Paper 3, Section I, B

Quantum Mechanics
Part IB, 2013

If α,β\alpha, \beta and γ\gamma are linear operators, establish the identity

[αβ,γ]=α[β,γ]+[α,γ]β[\alpha \beta, \gamma]=\alpha[\beta, \gamma]+[\alpha, \gamma] \beta

In what follows, the operators xx and pp are Hermitian and represent position and momentum of a quantum mechanical particle in one-dimension. Show that

[xn,p]=inxn1\left[x^{n}, p\right]=i \hbar n x^{n-1}

and

[x,pm]=impm1\left[x, p^{m}\right]=i \hbar m p^{m-1}

where m,nZ+m, n \in \mathbb{Z}^{+}. Assuming [xn,pm]0\left[x^{n}, p^{m}\right] \neq 0, show that the operators xnx^{n} and pmp^{m} are Hermitian but their product is not. Determine whether xnpm+pmxnx^{n} p^{m}+p^{m} x^{n} is Hermitian.