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If the photons of a laser beam all are coherent so that they all have the same momentum why don't we see them tunnel barriers very often ? Maybe I am misunderstanding what the coherence in laser beam really means: are they all in the same position or they vibrate the same way (momentum) if yes they should be able to quantum tunnel very often and quantum information converted into light would as well tunnel too...

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  • $\begingroup$ Why do you assume that coherence leads to quantum tunneling very often? $\endgroup$ – probably_someone Jul 11 '18 at 4:13
  • $\begingroup$ Since they are all coherent and behave in the same way if one photon tunnels then the others immediately tunnel too because they are all entangled. So if they are all coherent they will all tunnel contrarily to when there is decoherence and the chance of the not coherent photons tunneling all together is very small. $\endgroup$ – user198045 Jul 11 '18 at 4:16
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Coherence between between two light sources means that there is a fixed phase relationship between then. Therefore, they would form interference patterns that can be observed. One can have coherence between different parts of the same laser beam, so that the beam will interfere with itself when the different parts are brought together to overlap.

Coherence does not mean that different photons have the same momentum or that they are located at the same position.

Quantum coherence, is a little different. It includes the aspects of optical coherence mentioned above, but it also allows a larger variety of interference effects such as Hong-Ou-Mandel effect.

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Coherency with regards to lasers means the photons ability to be coherent temporally or spatially. Yes, the fundamental operation of a laser is to achieve stimulated emission where every photon has the same mode and such, but the concept of tunnelling is a more complicated matter then just smashing a photon with suitable attributes against a barrier (hah ... matter .... :) ).

Spatial coherence is how there seems to be a fixed phase relation to different spatial coordinated within the lasers coherent region, like different positions on the cross section of a beam wave front.

Temporal coherence is how the laser seems to behave predictably in time as well, where a high quality laser will exhibit almost sinusoidal operation for a long period of time with it's narrow linewidth.

There is another definition of quantum coherency involving the quantum states of the electronic states, but to my knowledge, and on the macroscopic level, that is not proportional or directly tied in with the ability to tunnel.

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