QHD that maintains quantum states for long enough. In 2015, one team managed to use a europium-doped yttrium crystal to store the nuclear spin states of photons for several hours. Late last year, a Chinese team says it succeeded in storing and retrieving the quantum states of photons in a similar device after an hour. So, the basic hypothesis seems plausible — astronomer Jonathan Bland-Hawthorn of the University of Sydney believes this technology could become the standard for deep space observations. There’s a lot more work to do before that happens, though.
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