On 25/08/16 17:35, jim bell wrote:
*From:* Georgi Guninski <[email protected]>
On Thu, Aug 04, 2016 at 02:22:05AM +0000, jim bell wrote:
>>
http://www.dailymail.co.uk/sciencetech/article-3720772/China-launch-unbreakable-quantum-spy-satellite-say-one-day-lead-megascope-size-Earth-spot-license-plate-Jupiter-s-moons.html
>> [quote]
>> China to launch unbreakable quantum spy satellite - and it could one
day lead to a megascope the size of Earth that could 'spot a license
plate on Jupiter's moons'
>China (Austria is also involved) launched this on 16 August 2016:
> https://en.wikipedia.org/wiki/Quantum_Experiments_at_Space_Scale
>Also in many news.
When I originally posted this, I briefly noted that I had a problem with
this
news item. As I recall, one of the problems was that they referred to
this
'megascope', without explaining the connection. It was as if two high-tech
articles collided, and bounced off each other, leaving a bit of detritus on
the other.
What does this quantum link have to do with building a super telescope? The
article was less than even unclear: It was totally silent on that matter.
Currently, the largest single-lens telescope mirrors are made in a rotating
furnace in Arizona, about 8.5 meters in diameter. the purpose of the
rotation is to make them very close to the idea curvature from the
beginning,
rather than polishing them out of a flat blank of glass as was the
previous process.
Other telescopes are going to use multiple-mirrors to increase the
light-collecting
area. That's important, but another factor is that the larger effective
diameter
of a telescope mirror, the smaller angular difference that can be
imaged. I
recall a data point: A 4.5 inch mirror has a resolution of about 1
second of arc.
(defined, I think, as a line/space pair, not merely a line.)
A telescope based on an 8.5 meter lens will have, ideally, a resolution
of 0.0134
arc seconds. Combine seven of them subtending a larger-diameter, and you'd
get perhaps 3 times the diameter, and one third the angular resolution:
About
0.00448 arc seconds. https://en.wikipedia.org/wiki/Giant_Magellan_Telescope
Would it be possible to 'mount' three such 8.5 meter mirrors in an
array where they
are millions of kilometers away from each other, and somehow combine
their images
and to produce and preserve the resolution of the larger diameter? It
wouldn't multiply
light-gathering ability, but it would increase the angular resolution
immensely, perhaps by
a factor of 100 million to one billion.
I speculate that this is what is being alluded to in the article's
reference to a 'super telescope'.
It would not be sufficient to merely detect the images generated by each
mirror; somehow
it would be necessary to combine the light signals to include phase
information. Perhaps this
could be done by some sort of quantum process.
As Sean said, this is both possible and done.
Some points here - first, the "signal addition" process may or may not
need to be quantum in nature. This depends on signal nature, strength
etc, ie a strong multi-photon CW or coherent signal would not need
quantum path addition, whereas a weak single-photon-at-a-time signal
would need quantum path addition.
If quantum path addition is needed, signals from two mirrors cannot be
converted or recorded separately, but must remain in their original
quantum state, ie as quasi-photons, until they reach the single
detector. In practice this can be done with optical cavities or fibres,
but these are very demanding as to precise length, dispersion etc.
Thinking of this as a sum-of-paths problem, there is a potential path
through one mirror/fibre, and a potential path through the other. The
photon takes both paths - think of the dual slit experiment. If it
doesn't take both paths but only goes through one mirror/fibre, then the
resolution advantage of the large mirror separation are lost.
Second, the "curse of the sparse array". The % of photons which are
detected is reduced from the % which would be captured by a mirror of
area a is further reduced by the fraction 2a/A, where A is the area of a
filled array of the same dimension as the synthetic aperture. The
resulting signal can be very dim.
Because of this signal dimming, and the engineering demands of
quantum-path addition hardware, the maximum practical synthetic optical
aperture is about 200 meters, though quantum-based synthetic radio
apertures up to 20km or so are used.
(vbli, very long baseline interferometry, which operates over thousands
of km, is not quantum-path-addition based - it operates on either
variations in a noisy signal or on the phase of CW signals, roughly
speaking)
Speculating about the megascope, about which I know nothing either, if
it is a simple telescope designed to collect a significant fraction of
single photons from a satellite for quantum cryptography purposes then
it should be near-filled (no gaps) in order to avoid sparse array
dimming problems. This can be done with many medium-large mirrors close
to each other.
As to resolution, The Daily Fail gives a distance of 750 miles, or a
close-ish LEO. Your 0.00448 arc second telescope would have a resolution
of about 1 inch at that distance, which would probably be plenty enough.
However the Fail seems to be talking about something else, a telescope
using quantum teleportation - but I don't know what that is.
-- Peter Fairbrother
