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pulse energies as low as 3 . 7 × 10 − $^{26}$J (0.01 poa) are still very well detectable. The power dependence of the cross-product is displayed in Figs. 2e–g. Down to 2 poa, the pulse region is clearly visible (Fig. 2f). For 1 poa, a Fourier transform (Fig. 2g) reveals that the signal component can still be identified. However, the associated peak has approximately the same amplitude as the largest noise peak in the spectrum. Hence, the detection limit of the cross-product (see also Fig. 2e) and therefore the one of the cross-variance is 1–2 poa. We note that this is not a fundamental limit, but is rather due to technical issues such as the 

