  Electron pairing is a rare phenomenon appearing only in a few unique physical
systems; e.g., superconductors and Kondo-correlated quantum dots. Here, we
report on an unexpected, but robust, electron "pairing" in the integer quantum
Hall effect (IQHE) regime. The pairing takes place within an interfering edge
channel circulating in an electronic Fabry-Perot interferometer at a wide range
of bulk filling factors, $2<{\nu} _B<5$. The main observations are: (a) High
visibility Aharonov-Bohm conductance oscillations with magnetic flux
periodicity ${\Delta}{\phi}={\varphi}_0/2=h/2e$ (instead of the ubiquitous
$h/e$), with $e$ the electron charge and $h$ the Planck constant; (b) An
interfering quasiparticle charge $e ^* {\sim} 2e$ - revealed by quantum shot
noise measurements; and (c) Full dephasing of the $h/2e$ periodicity by induced
dephasing of the adjacent edge channel (while keeping the interfering edge
channel intact) : a clear realization of inter-channel entanglement. While this
pairing phenomenon clearly results from inter-channel interaction, the exact
mechanism that leads to e-e attraction within a single edge channel is not
clear.
