# Could a photon activate multiple photomultipliers at once?

1. Mar 7, 2015

### Ghostcrown

At the risk of making my inexperience in quantum mechanics plainly obvious, I have a question to ask. Say you emit a photon from one side of a room. As i've come to learn, light is not only a particle, but a wave. Say there are two photomultipliers, both encompassed within the wave of light. Would that trigger both photomultipliers, or would it act like an electron does and "roll the quantum mechanical dice" so to speak, and hit one or the other out of pure probability?

2. Mar 7, 2015

### MisterX

The measurement states are one or the other photo-multiplier is triggered, but not both. Both being triggered is not in the set of outcomes.

Edit: I want to add that the photo-multipliers themselves can be in superpostion. In the Dirac notation we might call this $a\mid1,0 \rangle +b\mid0,1 \rangle$ where the first number is 1 if the 1st detector is triggered and so on. So these two seemingly mutually exclusive things can exist side by side, which might happen if the wave was incident on the two detectors but the detectors themselves were sufficiently isolated. But there is no $|1,1>$ state essentially.

Last edited: Mar 7, 2015
3. Mar 7, 2015

### Staff: Mentor

Light is neither. It is a quantum object, that can show particle or wave properties sometimes. The detection of a photon is related to particle-like behavior, only one detector can detect it.

Photomultipliers lead to decoherence within nanoseconds (or maybe even faster).

4. Mar 7, 2015

### MRBlizzard

It is possible for a virtual photon to trigger a lasing cascade. I wonder if a prepared (in the manner of MisterX) evanescent wave of a single photon could trigger two lasing cascades.

5. Mar 7, 2015

### Staff: Mentor

If you want a single photon to trigger more than one PMT then the usual approach is to put a scintillation crystal in front. The high energy photon will hit the crystal and turn into a shower of lower energy photons and those secondary photons can trigger multiple PMTs. This is how most PET and SPECT is done.

6. Mar 8, 2015

### e.bar.goum

Indeed, and if the photon doesn't deposit its full energy in the crystal (e.g. compton scattering followed by escape), you can create signals in multiple scintillation crystals too. (This is basically also how Compton suppression works as well - use a scintillator crystal surrounding your detector as a veto)

7. Mar 8, 2015

### Staff: Mentor

Okay, but I would not call that "the photon hit two detectors" because the original photon got converted to multiple new photons and only those hit the detectors.

8. Mar 8, 2015

### Staff: Mentor

Agreed.

As a practical matter this is often important because PMT tubes are relatively transparent to high energy photons so we need the crystals to detect enough photons at all. But we are interested in the first photon and not the shower of lower energy photons. So the triggering of multiple PMTs is somewhat of a nuisance.