Chinese satellite is one giant step for the quantum internet

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.

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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

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

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

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.

Exactly what I was thinking, if it's easy to tell they've been intercepted by a third party that infers it's possible to do it so what's to stop a DOS attack making it useless?

1. Yeah space so nothing disturbs the photons

2. "heart of their satellite is a crystal that produces pairs of entangled photons" (see https://en.wikipedia.org/wiki/Spontaneous_parametric_down-co...)

3. Mostly polarization

The quantum computers on earth work by cooling the processors to sub-zero (close to zero Calvin) temperatures. And space is dead cold. So that may be a factor. Also, the article mentions that light travels uninhibited in space.

No, it doesn't see the No Communication theorem. https://en.wikipedia.org/wiki/No-communication_theorem

Thanks, we updated the link from http://www.scientificamerican.com/article/chinese-satellite-..., which pointed to this.