  The phase of a single quantum state is undefined unless the history of its
creation provides a reference point. Thus quantum interference may seem hardly
relevant for the design of deterministic single-electron sources which strive
to isolate individual charge carriers quickly and completely. We provide a
counterexample by analyzing the non-adiabatic separation of a localized quantum
state from a Fermi sea due to a closing tunnel barrier. We identify the
relevant energy scales and suggest ways to separate the contributions of
quantum non-adiabatic excitation and backtunneling to the rare non-capture
events. In the optimal regime of balanced decay and non-adiabaticity, our
simple electron trap turns into a single-lead Landau-Zener-backtunneling
interferometer, revealing the dynamical phase accumulated between the particle
capture and leakage. The predicted "quantum beats in backtunneling" may turn
the error of a single-electron source into a valuable signal revealing
essentially non-adiabatic energy scales of a dynamic quantum dot.
