Speed Control for Microwaves
A wave-like signal moves with a so-called group velocity and, in certain circumstances, exhibits nonreciprocal propagation—that is, propagation at different speeds in the forward and backward directions. The simultaneous control of those two properties is of interest to many fields of science and technology, including quantum communication, neuromorphic computing, and classical signal processing. However, until now, implementing these concepts in a practical device in the optical and microwave domains has represented an open challenge. Jiguang Yao at the University of Manitoba in Canada and colleagues have now met this challenge by leveraging the techniques of cavity magnonics [1] (Fig. 1). Such techniques combine magnetic elements with microwave resonators [2–4]. The researchers have demonstrated that the clever combination and placement of the subsystems enable the speed of microwave pulses to be increased or decreased, even to the point of qualifying as so-called slow light. Thanks to the use of magnetic elements, the researchers also achieve nonreciprocal signal propagation. While slow-light phenomena and nonreciprocity have been achieved in various solid-state settings [5–9], the new work combines these two aspects in a single platform. The ability to control a signal’s group velocity based on propagation direction and magnetization will be important for any applications that depend on the synchronization of pulses and pulse trains.
The concepts of slow light and nonreciprocity underlie many optical and microwave signal routing and filtering applications that are critical to our modern society. Original classical realizations of these concepts have been adapted for quantum signals, mainly by utilizing so-called hybrid quantum systems. These systems combine excitations or transitions of mechanical, magnetic, electromagnetic, and atomic elements in a way that enables the control of their interactions and energy exchanges such that quantum signals can be manipulated.
Slow-light phenomena have been demonstrated in solid-state quantum hybrid systems using optomechanical platforms with multiple optical or microwave drives [5, 6], as well as in optical cavities filled with doped crystals [10]. On the other hand, nonreciprocal elements are typically realized using magnetic elements, which offer the broken time-reversal symmetry necessary for nonreciprocity. Prominent examples include microwave and optical circulators and spin-wave-based schemes in which bulk magnetic crystals such as yttrium iron garnet are integrated into a microwave circuit. Even more compact hybrid systems for nonreciprocity can be realized by altering the propagation properties of surface acoustic waves by interfacing them with magnetic thin films, paving the way toward more densely integrated microwave circuits [8, 9].
Yao and co-workers integrate both of these concepts on a single platform by combining a dielectric resonator with a magnetic yttrium iron garnet (YIG) sphere and a microwave circuit. Specifically, they achieve slow-light functionality by tailoring the interaction between the microwave excitations within the resonator and the magnetic excitations of the sphere. They then introduce an additional coupling to the microwave circuit to implement a direction-dependent phase shift. This addition achieves a propagation-direction-dependent slowdown or speedup of microwave pulses by up to 25 nanoseconds. The underlying design concept is attractive, as the operating frequency can be easily tuned over a wide range by selecting the geometry of the dielectric resonator. The magnetic system can also be frequency tuned by applying an external magnetic field, which, additionally, can invert the properties of the device, switching the slow and fast propagation directions.
The researchers conclude that their device concept could potentially improve or enable applications ranging from neuromorphic networks to the distribution of entanglement in quantum networks. Moreover, they emphasize that transferring this approach to the ultrafast optical excitation of magnetic systems could enable nonreciprocal slow- and fast-light phenomena in the optical domain.
But this exciting work also raises further experimental and theoretical questions. YIG is currently the go-to material for magnonic applications because of its exceptional magnetic damping properties. These damping properties determine the bandwidth within which nonreciprocity and slow- and fast-light phenomena arise. However, it is not known whether YIG is the most promising material for such device concepts. YIG cannot be easily integrated into microwave circuits, warranting the question of whether other materials—such as metallic thin films—can exhibit similar magnetic properties. On the other hand, YIG and similarly magnetic alternatives might be replaced by antiferromagnetic systems to translate this concept to frequencies above 100 GHz. This would allow for applications in optics, which are beyond the reach of the current device. But do we even need materials with large bandwidths in order to operate at these frequencies, or could we develop devices that can be tuned to specific frequencies within that broad range? We might also ask whether there are faster and more energy-efficient ways to control the nonreciprocal slow- and fast-light phenomena; for example, via an electric field in multiferroic heterostructures and materials. When thinking about quantum applications, what role does the vacuum noise play (which will be added to the signal), and how does this affect the propagating state? How do quantum states change when subjected to slow- and fast-light phenomena? And finally, could it be possible to translate this concept to quantum memories, which would benefit from the directional emission of quantum states? We are confident that the future is bright for device concepts of this kind, which can have a lasting impact on quantum computing, communication, and signal conditioning.
References
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