A “light diode” that passes photons in only one direction would be an essential element in nanoscale integrated optical circuits. Now a research team has built a prototype of such a device that works even for single photons. Light traveling through an optical nanofiber in one direction is mostly absorbed by atoms trapped just outside the fiber, while light traveling in the reverse direction can pass freely.
Light-based circuits could potentially send data faster and with lower power consumption than electronic circuits. To realize such schemes, optical engineers will need many of the same device types found in electronic circuits, such as diodes and transistors. Optical diodes, which allow light to travel in one direction but not the other, exist already, but they can't work with the low-intensity light signals, down to just a few photons, that optical circuits will require.
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