  We experimentally study the time-optimal construction of arbitrary
single-qubit rotations under a single strong driving field of finite amplitude.
Using radiation-dressed states of nitrogen vacancy centers in diamond, we
realize a strongly-driven two-level system and achieve driving frequencies four
times larger than its Larmor frequency. We implement time optimal universal
rotations on this system, characterize their performance using quantum process
tomography, and demonstrate a dual-axis ac magnetometry sequence with pulses at
sub-Larmor time scales. Our results pave the way for applying fast qubit
control and high-density pulse schemes in the fields of quantum information
processing and quantum metrology.
