Past exam of the theoretical physics II course of the University of Cambridge / 2017 / 1 / c / Solution

At fixed particle number $N$, write $n_1=n$ and $n_2=N-n$. The contact interaction between two ring modes has energy eigenvaluesFor $g>0$, the nonnegative product $n(N-n)$ is minimized at the endpoints. ConsequentlyFor $N\ge 1$ this is a two-dimensional degenerate ground state subspace. $N=0$ instead has the unique Fock vacuum. In this projected model, putting all particles into one traveling mode avoids the extra exchange contribution paid when both distinct modes are occupied.

For the alternative coefficients, put $\alpha =\beta =A<0$ and $\gamma =0$. Then $E(n)=A[N(N-1)-2n(N-n)]$ is again minimized at $n=0,N$, with $E_{\min }=AN(N-1)$. Evenness of $N$ is unnecessary for this result; the balanced Fock state maximizes this attractive-interaction energy instead of minimizing it.