The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier [1]. This current is driven by a superconducting phase difference $phi$ between the leads. In the presence of chiral and time reversal symmetry of the Cooper pair tunneling process [2] the current is strictly zero when $phi$ vanishes. Only if these underlying symmetries are broken the supercurrent for $phi=0$ may be finite [3-5]. This corresponds to a ground state of the junction being offset by a phase $phi_{0}$, different from 0 or $pi$. Here, we report such a Josephson $phi_{0}$-junction based on a nanowire quantum dot. We use a quantum interferometer device in order to investigate phase offsets and demonstrate that $phi_{0}$ can be controlled by electrostatic gating. Our results have possible far reaching implications for superconducting flux and phase defined quantum bits as well as for exploring topological superconductivity in quantum dot systems.
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