Fig. 3.

Measurement of the noise of a bead trapped by two colocalized beams in a split-path instrument. (A) A strong and a weak trap were formed in a split-path instrument and were focused onto the same bead. The recorded displacement signal in the two traps shows that the vast majority of the noise occurs in the weak trap signal, suggesting that the anticorrelated signal arises because the two traps are physically drifting relative to each other. (B) The power spectrum of the weak trap signal measured in A exhibits low-frequency pink noise comparable to the one measured on the tethered construct (Fig. 2), whereas no pink noise component is observable in the power spectrum of the strong trap signal. Similarly as in A, this suggests that the increased low-frequency noise in the differential signal is due to relative trap drift.