Characterizing a quantum process is the critical first step towards applying such a process in a quantum information protocol. Full process characterization is known to be extremely resource-intensive, motivating the search for more efficient ways to extract salient information about the process. An example is the identification of "decoherence-free subspaces", in which computation or communications may be carried out, immune to the principal sources of decoherence in the system. Here we propose and demonstrate a protocol which enables one to directly identify a DFS without carrying out a full reconstruction. Our protocol offers an up-to-quadratic speedup over standard process tomography. In this paper, we experimentally identify the DFS of a two-qubit process with 32 measurements rather than the usual 256, characterize the robustness and efficiency of the protocol, and discuss its extension to higher-dimensional systems.
@article{arxiv.1206.4510,
title = {Identification of Decoherence-Free Subspaces Without Quantum Process Tomography},
author = {D. H. Mahler and L. Rozema and A. Darabi and A. M. Steinberg},
journal= {arXiv preprint arXiv:1206.4510},
year = {2015}
}