Impact of modulation on CMB B‐mode polarization experiments

We investigate the impact of both slow and fast polarization modulation strategies on the science return of upcoming ground-based experiments aimed at measuring the B-mode polarization of the cosmic microwave background. Using detailed simulations of the Cl OVER experiment, we compare the ability of modulated and un-modulated observations to recover the signature of gravitational waves in the polarized CMB sky in the presence of a number of anticipated systematic effects. The general expectations that fast modulation is helpful in mitigating low-frequency detector noise, and that the additional redundancy in the projection of the instrument's polarization sensitivity directions on to the sky when modulating reduces the impact of instrumental polarization, particularly for fast modulation, are borne out by our simulations. Neither low-frequency polarized atmospheric fluctuations nor systematic errors in the polarization sensitivity directions are mitigated by modulation. Additionally, we find no significant reduction in the effect of pointing errors by modulation. For a Cl OVER-like experiment, pointing jitter should be negligible but any systematic miscalibration of the polarization coordinate reference system results in significant E–B mixing on all angular scales and will require careful control. We also stress the importance of combining data from multiple detectors in order to remove the effects of common-mode systematics (such as un-polarized 1/f atmospheric noise) on the measured polarization signal. Finally we compare the performance of our simulated experiment with the predicted performance from a Fisher analysis. We find good agreement between the (optimal) Fisher predictions and the simulated experiment except for the very largest scales where the power spectrum estimator we have used introduces additional variance to the B-mode signal recovered from our simulations. In terms of detecting the total B-mode signal, including lensing, the Fisher analysis and the simulations are in excellent agreement. For a detection of the primordial B-mode signal only, using an input tensor-to-scalar ratio of r= 0.026, the Fisher analysis predictions are ∼20 per cent better than the simulated performance.