# How can you know that 2 photons are occupying the same location?

Someone told me once that a photon can be in the same exact location of another photon. Because they are bosons and have spin 1 you can have billions of photons occupy the same location.

I was wondering if that is true, how can you tell you are dealing with two or more photons and not a single photon. The energy level of that "bunch" of photons doesn't change at that location since having many photons occupy a same one location doesn't change their wavelengths.

And by having photons at the same location it becomes indistinguishable to tell them apart. How then can we say that there are many photons at this location and not be able to measure their quantity?

• A photon is an amount of electromagnetic field energy. The electromagnetic field allows for linear superposition. Why is it so strange that two identical amounts of em field energy can be in the same state, including that they can be measured in the same location at the same time? I find it much stranger how spin 1/2 fermions "know" how not to overlap more than two at a time. I could never develop an intuition for that. I just shut up and (let other people) calculate. Commented Apr 29, 2023 at 4:55

A BEC is a matter of dilute gas of low densities called bosons cooled to temperatures close to absolute zero. Under such conditions, a large fraction of the bosons occupy the same quantum state, at which point quantum phenomena, like wavefunction interference become apparent macroscopically.

https://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate

Theoretically nothing prevent the bosons from occupying the same location. The state is not perfectly localized, because their state is not exactly zero momentum. The HUP puts a lower limit on their localization.

What prevents bosons from occupying the same location?

It is possible to create a BEC of just photons, in this case they are using optical microcavity.

Can a system entirely of photons be a Bose-Einsten condensate?

We correspondingly expect a Bose-Einstein condensation when the photon wave packets spatially overlap at low temperatures or high densities, i.e. the phase space density 2 nth exceeds a value near unity. Here n denotes the number density, photons per area, andth  h/ 2mph kBT  1.58m (defined in analogy to e.g. a gas of atoms17) the de Broglie wavelength associated with the thermal motion in the resonator plane.

Now you are asking about the number of photons in the condensate. In this case, they had 77000 photons.

At room temperature (T = 300 K), we arrive at 77000 Nc .

https://arxiv.org/ftp/arxiv/papers/1007/1007.4088.pdf