  A precise measurement of dephasing over a range of timescales is critical for
improving quantum gates beyond the error correction threshold. We present a
metrological tool, based on randomized benchmarking, capable of greatly
increasing the precision of Ramsey and spin echo sequences by the repeated but
incoherent addition of phase noise. We find our SQUID-based qubit is not
limited by $1/f$ flux noise at short timescales, but instead observe a
telegraph noise mechanism that is not amenable to study with standard
measurement techniques.
