Gaining Quantum information without destroying the coherence

Question

I've read about new optical interference among several telescopes all around the globe. It is supposed to work by combining the quantum states of a photon A arriving at once in every telescope of the grid.

Now you may think ... Well, if the quantum state is measured, the photon will collapse... So ... How can they read and save the quantum state in some kind of quantum hard disk without destroying the quantum superposition of the photon itself? Because if you measure the photon in some location, in some telescope located in some place, the photon itself is not any longer in other telescope located far away from the first one. And you would need the information of both "photons" to make the interference.

So, how does this work?

This problem maybe better explained in this link.

Answer 1

I read the link you give, and sounds like science fiction to me, but that is what our present day technology was for people a hundred years ago.

It is supposed to work by combining the quantum states of a photon A arriving at once in every telescope of the grid.

Not the same photon. They want to get an interference pattern analogous to the interference pattern seen in a double slit experiment one photon at a time , for example.

Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

Each photon leaves one dot, but the summed photons show the probability distribution and the wave is a probability wave .For this simple experiment it is not necessary to have the photons accumulate on one screen, If one recorded the x,y of the photon footprint on the screen and changed screens, the interference would still be there adding single photons screens.

The article you link to says:

Now imagine that, instead of two slits, you have two telescopes. When a single photon from the cosmos arrives on Earth, it could hit either telescope. Until you measure this — as with Young’s double slits — the photon is a wave that enters both.

It is at this point that I cannot follow the logic. The size of the slits and the distance between them is very strict and related to the wavelength of light, otherwise there is no interference. ( let alone that in my opinion the photon is not a wave, the probability of interacting has the wavefunction properties).

I think that the popularized description does not describe correctly how they expect to make two telescopes into one telescope.

Bland-Hawthorn, Bartholomew and Sellars suggest plugging in a quantum hard drive at each telescope that can record and store the wavelike states of incoming photons without disturbing them.

Just proving the wavelike state of individual photons is mind boggling Afaik a photon is an elementary point particle of spin + or - 1 , mass zero and energy $hν$ where $ν$ is the frequency of the classical electromagnetic wave built up by many such photons.

Answer 2

Consider you have telescope (T1) and you observe a spec of light which you know to be 2 stars ... but your telescope just can't resolve it. With some study you realize you are diffraction limited (Airy disk) and you need a bigger aperture so you call a friend to come over and help. She brings her telescope (T2) over but you realize it has the same aperture size as your T1. Being determined she hopes that if the images are combined digitally you just might be able to resolve it. To her dismay the digital technique doesn't work. After some thought she comes up with another innovative idea, she suggests moving T1 and T2 directly beside each other thereby having a net larger aperture, the hope is that the images of each telescope will improve and finally resolve the 2 stars! After observing the individual images as well as digitally combined ones, it still does NOT work ... something is still a miss.

So you then you remember a physics stack exchange question you had submitted and that some how things had to be combined quantum mechanically!! So you state that that T1 and T2 must be combined in a way so that the incoming photons see one complete optical system where their respective EM fields can interact with a bigger aperture and detector ... You state the wave functions must collapse so that some photons that enter the left aperture can actually be detected by the right camera (and vica versa). Well she is very impressed and although you do not know how to make such a system she suggest using mirrors and lenses so that each telescope makes an overlapping image on the wall ..... after a while low and behold you observe 2 specs of light!

You guys are very impressed with one another, you realize that not only are the wave functions seeing the bigger apparatus but that each photon is flying a little straighter .... i.e. less diffracted. In another experiment you combine the images onto a single ccd ... yes the 2 stars are resolved.

The challenge with the proposed system in your link is that photons have to be stored in he same quantum state as they exist in the "wild" some how they are stored without being detected or collapsed ... it is not until they are collapsed that the required information is gleaned.