Characterizing a quantum process is the critical first step towards applyingsuch a process in a quantum information protocol. Full process characterizationis known to be extremely resource-intensive, motivating the search for moreefficient ways to extract salient information about the process. An example isthe identification of "decoherence-free subspaces", in which computation orcommunications may be carried out, immune to the principal sources ofdecoherence in the system. Here we propose and demonstrate a protocol whichenables one to directly identify a DFS without carrying out a fullreconstruction. Our protocol offers an up-to-quadratic speedup over standardprocess tomography. In this paper, we experimentally identify the DFS of atwo-qubit process with 32 measurements rather than the usual 256, characterizethe robustness and efficiency of the protocol, and discuss its extension tohigher-dimensional systems.
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