I've been playing around with vacuum Rabi oscillation on QuTip and found an odd behavior.
My Hamiltonian is as follows:
$$ H=\omega_n n^\dagger n + \omega_c c^\dagger c -6K(n+n^\dagger)^4 - g(n^\dagger-n)(c^\dagger-c). $$ where $w_c=w_n=1 \cdot 2 \cdot \pi$, $ g= 0.05 \cdot 2 \cdot \pi$, and $ K=0.001 \cdot 2 \cdot \pi $ in which all units are in GHz.
When I truncate the Hilbert space into any dimensions other than 15 x 15 or 30 x 30, I get an expected vacuum Rabi oscillation for an initial state of $ |0\rangle|1\rangle $ in the Fock basis. However, for 15 x 15 truncation of the Hilbert space, I get 
and for 30 x 30 truncation of the Hilbert space, I get 
.
Could anyone familiar with QuTip give any insight on why this bug is happening?
