A Josephson diode (JD) is a special type of Josephson junction (JJ), which conducts (super)current in one direction better than in the opposite direction. In other words, it has asymmetric current-voltage characteristic. Since the Josephson diode is a superconducting device, the asymmetry of the supercurrent transport is the main focus of attention. Opposite to conventional Josephson junctions, the critical (maximum) supercurrents ${\displaystyle I_{c+}}$ and ${\displaystyle I_{c-}}$ for opposite bias directions are different by absolute values (${\displaystyle I_{c+}\neq |I_{c-}|}$). In the presence of such a non-reciprocity, the bias currents of any magnitude in the range between ${\displaystyle I_{c+}}$ and ${\displaystyle |I_{c-}|}$ can flow without resistance in only one direction.

This asymmetry, characterized by the ratio of critical currents ${\displaystyle {\mathcal {A}}}$, is the main figure of merit of Josephson diodes and is the subject of new developments and optimizations. The Josephson diode effect can occur, e.g., in superconducting devices where time reversal symmetry and inversion symmetry are broken.

Josephson diodes can be subdivided into two categories, those requiring an external (magnetic) field to be asymmetric and those not requiring an external magnetic field --- the so-called "field-free" Josephson diodes (more attractive for applications). In 2021, the Josephson diode was realized in the absence of applied magnetic field in a non-centrosymmetric material, followed shortly by the first realization of the zero-field Josephson diode in an inversion-symmetric device.

Since decades the physicists tried to construct Josephson junction devices with asymmetric critical currents. This didn't involve new physical principles or advanced (quantum) material engineering, but rather electrical engineering tricks like combining several JJs in a special way (e.g. asymmetric 3JJ SQUID) or specially designed long JJs or JJ arrays, where Josephson vortex transport was asymmetric in opposite directions. After all, if one does not look inside the device, but treats such a device as a black box with two electrodes, its current-voltage characteristic is asymmetric with ${\displaystyle I_{c+}\neq |I_{c-}|}$. Such devices were especially popular in the context of Josephson ratchets — devices used to rectify random or deterministic signals with zero time-average. These devices can be subdivided into several classes:

Starting from 2020 one observes a new wave of interest to the systems with non-reciprocal supercurrent transport based on novel materials and physical principles.

In-depth review of recent developments.

The superconducting diode effect is an example of nonreciprocal superconductivity, where a material is superconducting in one direction and resistive in the other. This leads to half-wave rectification when a square wave AC-current is applied. In 2020, this effect was demonstrated in an artificial [Nb/V/Ta]_n superlattice. The phenomenon in the Josephson diode is believed to originate from asymmetric Josephson tunneling. Unlike conventional semiconducting junction diodes, the superconducting diode effect can be realized in Josephson junctions as well as junction-free bulk superconductors.

Currently, the precise mechanism behind the Josephson diode effect is not fully understood. However, some theories have emerged that are now under theoretical investigation. There are two types of Josephson diodes, relating to which symmetries are being broken. The inversion breaking Josephson diode, and the Josephson diode breaking inversion breaking and time-reversal. The minimal symmetry breaking requirement for forming the Josephson diode is inversion symmetry breaking, and is required to obtain nonreciprocal transport. One proposed mechanism originates from finite momentum Cooper pairs. It may also be possible that the superconducting diode effect in the JD originates from self-field effects, but this still has to be rigorously studied.