Multiple electron spin resonance echoes observed for paramagnetic defects in diamond at room temperature

Magnetic resonance offers an invaluable testbed for observing and studying the fundamental concepts of quantum cavity interactions with two-level systems in the microwave regime. Typically, these experiments are conducted at low cryogenic temperatures, utilizing spin systems embedded within a high-quality (Q-factor) superconducting cavity. Recent studies indicate that under these conditions, especially in a high-cooperativity regime with strong collective coupling between an electron spin system and a microwave cavity, multiple spin echoes can be detected. These echoes are interpreted as manifestations of coherent quantum effects. To put it simply, photons within the cavity can excite the spin system, which subsequently can stimulate the cavity, creating a feedback loop. In our research, we demonstrate that a specially designed moderate-Q cavity, paired with diamond crystals rich in nitrogen vacancy (NV) centers, allows us to observe such nonlinear quantum phenomena, even at ambient temperatures. Crucially, our experimental design necessitates amplifying the net number of spins for a specific, limited spin concentration. This is achieved by lowering the spins' thermodynamic temperature (as opposed to their physical temperature) to a few kelvins. Notably, we find that maintaining high cooperativity or strong coupling is not essential for these observations. The potential to observe significant microwave cavity quantum effects at room temperature could be useful for future applications, such as quantum memories and quantum sensing.