  A single electron shared between two levels threaded by a magnetic flux is an
irreducibly simple quantum system in which interference is predicted to occur.
We demonstrate tuning of the tunnel coupling between two such electronic levels
with flux, implemented in a loop comprising two quantum dots. Using
radio-frequency reflectometry of the dots' gate electrodes we extract the
inter-dot coupling, which exhibits oscillations with a periodicity of one flux
quantum. In different tunneling regimes we benchmark the oscillations'
contrast, and find that their amplitude varies with the levels involved, while
tunneling is generically not suppressed at oscillation minima. These results
establish the feasibility and limitations of parity readout of qubits with
tunnel couplings tuned by flux.
