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Abstract

In order to control a quantum system, we need the full information on its Hamiltonian. Yet, how can we know all the entries of a Hamiltonian matrix, especially when the system is large and our access is limited? The problem of Hamiltonian identification under limited access has recently studied quite actively, and some partial and positive results have been obtained. Here, we ask a more ambitious question along the same lines; what if we don’t know anything about the system and we still attempt to estimate its Hamiltonian through a small gateway? We shall discuss how we can probe such an ‘untouchable’ system E through a small gateway system S, paying a close attention to the equivalence class induced by the limitedness of access. The insight obtained hereby would open up a possibility of controlling a large quantum system with only a few parameters.