  We perform a new test of general relativity (GR) with signals from GWTC-2,
the LIGO and Virgo catalog of gravitational wave detections. We search for the
presence of amplitude birefringence, in which left versus right circularly
polarized modes of gravitational waves are exponentially enhanced and
suppressed during propagation. Such an effect is present in various beyond-GR
theories but is absent in GR. We constrain the amount of amplitude
birefringence consistent with the data through an opacity parameter $\kappa$,
which we bound to be $\kappa \lesssim 0.74 \textrm{ Gpc}^{-1}$. We then use
these theory-agnostic results to constrain Chern-Simons gravity, a beyond-GR
theory with motivations in quantum gravity. We bound the canonical Chern-Simons
lengthscale to be $\ell_0 \lesssim 1.0 \times 10^3$ km, improving on previous
long-distance measurement results by a factor of two.
