Quantum particles can work together to produce powerful signals. However, those signals are unstable and usually vanish almost instantly.<\/p>\n
For decades, one striking example of this problem has been superradiance\u2014a collective effect where many quantum particles emit energy together, producing an intense burst of radiation that rapidly dies out.\u00a0<\/p>\n
Now, researchers have shown that this long-standing weakness can be turned into a strength. In a new study, scientists demonstrate that superradiance can sustain<\/a> itself, producing long-lived, highly coherent microwave signals without any external driving.\u00a0<\/p>\n \u201cThis discovery changes how we think about the quantum world. We\u2019ve shown that the very interactions once thought to disrupt quantum behavior can instead be harnessed to create it. That shift opens entirely new directions for quantum technologies,\u201d Kae Nemoto, one of the study authors and Center Director of the Okinawa Institute of Science and Technology in Japan, said<\/a>.\u00a0<\/p>\n Using a diamond crystal for superradiance<\/p>\n Superradiance occurs when many quantum particles, such as atomic spins, act in unison. Instead of emitting energy independently, they synchronize and release it together, creating a signal much stronger than the sum of its parts.\u00a0<\/p>\n The problem is that this process usually burns through energy extremely fast, making it impractical for real technologies like clocks or communication devices. To explore whether this limitation could be overcome, researchers from TU Wien and OIST built a carefully designed quantum system.\u00a0<\/p>\n They used a diamond crystal filled with a dense ensemble of nitrogen-vacancy<\/a> (NV) centers\u2014tiny defects in the diamond lattice where a nitrogen atom sits next to a missing carbon atom. Each NV center hosts an electron spin, which behaves like a tiny magnet that can switch between quantum states.\u00a0<\/p>\n The researchers placed this diamond inside a microwave cavity, a structure that traps microwave radiation and allows it to interact strongly with the spins. When the spins were initially excited, the researchers observed a familiar phenomenon: a powerful superradiant burst of microwave emission.\u00a0<\/p>\n Then something unexpected happened<\/p>\n Instead of fading away, the system began producing a series of narrow, long-lived microwave pulses<\/a>. These signals appeared without any external pumping or continuous energy input.\u00a0<\/p>\n \u201cAfter an initial fast superradiant burst, we observe a train of subsequent emission pulses followed by quasi-continuous masing for up to one millisecond,\u201d the study authors note.\u00a0<\/p>\n To understand why this happened, the researchers turned to large-scale computer simulations. The calculations revealed that spin\u2013spin interactions inside the diamond<\/a> were dynamically repopulating the energy levels of the system.\u00a0<\/p>\n In simple words, the spins were constantly reshuffling energy among themselves, triggering new emissions again and again. This process, known as self-induced superradiant masing, means the system effectively drives itself.\u00a0<\/p>\n The same interactions that usually introduce noise and destroy coherence were instead organizing the spins, creating an extremely stable and coherent microwave signal. \u201cThe system organizes itself, producing an extremely coherent microwave signal from the very disorder that usually destroys it,\u201d Wenzel Kersten, lead researcher, said.<\/p>\n The significance of stable quantum signals<\/p>\n Stable, self-sustained microwave signals are essential for technologies such as atomic clocks, navigation systems, radar, and communication networks.\u00a0<\/p>\n Plus, \u201cthe principles we observe here could also enhance quantum sensors capable of detecting minute changes in magnetic or electric fields. Such advances could benefit medical imaging, materials science, and environmental monitoring,\u201d J\u00f6rg Schmiedmayer, one of the study authors and an expert in quantum technology at TU Wien, said.<\/p>\n At a deeper level, the work changes how physicists think about quantum<\/a> interactions. Instead of viewing complex spin interactions as a source of unavoidable noise, this study shows they can be engineered as a resource.\u00a0<\/p>\n Further research will focus on exploring how universal this effect is, whether it can be realized in other quantum platforms, and how precisely the emitted microwaves can be tuned and controlled.<\/p>\n