  The superconducting analog to the semiconducting diode, the Josephson diode,
has long been sought, with multiple avenues to realization proposed by
theorists. Exhibiting magnetic-field free, single directional superconductivity
with Josephson coupling of the supercurrent across a tunnel barrier, it would
serve as the building-block for next-generation superconducting circuit
technology. Here we realized the field-free Josephson diode using an inversion
symmetry breaking heterostructure of $\mathrm{NbSe_2/Nb_3Br_8/NbSe_2}$. We
demonstrate, for the first time without magnetic field, the junction can be
superconducting in one direction while normal in the opposite direction. Based
on that, half-wave rectification of a square-wave excitation was achieved with
low switching current density ($~2.2\times 10^2 \mathrm{A/cm^2}$), high
rectification ratio ($~10^4$) and high robustness (at least $10^4$ cycles). We
also demonstrate symmetric $\Delta I_\mathrm{c}$ (the difference between
positive and negative critical currents) behavior with field and the expected
Fraunhofer current phase relation of a Josephson junction. This realization
raises fundamental questions about the Josephson effect through an insulator
when breaking symmetry, and opens the door to ultralow power, high speed,
superconducting circuits for logic and signal modulation.
