  The unique properties of quantum Hall devices arise from the ideal
one-dimensional edge states that form in a two-dimensional electron system at
high magnetic field. Tunnelling between edge states across a quantum point
contact (QPC) has already revealed rich physics, like fractionally charged
excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we
show that a single QPC can turn into an interferometer for specific potential
landscapes. Spectroscopy, magnetic field and temperature dependences of
electron transport reveal a quantitatively consistent interferometric behavior
of the studied QPC. To explain this unexpected behavior, we put forward a new
model which relies on the presence of a quantum Hall island at the centre of
the constriction as well as on different tunnelling paths surrounding the
island, thereby creating a new type of interferometer. This work sets the
ground for new device concepts based on coherent tunnelling.
