Breaking Lorentz reciprocity to overcome the time-bandwidth limit in physics and engineering

Resonant systems with high bandwidth The performance of an active system, whether it is optical, electrical, or mechanical, is often described by its quality (Q) factor. Typically, one learns the rule that the higher the Q factor, the sharper the resonance—that is, the bandwidth of the device is reduced. Tsakmakidis et al. show that this is indeed the case, but only for symmetric systems. However, for the case of asymmetric (or nonreciprocal) systems, the rule need not be obeyed. They show theoretically that the more asymmetric a system with high Q is, the wider the bandwidth can be. The effect raises the prospect of designing high-Q devices operating over large bandwidths. Science, this issue p. 1260 Asymmetric materials provide the prospect of designing high–quality-factor devices with high bandwidth. A century-old tenet in physics and engineering asserts that any type of system, having bandwidth Δω, can interact with a wave over only a constrained time period Δt inversely proportional to the bandwidth (Δt·Δω ~ 2π). This law severely limits the generic capabilities of all types of resonant and wave-guiding systems in photonics, cavity quantum electrodynamics and optomechanics, acoustics, continuum mechanics, and atomic and optical physics but is thought to be completely fundamental, arising from basic Fourier reciprocity. We propose that this “fundamental” limit can be overcome in systems where Lorentz reciprocity is broken. As a system becomes more asymmetric in its transport properties, the degree to which the limit can be surpassed becomes greater. By way of example, we theoretically demonstrate how, in an astutely designed magnetized semiconductor heterostructure, the above limit can be exceeded by orders of magnitude by using realistic material parameters. Our findings revise prevailing paradigms for linear, time-invariant resonant systems, challenging the doctrine that high-quality resonances must invariably be narrowband and providing the possibility of developing devices with unprecedentedly high time-bandwidth performance.