Fock state

Imagine you have a bunch of balls, each with different colors. Now, let's say you want to only have one ball of each color. This means you have one red, one blue, one green, and so on. This is similar to a Fock state in quantum mechanics.

In quantum mechanics, particles such as photons can be in different states at the same time. However, a Fock state is when there is only one photon in a specific state.

To give an example, let's say you have two boxes, one labeled "empty" and the other labeled "occupied". You have a specific number of photons in a random order, and you want to sort them into each box. In a Fock state, each box will only have one photon, and all of the photons will be in different states.

So, a Fock state is when there is only one particle in a specific state, and all other states are unoccupied. It's like having only one ball of each color, or one photon in each box labeled by its state.