Figure 3: Impact of finite anharmonicity on Bell-frame dynamics.
From: Hidden two-qubit dynamics of a four-level Josephson circuit
The transition from SU(4) to SO(4) dynamics is measured by scanning (vertical) along the diagonal (p=q) of the pq plane (with a distance 
) and sweeping (horizontal) the duration of the three-tone pulse in the range 1–200 ns. (a) Population of the third qudit level. At low amplitudes excitation peaks move towards the origin so that 
, consistent with Pythagorean dynamics. At high amplitudes the dynamics transition from SO(4) to SU(4) because of the finite qudit anharmonicity. The larger group SU(4) does not factor into SU(2)⊗SU(2), but rather includes a term that couples together the Bell-frame qubits. The coupling terms shift their respective resonance frequencies, resulting in the chevron pattern. (b) Calculated von Neumann entanglement entropy of the Bell-frame qubits using the relation S= −Tr(ρA log2 ρA), where ρA is the reduced density matrix of one of the Bell-frame qubits. For low drive amplitudes where the dynamics factor into SU(2)⊗SU(2) the entanglement entropy is constant and maximum. The coupling terms become visible at high drive amplitudes and induce disentanglement–entanglement oscillations in the time domain.
