Indirect Hamiltonian Identification through a small gateway

Identifying the nature of interactions in a quantum system is essential in understanding any physical phenomena. Acquiring information on the Hamiltonian can be a tough challenge in many-body systems because it generally requires access to all parts of the system. We show that if the coupling topology is known, the Hamiltonian identification is indeed possible indirectly even though only a small gateway to the system is used. Surprisingly, even a degenerate Hamiltonian can be estimated by applying an extra field to the gateway.