  Superconducting diode effect, in analogy to the nonreciprocal resistive
charge transport in semiconducting diode, is a nonreciprocity of
dissipationless supercurrent. Such an exotic phenomenon originates from
intertwining between symmetry-constrained supercurrent transport and intrinsic
quantum functionalities of helical/chiral superconductors. In this article,
research progress of superconducting diode effect including fundamental
concepts, material aspects, device prospects, and theoretical/experimental
development is reviewed. First, fundamental mechanisms to cause superconducting
diode effect including simultaneous space-inversion and time-reversal symmetry
breaking, magnetochiral anisotropy, interplay between spin-orbit interaction
energy and the characteristic energy scale of supercurrent carriers, and
finite-momentum Cooper pairing are discussed. Second, the progress of
superconducting diode effect from theoretical predictions to experimental
observations are reviewed. Third, interplay between various system parameters
leading to superconducting diode effect with optimal performance is presented.
Then, it is explicitly highlighted that nonreciprocity of supercurrent can be
characterized either by current-voltage relation obtained from resistive
direct-current measurements in the metal-superconductor fluctuation region
($T\approx T_c$) or by current-phase relation and nonreciprocity of superfluid
inductance obtained from alternating-current measurements in the
superconducting phase ($T<T_c$). Finally, insight into future directions in
this active research field is provided with a perspective analysis on
intertwining between band-topology and helical superconductivity, which could
be useful to steer the engineering of emergent topological superconducting
technologies.
