For a quantum system of $N$ identical, harmonically interacting particles in a one-dimensional harmonic trap we calculate for the bosonic and fermionic ground state the corresponding one-particle reduced density operator ${\widehat{\rho }}_1$ analytically. In case of bosons ${\widehat{\rho }}_1$ is a Gibbs state for an effective harmonic oscillator. Hence the natural orbitals are Hermite functions and their occupation numbers obey a Boltzmann distribution. Intriguingly, for fermions with not too large couplings the natural orbitals coincide up to just a small error with the bosonic ones. In case of strong coupling this still holds qualitatively. Moreover, the decay of the decreasingly ordered fermionic natural occupation numbers is given by the bosonic one, but modified by an algebraic prefactor. Significant differences to bosons occur only for the largest occupation numbers. After all, the “gap” at the “Fermi level” decreases with increasing coupling strength but remains well pronounced even for strong interaction.

• Received 28 July 2013

DOI:https://doi.org/10.1103/PhysRevA.88.042105