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Comment by xgbi | Hacker News Reader

> Eventually, quantum teleportation in space could even allow researchers to combine photons from satellites to make a distributed telescope with an effective aperture the size of Earth — and enormous resolution. “You could not just see planets,” says Kwiat, “but in principle read licence plates on Jupiter’s moons.”

Uh, what? What does entangled photons have to do with interferometry-based astronomy? Can somebody explain to me how this could be achieved?

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It lets you bypass some classical limits. It's a well known idea. For example, John Preskill mentioned entangled telescopes in a non-technical talk back in February [1].

A specific case is "NOON" states [2]:

> NOON states are an important concept in quantum metrology and quantum sensing for their ability to make precision phase measurements when used in an optical interferometer.

---

Here's an example that doesn't exactly use entanglement, but does use quantum stuff to give you the general flavor. Suppose you have an optical setup like this:

          B
          |
          v
    A --> ◩ -----> D1
          |
          |
          v
          D2

When a photon is emitted from A, or B, it passes through a beam splitter then continues on to the two detectors and triggers one of them. If the detectors are classical, then there's no way for you to distinguish whether A or B emitted the photon. Both are a 50/50 split. But if the detectors can store their readings at various times as quantum information and keep that information coherent, then you can bring the stored qubits together, simulate un-applying the beam splitter, and voila! The same basic idea applies to telescopes: photons from different sources spread out in slightly different ways, and we can undo some of that spreading with quantum computation.

1: https://youtu.be/lN8zT_Yk5sg?t=3m58s

2: https://en.wikipedia.org/wiki/NOON_state

Thanks, I actually understood that!

I can already read all of the license plate data on Jupiter's moons. The log is attached below:

Well, it is theoretically possible that life exists deep inside Jupiter. They may even have cars and license plates.

But then again, even a quantum telescope might not be able to see it either.

This also made me raise an eyebrow. I found a preprint[0] of an article on the subject after some googling. I've only skimmed it so far but it seems promising.

[0] http://arxiv.org/abs/1403.6681

Really cool. Here is some more recent work (2016) by the same author, Aglae Kellerer.

https://arxiv.org/abs/1604.06928

Taking a guess here -- the entangled photons would act as nodes between satellites, enabling the combining of information collected by the satellite array. Essentially, take an interferometric array such as CHARA, replace fiber with empty space, and voila. I remember some of the technical obstacles CHARA had to overcome, one of which was extremely precise path lengths from each telescope to the central collector. Not sure if they could achieve that in space where no solid anchoring material exists.

I believe they're referring to this proposal by Gottesman [0]. I will try to explain in layman's terms what is happening: the simplest telescope array you can imagine is just two telescopes separated by what is called the baseline. The telescopes could either just detect light from a star independently and then autocorrelate these signals afterwards, or they could interfere their signals before detection. The latter is called an interferometric telescope array and has a resolution advantage over the former, simpler, solution. The length of the baseline is an important factor in determining the size of the objects you can resolve with such a telescope, and the goal is generally to make it as large as possible. Unfortunately, as the baseline becomes larger the signals from the two telescopes are likely to become distorted or lost before you can interfere them due to the distance you have to send one to reach the other. The Gottesman paper offers one solution to this, which is to use a quantum repeater (essentially quantum teleportation) to send one of the telescope's signals to the other telescope without any loss or degradation, in principle allowing for arbitrarily long baselines in telescopes.

[0] https://arxiv.org/abs/1107.2939

It sounds like observing only entangled photons lifts the diffraction angular resolution limit of the classical telescopes.

I was wondering the exact same thing.

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