  The tunability and spatial precision of paramagnetic molecules makes them
attractive for quantum sensing. However, usual microwave-based detection
methods have poor temporal and spatial resolution, and optical methods
compatible with room-temperature solutions have remained elusive. Here, we
utilize pump-probe polarization spectroscopy to initialize and track electron
spin coherence in a molecule. Designed to efficiently couple spins to light,
aqueous $K_{2}IrCl_{6}$ enables detection of few-picosecond free induction
decay at room temperature and micromolar concentrations. Viscosity is found to
strongly vary decoherence lifetimes. This work redefines the meaning of
room-temperature coherence by improving experimental time resolution by up to
five orders of magnitude. Doing so unveils a new regime of electron spin
coherence, opening the door to new synthetic design and applications of
molecular quantum bits.
