Non-reciprocity: The physics of letting waves go one way but not the other Premium

There are many technologies that need signals to be amplified, and are thus susceptible to strong back-reflections. They will all benefit from the use of non-reciprocity, a new and emerging area of physics research that is focused on preventing back-reflection, backpropagation, and, in general, on allow electromagnetic signals to travel one way in a system but not the other.

If you can hear someone on the other side of a wall, then that someone can hear you as well. This is something to keep in mind when you’re snooping on your neighbour – but this straightforward statement also embodies a profound principle of physics called reciprocity.

Simply put, the principle asserts that if a signal can be transmitted from Point A (the source) to Point B (the destination), the same signal can also be transmitted from Point B to Point A, simply by exchanging the positions of the source and the destination.

It is an intuitive principle that we often encounter in our daily lives. For example, if you can shine a torchlight at your friend, they can shine the torch back at you as well. The air in between you two that allowed the light to pass one way will have no qualms about letting it pass the other way as well.

(Readers are encouraged to consider some counterintuitive examples of reciprocity as well. For example, in some films, a person being interrogated can’t see the police officers through a window pane but the officers can see the person. Similarly, when you sit in darkness, you can see someone walking under a streetlight, but the other person can’t see you.)

Scientists and engineers have discovered more exciting applications of this principle. Consider antennas, like the dish-shaped ones used to receive signals for your television, or those erected on Wi-Fi routers and radio sets, or discreetly packed into the body of a smartphone. Antennas can both send and pick up signals in the form of electromagnetic energy. If you want to know how well an antenna is able to receive signals from different directions, you can place multiple sources in different random directions and estimate the reception quality.

Then again, you can make things easier – and cheaper – by using the principle of reciprocity. You just need to feed a single signal into the antenna and then observe how it transmits that signal. Thanks to reciprocity, the antenna should be able to send signals in whichever direction in which it is able to receive signals.

So by studying the way the antenna sends out a signal, we can figure out how well it can pick up signals from that direction – or, in technical parlance, its far-field pattern. This is a handy shortcut that considerably simplifies the testing process and renders it more cost-effective. Engineers use it to test as well as operate radars, sonar, seismic surveys, and magnetic resonance imaging (MRI) scanners as well.