# Why is <n|n-2> = 0?

1. Jan 23, 2015

### Kidiz

1. The problem statement, all variables and given/known data

This is not really homework. I'm just studying and came across this question. So, I'm not sure if I should post this question here on one of the physics sections.

On computing $<n|\hat{a}^2|n>$, one arrives at $\sqrt{n}*\sqrt{n-1}<n|n-2>=0$ because $<n|n-2>=0$

Now, why is that so?

2. Jan 23, 2015

### Fightfish

Isn't that just because the two Fock states are orthogonal?