[Perspective] Optical circulators reach the quantum level
Optical circuits for information processing offer higher data rates (channel capacities) and lower power consumption as compared with those of electronic circuits. Many optical devices (such as switches and isolators) have analogous electronic components (transistors and diodes), but some have special capabilities. For example, optical circulators form part of an unusual class of components known as nonreciprocal devices (1)—their optical properties depend on the direction of light passing through them. A three- or four-port optical circulator routes light entering from any port to exit only from the next. Bulk optical implementations rely typically on nonreciprocal polarization rotation via the Faraday effect, in which a magnetic field breaks symmetry (2). However, the drive to miniaturization that has led to nanophotonic integrated circuits and waveguides has not included optical circulators until now. On page 1577 of this issue, Scheucher et al. (3) demonstrate a fiber-integrated photonic circulator that can work even at...