Big telescopes are all the rage; better because they collect more light and produce higher-resolution images. The largest optical telescope is currently the Gran Telescopio Canarias in the Canary islands, which has a primary mirror with a diameter of 10.4 meters.
This will soon be dwarfed by the Extremely Large Telescope currently under construction in Chile, which, when it switches on in 2024, will have a primary mirror almost 40 meters in diameter. It will also have cost about $1 billion to build.
But there is a cheaper way to make telescopes bigger—build an array of smaller ones and combine the light from them using an interferometer. The most powerful of these is CHARA, which is situated on Mount Wilson in California. CHARA consists of six one-meter telescopes separated in a way that gives them a resolution equivalent to a 330-meter mirror.
This produces much higher-resolution images than any conventional telescope. In 2013, CHARA took the first images of starspots on the surface of another sun, Zeta Andromedae, some 180 light-years from here.
But there is a problem with giant optical arrays. The light collected at each telescope must be fed to a central interferometer that combines the photons to create an image. Photons are inevitably lost in the transmission process, however, and this severely limits the imaging performance.
As a result, CHARA and other similar arrays can image only bright stars. And the prospects for building bigger arrays look bleak.
Enter Emil Khabiboulline and colleagues at Harvard University in Cambridge, Massachusetts, who today show how the strange laws of quantum mechanics can help solve this problem. They say that quantum-assisted telescopes could significantly increase the maximum size of these arrays and the resolution of the images they can produce.
First, some background. Physicists have long known that quantum particles created at the same point in the universe share the same existence. This creates a connection between them that survives even when they are separated by huge distances. This connection is called entanglement, and physicists have already exploited it to send quantum information across space and to teleport quantum particles from one location to another.
Teleportation begins with a pair of entangled particles, call them A and B. When one of this pair, A, interacts with a third particle, the quantum information from this third particle is transmitted across the entangled link to particle B, which takes on its identity.
It’s as if the third particle has traveled from one location to another without passing through the space in between. That’s why physicists call it teleportation.
It is this process of teleportation that quantum-assisted telescopes will exploit. The idea, first proposed in 2011, is to create a constant stream of entangled pairs. One of the pair resides at the telescope, while the other travels to the central interferometer.
When a photon arrives from a distant star, it interacts with one of this pair and is immediately teleported to the interferometer, where it can create an image. In this way, an image can be created without the losses that normally limit performance.
When this idea was first proposed in 2011, physicists immediately realized that it would require a huge number of entangled pairs, one for each incoming photon. That’s in the region of 1011 per second at CHARA and orders of magnitude more than is possible with current technology.
Because of this, the idea of using teleportation-assisted telescopes has languished. Until now.
The breakthrough that Khabiboulline and colleagues have made is to work out how the quantum information from starlight can be compressed and stored and how this dramatically reduces the amount of entanglement required. “The necessary rate of entanglement distribution is reduced by several orders of magnitude, which opens up realistic prospects for employing near-term quantum networks for high-resolution imaging,” they say.
The technology that makes this possible is quantum memory. These are devices that can store a quantum state and then transmit it. “[This yields] an exponential reduction in the consumption of entangled resources, as compared to memoryless schemes,” they say.
Physicists have recently made significant strides in developing quantum memories, driven by the idea that these devices will enable technologies such as a quantum internet. Quantum telescopes are significantly more demanding because of the required rate of entangled particles. But Khabiboulline and colleagues say that this now looks more practical.
It’s interesting work that opens up an entirely new approach to astronomical imaging. The suggestion is that it will make possible an array with a baseline in the region of 30 kilometers. That will significantly increase the resolution of the images.
But in principle, it should be possible to build arrays that are significantly larger still, perhaps even the diameter of Earth. That’s an exciting prospect for the astronomers of the future.
Ref: arxiv.org/abs/1809.03396 : Quantum-Assisted Telescope Arrays