Time-Optimal Universal Control of Two-Level Systems under Strong Driving
  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. 
Chen Avinadav, Ran Fischer, Paz London, David Gershoni
quant-ph