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I’m not sure if such a device exists, or if it could.

Is it possible for a photon detector to be one-way? Like a one-way mirror, where one side can be seen through, but the other side is a mirror.

This is the same concept, but one side can be passed through, whereas the other side can detect photons.

Does such a device exist?

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  • $\begingroup$ This sort of nonreciprocal device is certainly possible: "circulators" are common microwave components. I'm a bit stumped as to how you'd make it polarization-independent in the optical, but maybe somebody else knows. $\endgroup$
    – John Doty
    Commented Sep 5, 2023 at 21:23
  • $\begingroup$ globalspec.com/learnmore/optics_optical_components/fiber_optics/…. Put your photon detector on one of the ports, and you have it. $\endgroup$
    – John Doty
    Commented Sep 5, 2023 at 21:33
  • $\begingroup$ You can make a polarization independent circulator, based on a Faraday rotator. $\endgroup$
    – JQK
    Commented Sep 5, 2023 at 21:45
  • $\begingroup$ @JQK Indeed, and the link above has the details. $\endgroup$
    – John Doty
    Commented Sep 5, 2023 at 23:00

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This statement holds for all devices that are reciprocal, or time reversal invariant, One Way Mirrors do not exist. Thus your detector doesn't exist.

However, let's look at 2 situations where you could make this work:

  1. Differentiating between modes of photons. You can craft a device that allows a certain polarization to be transmitted but it's orthogonal polarization to be detected. Or you can transmit photons of a certain wavelength and detect photons of other wavelengths.

  2. If you had a non-reciprocal device, such as a Faraday rotator, you can perhaps think of a system that may perform as you wish.

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    $\begingroup$ Nonreciprocal optical devices are common. $\endgroup$
    – John Doty
    Commented Sep 5, 2023 at 21:41
  • $\begingroup$ Im not sure if I misunderstand; but I believe one-way mirrors are commonly used in, for example, police interrogation rooms. $\endgroup$
    – Loic Stoic
    Commented Sep 5, 2023 at 23:00
  • $\begingroup$ You do misunderstand. That is not a one way mirror. The mirror that you are referring to is a normal mirror. How it works is that there is less light in the room where the observers are relative to where the suspect is. $\endgroup$
    – JQK
    Commented Sep 5, 2023 at 23:04
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    $\begingroup$ @LoicStoic Many so-called one-way mirrors are simply partly reflective. If you have a brightly illuminated room on one side, and an unilluminated room on the other, someone on the dark side can see the leakage from the bright side, but the other way there isn't enough. $\endgroup$
    – John Doty
    Commented Sep 5, 2023 at 23:04
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    $\begingroup$ @LoicStoic those aren't truly one-way mirrors. They work as follows. They are windows with moderately low transmission and decent reflection. But the reflection and transmission coefficients are the same whichever way light is going. Then one room is kept very dark and the other room is kept very bright. The dark room can see light from bright room because the room is brightly lit. The bright room can't see the dark room because (1) not much light is coming out of the dark room and (2) a lot of light from the bright room is reflecting off the window, washing out the light from the dark room. $\endgroup$
    – Jagerber48
    Commented Sep 5, 2023 at 23:04

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