Most devices for the transmission of electromagnetic signals obey Lorentz reciprocity, meaning that signals propagate freely in both directions through circuits. A microwave pulse, for example, can travel in either direction along a waveguide and a light signal can move both ways along an optical fibre. This two-way traffic can cause problems and current technologies for avoiding reciprocity tend to be large and unwieldy. But now physicists in the US have come up with a more practical solution.
Lorentz reciprocity creates challenges for circuit designers because backward-propagating reflections can inject noise into circuits and even damage devices such as lasers. Isolators currently used in radar microwave transmitters, for example, get around this problem using a large external magnetic field. Waves propagating in the opposite direction see the opposite field and are therefore affected differently. However, this requires large, heavy magnets and adds considerably to the circuit's power consumption. While nonmagnetic isolators have been developed, their performance has been lacking.
Now, Andrea Alù and colleagues at the University of Texas at Austin and City University of New York have shown that two nonmagnetic isolators can be combined to produce a device that transmits a signal almost perfectly in one direction, but has near-zero transmission in the opposite direction.
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