  Superconducting magnets enable precise control of nuclear and electron spins,
and are used in experiments that explore biological and condensed matter
systems, and fundamental atomic particles. In high-precision applications, a
common view is that that slow (<1 Hz) drift of the homogeneous magnetic field
limits control and measurement precision. We report on previously undocumented
higher-frequency field noise (10 Hz to 200 Hz) that limits the coherence time
of 9Be+ electron-spin qubits in the 4.46 T field of a superconducting magnet.
We measure a spin-echo T2 coherence time of ~6 ms for the 9Be+ electron-spin
resonance at 124 GHz, limited by part-per-billion fractional fluctuations in
the magnet's homogeneous field. Vibration isolation of the magnet improved T2
to ~50 ms.
