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Comment by xhrpost | Hacker News Reader

> To optimize this ratio in practice, we used the multi-pixel sensor of an electron multiplying charge-coupled device (EMCCD) camera as our idler detector (Fig. 1a). As the EMCCD can detect multiple photons simultaneously, it allowed us to identify and reject, that is, post-select, all events other than those where a single-photon pair was generated with a higher efficiency than with more traditional single-photon avalanche diodes (SPAD)

How is it possible to both detect a photon and then allow it to travel to the human eye? Wouldn't detection require absorption of the photon?

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There is a process called spontaneous parametric downconversion (SPDC), where if you shine laser of frequency f at a crystal, it will with small probability p emit two photons of frequency f/2 (energy conservation in play here).

If done correctly, the outgoing laser light and the two photons all travel in different directions and so can be separated and further directed using mirrors or optical fiber cables.

Because the process is non-deterministic, what we usually do is direct one of the photon beams towards a "heralding" [1] detector, while the other is directed towards the optical setup where we need a single photon [2]. If at a given moment a photon pair is produced, then the heralding detector will click; which tells us that is also a photon currently in our optical setup.

Finally, there is a ~p^2 probability that two photon-pairs will be produced at the same time by this process (and p^3 etc). To eliminate this possibility, in this experiment their heralding detector can detect how many photons landed on it any given moment. So if they see 2 or more photons in their heralding detector, then they discard this run, because now there are multiple photons heading towards the human eye.

[1] Herald as in the guy who announced that the King was approaching.

[2] In this case, towards the human eye.

I drew a schematic using https://asciiflow.com/

                                        +---------------------+
                                        | Heralding detector  |
                                        |                     |
                                        +---------------------+
                                       /
                                      /
                       +----------+  /  Photon 1
                       |          | /
    Laser beam ------> | Crystal  | ------------> Outgoing laser beam
                       |          | \
                       +----------+  \ Photon 2
                                      \
                                       \
                                        To human eye

"SPDC is a quantum optical technique in which correlated pairs of photons (called signal and idler) are produced probabilistically from a higher energetic pump photon in a non-linear crystal following energy and momentum conservation16,17 (Fig. 1a). By detecting one of the photons (idler) and sending the other (signal) to the observer’s eye..." I suppose conservation of momentum allows them to guarantee pairs, in which case a photon in one direction guarantees a photon in the opposite direction.

> How is it possible to both detect a photon and then allow it to travel to the human eye?

Leonard Susskind explained it like this in one of his lectures (they are on YouTube, he's an excellent explainer):

From the moment the photon is emitted, to the moment it's detected, the photon exists in entanglement with all the intermediary things it "touched". Only at the final location it's "absorbed" (with a probability). At the intermediary locations the probability ended on the low side so it passed through.

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