In the world of physics, few discoveries challenge our intuition as much as the idea that light could travel at different speeds depending on its direction. For decades, efforts to demonstrate nonreciprocal group delay have been hindered by excessive losses and limited effects. Yet a recent breakthrough in cavity magnonics has achieved a significant demonstration of this phenomenon, without violating fundamental laws like the Kramers–Kronig relations. This discovery taps into the fascinating hybrid interaction between photons and magnons, leveraging magnism to manipulate light in ways previously out of reach.
This pioneering work comes from the Spintronics Group at the Department of Physics & Astronomy, University of Manitoba, Canada, led by Dr. Can-Ming Hu (FAPS, Distinguished Professor, Habil.). The group specializes in cutting-edge research into magnetism, spintronics, and metamaterials, exploring how spin, charge, and photons interact in hybridized systems.
In this interview, we speak with Jiguang Yao, one of the researchers working under Dr. Hu. His responses not only unpack the physics behind the discovery, but also reflect a deep curiosity about the nature of waves, information, and the elegance of experimental design. Scroll down to read the full interview:
Your team’s experiment showed that light can travel at different speeds depending on direction, something previously thought impossible due to the Kramers-Kronig relations. What was going through your mind when the data confirmed it? Did it feel like you were rewriting the rules of physics?
The Kramers–Kronig relation intuitively implies that wave speed and energy loss are intrinsically connected. This means that while slow light can be achieved with low loss, fast light cannot fundamentally reach the same low-loss level. What we observe here is essentially a trade-off between the two effects — that is, limiting the losses for both to a reasonably acceptable extent. There is no violation of the Kramers–Kronig relation; rather, this is an exploration demonstrating that such a trade-off can be achieved through appropriate parameter design. Because of this, the extent to which this effect can be further optimized remains highly challenging, and whether it can be translated into practical applications is still an open question.
You mentioned using a hybrid system of photons and magnons. That sounds like a lightwave-meets-spinwave crossover. Can you tell us what happens when these two very different entities interact inside your device?
Photons are the energy quanta of light, while magnons are the energy quanta of collective spin excitations in magnetic materials. In this sense, photons and magnons are essentially the quantum counterparts of “light waves” and “spin waves,” respectively. This is how physicists understand them from a quantum perspective. When they interact, energy can be exchanged between the two via interference effect, giving rise to a phenomenon known as electromagnetically induced transparency (EIT). This effect opens a transparent window for light pulses and slows their propagation through the medium.
Many previous studies focused on manipulating the amplitude of light. But your research tackled its phase. Why does phase matter so much, and how does it influence how light carries information?
Wave pulses serve as carriers of information in various fields. It arises from the constructive and destructive interference among monochromatic waves of different frequencies. The relative phase shifts of these frequency components modify the speed of the wave packet, which is called as group velocity.
In fact, control over group velocity has been studied for nearly a century. However, practical manipulation remained limited for decades until the discovery of electromagnetically induced transparency (EIT), which significantly advanced the development and application of slow light. On the other hand, nonreciprocal amplitude control underpins essential components in various communication systems. Hence, we aim to combine nonreciprocity with phase control based on the EIT effect to achieve greater flexibility in light manipulation.
How do you envision this technology supporting neuromorphic computing? Can nonreciprocal phase control help mimic the way the human brain processes signals?
One approach to exploring neuromorphic computing is through pulse-based technologies. For example, in optical neuromorphic computing, wave pulses are used to simulate the spikes by which neurons transmit information. The ability to control pulse speed is crucial, and nonreciprocal phase control offers a novel means to enhance this flexibility and potentially unlock new possibilities. However, its practical application in this area remains unclear at this stage.
Looking ahead, what’s your dream application of this technology? If you had unlimited funding and a team of engineers, where would you take it next?
Although our experiment has demonstrated a novel concept, the time delay or advance achieved so far remains relatively modest. And personally, I am more passionate about fundamentally physical concepts than technical details. Therefore, I would still like to focus more on addressing the general challenges in pulse control, such as enhancing group delay using new techniques and overcoming bandwidth limitations.
Quick bits:
Photons or magnons, who wins the quantum showdown?
Both play essential roles. Photons are central to quantum communication and quantum computing, while magnons offer unique advantages such as long coherence times and high tunability. More interestingly, the hybridization of the two gives rise to a quasiparticle known to physicists as a polariton, which provides advantages neither can achieve on its own-much like parents having a child who outshines them both. This field, known as cavity magnonics, is what we are working on right now. Therefore, rather than being in competition, I believe photons and magnons can complement each other, and cooperate through coupling, to overcome current limitations and push the boundaries of next-generation quantum technologies.
Building the experiment or watching the data come in?
Once again, both play essential roles, yet each offers its own unique kind of delight. The design of an experiment is grounded in specific physical hypotheses. After careful and deliberate thought, physicists craft experiments as a way to pose questions to nature. Ingenious experimental design brings a deep sense of achievement. Observing the data feels like drawing a lottery ticket—an experience filled with tension and anticipation. When the outcome isn’t as hoped, we must reflect and redesign the experiment and pick up another lottery ticket from nature.
Which tech sector do you think will adopt this first – quantum computing or high-speed telecom?
As for potential practical applications in specific fields, the challenges would be multifaceted, including technical aspects such as integration. I am not able to make a definitive judgment before thoroughly exploring the specific technical challenges. Moreover, I want to clarify that, unlike slow light, fast light cannot be used for information communication or so-called ‘high-speed telecom,’ as it is merely a unique interference phenomenon resulting from the wave nature of light rather than a mechanism for superluminal information transmission. Over the years, this phenomenon has led to debate and misunderstanding, so I want to clarify it here.
What do you tell your non-scientist friends you do all day?
To my non-scientist friends, my work may seem like nothing more than a daily routine of experiments and calculations—which looks boring from the outside. I only explain specific physics concepts to those who genuinely show interest; with those who aren’t, I’d rather share other kinds of joy. When I do explain, I tailor my explanations to the listener’s background, making them as intuitive as possible and connecting the ideas to broader, more familiar phenomena. However, the true pleasure of physics also lies in its precision. That’s why I believe the joy of doing physics is a unique reward, one granted only to those who experience it firsthand.
Photons in a dielectric resonator (yellow) interact with magnons in a YIG sphere (violet) via a microstrip (grey). 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.
(Wow! Thank you, Jiguang Yao, for such a thoughtful and illuminating conversation. Your passion for the fundamental questions of physics shines through every answer, and your work on cavity magnonics is a compelling glimpse into the future of light manipulation. We look forward to following your continued exploration at the frontier of quantum technologies. Wishing you every success in the challenges and discoveries ahead!)
