Observation of Faraday rotation from a single confined spin

The ability to read out the state of a single confined spin lies at the heart of solid-state quantum-information processing(1). Although spin measurements using Faraday rotation of light polarization have been implemented in semiconductor spin ensembles(2-4), single-spin read-out has only been achieved using transport measurements(5,6). Here, we demonstrate an all-optical dispersive measurement of the time-averaged spin state of a single electron in a quantum dot. We obtain information on the spin state through conditional Faraday rotation of a spectrally detuned laser, induced by the polarization- and spin-selective trion (charged quantum dot) transitions. To assess the sensitivity of the technique, we use an independent resonant laser for spin-state preparation(7). We infer that there are similar to 10 spin-flip Raman scattering events (that is, back-action) within our measurement timescale. Straightforward improvements such as incorporating solid-immersion lenses(8,9) and higher efficiency detectors should allow for back-action-evading spin measurements, without the need for a cavity.