  The measurement of multiple ringdown modes in gravitational waves from binary
black hole mergers will allow for testing fundamental properties of black holes
in General Relativity, and to constrain modified theories of gravity. To
enhance the ability of Advanced LIGO/Virgo to perform such tasks, we propose a
coherent mode stacking method to search for a chosen target mode within a
collection of multiple merger events. We first rescale each signal so that the
target mode in each of them has the same frequency, and then sum the waveforms
constructively. A crucial element to realize this coherent superposition is to
make use of a priori information extracted from the inspiral-merger phase of
each event. To illustrate the method, we perform a study with simulated events
targeting the l=m=3 ringdown mode of the remnant black holes. We show that this
method can significantly boost the signal-to-noise ratio of the collective
target mode compared to that of the single loudest event. Using current
estimates of merger rates we show that it is likely that advanced-era detectors
can measure this collective ringdown mode with one year of coincident data
gathered at design sensitivity.
