Figure 2: Measuring the two-colour spin noise correlator C2(Δω) and QD homogeneous linewidth γh.
(a) Experimental schematic: narrowband probe lasers 1 and 2 are combined in a single-mode polarization-maintaining fibre (F). The probe light is weakly focused through the ensemble of singly charged QDs, where hole spin fluctuations impart FR noise on the transmitted light, which is measured by balanced photodiodes. Here, MS are mechanical shutters, LP is a linear polarizer, HWP is a half-wave plate and WBS is a Wollaston beam splitter. HWP can be replaced by a quarter-wave plate to measure ellipticity noise. (b,c) Raw spin noise power spectra for the case of large and small laser detuning (Δω>γh and Δω<γh), giving uncorrelated and correlated spin noise, respectively; T=5 K. The red and blue spectra were detected using only laser 1 or laser 2, respectively. The green trace is their mathematical sum. The black trace was measured with both lasers on simultaneously. (d) Two-colour spin noise correlator C2 as a function of the detuning between the probe lasers. The solid line is a Lorentzian fit, and its HWHM reveals the underlying homogeneous linewidth γh of the QDs.
