  The interstellar medium scatters radio waves which causes pulsars to
scintillate. For intrinsically short bursts of emission, the observed signal
should be a direct measurement of the impulse response function. We show that
this is indeed the case for giant pulses from PSR B1957+20: from baseband
observations at 327 MHz, we demonstrate that the observed voltages of a bright
pulse allow one to coherently descatter nearby ones. We find that while the
scattering timescale is $12.3\,\mu$s, the power in the descattered pulses is
concentrated within a span almost two orders of magnitude shorter, of
$\lesssim\!200\,$ns. This sets an upper limit to the intrinsic duration of the
giant pulses. We verify that the response inferred from the giant pulses is
consistent with the scintillation pattern obtained by folding the regular
pulsed emission, and that it decorrelates on the same timescale, of~$84\,$s. In
principle, with large sets of giant pulses, it should be possible to constrain
the structure of the scattering screen much more directly than with other
current techniques, such as holography on the dynamic spectrum and cyclic
spectroscopy.
