A new project will use radio waves and microwaves to actively control individual muons—short-lived subatomic particles—rather than simply watching them wobble inside a material. Muons are exquisitely sensitive magnetic probes, making them ideal for studying exotic magnetic states called spin liquids, where electrons refuse to settle into orderly magnetic patterns even at extremely low temperatures. These disordered states are thought to harbour topological order, a form of quantum entanglement that could be useful for future quantum computing. But current muon experiments are passive: researchers fire muons into a sample and observe how they precess, like watching a spinning top slow down. This project aims to change that by combining muon techniques with pulsed nuclear magnetic resonance and electron spin resonance methods, allowing researchers to actively manipulate the spins in the sample using precisely timed sequences of radio frequency and microwave pulses. If successful, the project will transform how muons are used to study spin liquids, enabling entirely new experiments that probe the quantum entanglement at the heart of these materials. This is fundamental science—there is no immediate practical application. But understanding how quantum order emerges from disorder could, over the long term, inform the design of materials for quantum computing or other technologies that exploit exotic quantum states.
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A central goal of condensed matter physics is to understand systems which are on the verge of long-range order. For example, the term spin liquid describes a strongly-interacting magnetic state of matter in which the strong interactions do not lead to a low-temperature long-range ordered state (such as ferromagnetism or antiferromagnetism) but instead result in a non-magnetic 'quantum disordered' ground state. Intriguingly, despite showing no long-range magnetic order, such systems can often show topological order resulting from an anomalously high degree of entanglement in which the ground state consists of a quantum superposition of pairs or collections of spins. To understand such systems, it is important to have a highly-sensitive local magnetic probe, ideally one operating with single spin detection. Spin-polarized muons provide just such a sensitive local magnetic probe, and this project is centred around advancing the muon technique for studying delicate magnetic systems such as spin liquids. One strand of the new project will lead to some new methods for understanding and extracting muon sites in materials, including spin liquids. A second strand of the project will marry muon methods with established pulsed nuclear magnetic resonance and electron spin resonance techniques. Rather than sitting back and watching the muons precess in the sample (as we do at present), we will then be able to use highly-specified sequences of radio frequency and microwave pulses to directly control the spins in the sample. This will allow entirely new experiments to be attempted and this project has the potential to radically alter the way we use muons to study spin liquid physics.
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