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This article discusses the recent invention of a "quantum radar" that works by bouncing one photon off of something, storing a photon that's entangled with it, and watching for interference on the stored photon that indicates the radar photon has bounced off an object.

Most of that makes sense, (about as much as anything in quantum physics makes sense, at least,) but... how exactly do you "store" a photon? To my mind at least, the concept of storage involves putting something in a location and keeping it there, unmoving, but photons are not capable of not moving at the speed of light. So what's going on in this scenario?

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  • $\begingroup$ "To my mind at least, the concept of storage involves putting something in a location and keeping it there, unmoving" - For the purposes of keeping a photon in "storage", there's no need for the photon to be "unmoving", is there? It could maybe just be kept busy bouncing back and forth between mirrors or something like that. The article you presented is about an experiment operating over a distance of just 1 meter so keeping the photon busy for the corresponding short amount of time shouldn't be difficult. $\endgroup$ – user93237 Aug 23 '19 at 17:45
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A common - and typically the simplest - way to "store" a photon for a limited amount of time (this is, distance travelled) is to send it into a rolled up optical fiber.

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  • $\begingroup$ Note that from a quick look it is not clear to me whether the original paper is really about single photons or not weak coherent light. Also, they don't talk about storage of photons. $\endgroup$ – Norbert Schuch Aug 23 '19 at 23:24
  • $\begingroup$ Would a rolled up optical fiber preserve the coherence of the stored photon so that it can show interference when combined with the reflected photon from the target object? I'm just imagining all of the reflections that can occur in an optical fiber when sending light through it and wondering what that does to the coherence of the light going through the fiber. $\endgroup$ – user93237 Aug 24 '19 at 1:34
  • $\begingroup$ @SamuelWeir Yes, just like a photon traveling through an optical medium (like a lens) keeps coherence. The polarization will be rotated, but this can be measured beforehand and drifts relatively slowly. I'm sure you can find details of that with a bit of searching, it is a pretty standard method. $\endgroup$ – Norbert Schuch Aug 24 '19 at 8:46
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In the article you cite, the researchers create entangled photon pairs via parametric down conversion. One of the photons is the signal, the other is the idler.

They shoot the signal photon towards the object they need to radar, and wait until it is reflected.

Now in the meantime, they need to keep the idler photon physically inside the lab. In your case, what they could do is they create special reflecting mirrors, and keep the idler photon reflecting, bouncing between the two mirrors.

Now when the signal photon returns, it interferes (the signal and idler are still entangled) with the idler photon, and the researchers can calculate the time and distance for the signal photon, thus getting the distance to the object.

Now in your article it says that this method is only effective for short distances. The reason for that is:

  1. photons can be kept bouncing between mirrors only for a limited time (as per QM, the mirrors not being perfect, will elastically scatter the photon for a while, but since it is all probabilities, after a while the photon will be absorbed)

  2. the photons need to be low frequency (otherwise they would melt the walls of the mirror when absorbed)

https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.113.133601

https://arxiv.org/abs/1307.2830

https://people.phys.ust.hk/dusw/wp-content/uploads/sites/7/2018/01/PhysRevLett_113_133601.pdf

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