Past exam of the physics A course of the University of Cambridge / 2022 / Paper 1 / Section B / B6 / a / Solution

Use the canonical commutation relations $[\hat{x},{\hat{p}}_x]=[\hat{y},{\hat{p}}_y]=i\hslash$, with cross-coordinate commutators zero. For either coordinate,Thus $2\alpha \beta \hslash =1$. To obtain the required Hamiltonian, the ratio of its position operator and momentum operator coefficients must be preserved:where the positive root follows from the requested signs. ConsequentlyThe annihilation operators are dimensionless; $\alpha$ has units of inverse length and $\beta$ has units of inverse momentum. Operators belonging to different coordinates commute automatically. Writing the number operators as ${\hat{N}}_j={\hat{a}}_j^{†}{\hat{a}}_j$, substitution givesThe constant is the sum of the two zero-point energies of this two-dimensional isotropic harmonic oscillator.