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Photons in a dielectric resonator (yellow) interact with magnons in a YIG sphere (violet) via a microstrip (gray). This interaction acts as a ‘traffic light’ for microwave pulses—speeding them up (green) in one direction and slowing them down (red) in the other, controllable by a magnetic field. Credit: Yao et al.

The reliable manipulation of the speed at which light travels through objects could have valuable implications for the development of various advanced technologies, including high-speed communication systems and quantum information processing devices. Conventional methods for manipulating the speed of light, such as techniques leveraging so-called electromagnetically induced transparency (EIT) effects, work by utilizing quantum interference effects in a medium, which can make it transparent to light beams and slow the speed of light through it.

Despite their advantages, these techniques only enable the reciprocal control of group velocity (i.e., the speed at which the envelope of a wave packet travels through a medium), meaning that a light beam will behave the same irrespective of the direction it is traveling in while passing through a device. Yet the nonreciprocal control of light speed could be equally valuable, particularly for the development of advanced devices that can benefit from allowing signals to travel in desired directions at the desired speed.

Researchers at the University of Manitoba in Canada and Lanzhou University in China recently demonstrated the nonreciprocal control of the speed of light using a cavity magnonics device, a system that couples microwave photons (i.e., quanta of microwave light) with magnons (i.e., quanta of the oscillations of electron spins in materials).

The magnonics-based methods they employed, outlined in a paper published in Physical Review Letters, could contribute to the advancement of microwave signal communications, neuromorphic computing and quantum circuits.

“In 2019, my group demonstrated a novel method to produce dissipative coupling in hybrid cavity magnonics systems,” Can-Ming Hu, head of the dynamic spintronics group at the University of Manitoba, told Phys.org.

“Our technique, presented in a paper published in Physical Review Letters, enables nonreciprocal signal transmission with a substantial isolation ratio and flexible controllability.”

As part of their earlier work, Hu and his colleagues specifically attempted to manipulate the amplitude of light (i.e., the maximum strength of a light wave’s electric or magnetic field) traveling in only one direction. Yet light also possesses another fundamental characteristic, known as its phase, which is essentially how ‘far along’ a light wave is relative to a specific reference location.

“Phase manipulation also has broad implications, as it determines the speed of pulses that carry information across various systems,” said Hu. “The primary objective of this new study was to address the following question: would nature allow us to nonreciprocally manipulate the phase of light while maintaining bi-directionally comparable transmission amplitude.

“There is a fundamental principle known as Kramers-Kronig relations which seems to prohibit it, but surprisingly, our experiment shows that nature is extraordinarily generous to us here.”

A key goal of attempts aimed at slowing down the speed of light is to significantly alter the velocity of light pulses without compromising their transmission efficiency. This is typically achieved via interference effects in hybridized resonant systems, known as a classical analog of EIT effects in quantum regimes.

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“In our work, we construct such a hybridized system using the photon mode of a dielectric resonator and the magnon mode of a magnetic yttrium iron garnet (YIG) sphere,” explained Jiguang Yao, senior Ph.D. student and first author of the paper.

“Beyond conventional resonators, the magnetic materials possess intrinsic chirality—its spin precesses in a fixed direction determined by the applied magnetic field. This chirality can be harnessed to induce nonreciprocity, enabled by an additional dissipative coupling introduced via a common microstrip. As a result, we achieved a nonreciprocal and controllable light propagation system.”

To demonstrate the potential of their proposed approach, the researchers sent a microwave pulse into the coupled cavity magnonics system they developed from two directions. When they compared the speed of this pulse with a reference path, they found that their method enabled striking delay and advance effects, —- nonreciprocally.

“Light and microwave pulses serve as carriers of information in various fields, ranging from signal communications to neuromorphic computing and quantum signal processing,” said Jerry Lu, junior Ph.D. student and co-author of the paper.

“Previous efforts in nonreciprocal control of electromagnetic waves have primarily focused on directional amplitude manipulation—allowing transmission in only one direction. That concept underpins essential components in communication systems, such as isolators and circulators. Our study revealed for the first time that light is allowed to propagate in both directions but at different speeds.”

The team’s promising new method for the nonreciprocal control of light speed could soon enable the development of various cutting-edge and previously unimaginable technologies. Meanwhile, Hu and his research group are working to further improve their methodology, with the hope of enhancing the delay and advanced effects it produces.

“Although the effect demonstrated in our work is exciting, the time delay/advance achieved so far remains relatively modest,” added Hu.

“Enhancing this effect is essential for enabling practical applications. As a first step, we plan to introduce a few new techniques to our device to enhance the effect. In the longer term, we intend to explore a wider range of application scenarios.”

More information: Jiguang Yao et al, Nonreciprocal Control of the Speed of Light Using Cavity Magnonics, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.196904. Journal information: Physical Review Letters

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