One-Way Waves

Imagine a string of pearls. You can start a wave by wiggling the first pearl or the last; the waves can travel either way because each pearl is coupled equally to both neighbors. But researchers have lately become interested in “unidirectional” coupling, in which the force between neighbors only allows waves to move in one direction. This can be seen as an extreme example of anisotropic media, in which the wave speed depends on the direction. Computer simulations have shown how waves will propagate through unidirectional arrays, and researchers have built electronic circuits that exhibit unidirectional coupling [1]. But these circuits had only three “pearls” in the array–too small to see all of the wave propagation effects predicted in the simulations.