  I perform an unprecedented template-based search for stimulated emission of
Hawking radiation (or Boltzmann echoes) by combining the gravitational wave
data from 65 binary black hole merger events observed by the LIGO/Virgo
collaboration. With a careful Bayesian inference approach, I found no
statistically significant evidence for this signal in either of the 3
Gravitational Wave Transient Catalogs GWTC-1, GWTC-2 and GWTC-3. However, the
data cannot yet conclusively rule out the presence of Boltzmann echoes either,
with the Bayesian evidence ranging within 0.3-1.6 for most events, and a common
(non-vanishing) echo amplitude for all mergers being disfavoured at only 2:5
odds. The only exception is GW190521, the most massive and confidently detected
event ever observed, which shows a positive evidence of 9.2 for stimulated
Hawking radiation. An optimal combination of posteriors yields an upper limit
of $A < 0.42$ (at $90\%$ confidence level) for a universal echo amplitude,
whereas $A \sim 1$ was predicted in the canonical model. The next generation of
gravitational wave detectors such as LISA, Einstein Telescope, and Cosmic
Explorer can draw a definitive conclusion on the quantum nature of black hole
horizons.
