  Three-dimensional entanglement of orbital angular momentum states of an
atomic qutrit and a single photon qutrit has been observed. Their full state
was reconstructed using quantum state tomography. The fidelity to the maximally
entangled state of Schmidt rank 3 exceeds the threshold 2/3. This result
confirms that the density matrix cannot be decomposed into ensemble of pure
states of Schmidt rank 1 or 2. That is, the Schmidt number of the density
matrix must be equal to or greater than 3.
