Some thirty years ago, Lubotzky, Phillips and Sarnak used deep number-theoretic insights to construct the first Ramanujan graphs, as well as optimal pseudo-random generators for the rotation group SO(3). In the last decade their work has seen several developments: On the discrete side, in the study of Ramanujan complexes, which are finite simplicial complexes with the spectral behavior of infinite buildings. On the continuous side, in the study of pseudo random generators for U(n), motivated by the problem of constructing optimal gates for fault-tolerant quantum computation. Results on both the continuous and discrete sides draw on the study of Ramanujan digraphs, which seem to be of independent interest. Based on joint works with Eyal Lubetzky, Alex Lubotzky and Peter Sarnak.

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