The goal of new telescopes is usually to resolve more details at greater distances. The most straightforward way to do this is to simply make telescopes larger. Unfortunately, the simple weight of the mirrors makes this a path fraught with difficulties. Despite the difficulties, there are consortiums attempting to do just this.

An alternative is to combine the light from different telescopes. The interference between the light fields captured by separate telescopes creates an image that has details that would ordinarily be resolved by a telescope with a size similar to the separation between the telescopes. This seems simple, but the light fields are very weak and losses involved in transporting and combining the light fields are such that separations of a few hundred meters are today's limits. A trio of Canadian researchers has proposed using technologies being developed for quantum key distribution to greatly extend the allowable distance between telescopes.

How does interference help?

Imagine we have a pair of objects very close to each other, but at a great distance from our two telescopes. According to the individual telescopes, there is actually only a single object, because they do not have sufficient resolving power to separate the pair. When we use the two telescopes together, though, things change. The light that travels to the first telescope has to travel a slightly different distance to the light that travels to the second telescope. That difference depends on the direction from which the light comes, which is slightly different for the two objects. When the light from the two telescopes is mixed, the brightness of the pattern depends on that path difference: if it is an integer number of half wavelengths, we get a bright pattern, and as we move away from that magic number, the pattern dims and eventually disappears.

When we mix the light from the two telescopes, we can introduce an additional delay to the light from one telescope, so that we can choose to get a bright pattern. In our case, with two objects, each one produces its own repeating pattern of bright and dark patterns as the delay is varied. The mixture of those two patterns tells us that there are two objects, not one. It also tells us the angular separation of the two objects. Hence our two (rather poor) telescopes have resolved objects that would normally only be separated by a much more expensive telescope.

That's already awesome—why add quantum-y stuff?

Assuming that the light from stars is purely classical, I was a little puzzled at how the physics behind quantum key distribution might help in this respect. It turns out that as you crank up the resolution of the telescope system, you see more features, but the total amount of light doesn't increase that much, so the total number of photons per mode (or feature) goes down until it is much less than one. This makes starlight very much like a quantum light source.

But with so few photons, it's impossible to build up the interference patterns used to separate objects. Indeed, every photon counts, so the last thing you want to do is throw them into an optical fiber and have them absorbed by a defect in the fiber. Instead, a Canadian team of researchers has proposed to use entangled photons to perform the interference measurement at each telescope without having to transport the starlight.



Quantum entanglement Quantum entanglement is one of the most misused concepts around. Entanglement is delicate, rare, and short-lived. At its heart, quantum entanglement is nothing more or less than a correlation between two apparently separate quantum objects. Having discovered that, you might ask "so what is all the fuss about?" The answer lies deep in quantum mechanics. Read more…

The basic idea is that, somewhere between the two telescopes, we generate entangled photons on Earth that are sent to a randomly chosen telescope over a fiber optical cable whose length can be varied. At each telescope, the generated photon is mixed with the starlight on a beam splitter. Half the time, the stellar photon and the generated photon go to the same telescope, and the result is thrown away. The other half of the time, the two photons go to different telescopes, resulting in one of two detectors clicking at each telescope. The interference pattern is built up by looking for simultaneous clicks at pairs of detectors as a function of the fiber optic cable length.

This scheme has the advantage that the photons sent between the telescopes are generated here on Earth and can be easily replaced. Indeed, single photons can be sent over lengths of about 70-100km reliably at the moment, a massive increase over the few hundred meter limitation in stellar interferometry. But the scheme automatically throws away half of the stellar light—enough to make any astronomer cry. And single photon sources are simply not very good at the moment; they don't produce useable photons at the rate required. Indeed, the researchers acknowledge this, and calculate that the single photon source should produce photons at about 150GHz to end up with a sensitivity comparable with today's stellar interferometers. For reference, current single photon sources are in the MHz range.

So why is this interesting if it's so far away from a practical implementation? The technology that the team is proposing is exactly what people are working on in quantum information technology, but the requirements are much more stringent for stellar interferometry. These ideas, then, act as a technology driver for those working on the practical side of quantum information technology.

Physical Review Letters, 2012, DOI: 10.1103/PhysRevLett.109.070503