Diamond Spins Amplify Signals for Enhanced Sensing Technologies

Researchers demonstrate room temperature superradiant echoes in diamond containing nitrogen-vacancy (NV) centres, achieved via laser illumination and microwave pulses. These pulses create a phase grating amongst NV spin sub-ensembles, enabling tailored, multiple re-phasing dynamics analogous to those observed in clock systems, potentially advancing sensing technologies.

The collective behaviour of quantum systems frequently manifests in unexpected ways, and recent research explores the generation of superradiant echoes within diamond structures containing nitrogen-vacancy (NV) centres. These defects, created when a nitrogen atom replaces a carbon atom in the diamond lattice, possess quantum properties that make them promising candidates for applications in sensing and quantum information processing. Qilong Wu, Yuan Zhang, and colleagues report in their Letter, ‘Superradiant Echoes Induced by Multiple Re-phasing of NV Spin Sub-ensembles Grating at Room Temperature’, a method for inducing these echoes at room temperature using a combination of laser illumination and microwave pulses applied to a diamond containing a high concentration of NV centres placed within a dielectric microwave cavity. The team demonstrate that carefully timed microwave pulses create a ‘phase grating’ amongst the NV centre spin states, leading to multiple re-phasing events and the subsequent emission of superradiant echoes, a phenomenon where the collective emission of photons is significantly enhanced. This work, conducted by researchers at the Henan Academy of Sciences and the Niels Bohr Institute, suggests a pathway towards actively controlling these echoes and optimising them for practical applications.
Researchers report the generation of multiple superradiant echoes from nitrogen-vacancy (NV) centres within diamond at ambient temperature, a feat achieved by coupling these defects with a dielectric microwave cavity and utilising both laser illumination and microwave Hahn echo sequences. A key element of the observed phenomenon is the establishment of a phase grating, imprinted amongst sub-ensembles of NV centre spins in frequency space through the combined application of microwave pulses and natural spin evolution. This grating facilitates the generation of superradiant echoes via collective coupling with the cavity, and active control of these echoes proves possible through precise manipulation of microwave pulses and laser illumination, effectively modifying the grating’s parameters. The resulting behaviour exhibits a notable resemblance to the mechanisms underpinning superradiant beats observed in clock systems.

The investigation confirms the critical role of cavity quantum electrodynamics (QED) in amplifying the interaction between NV centres and microwave photons. Cavity QED involves confining electromagnetic fields within a resonant cavity, enhancing light-matter interactions. Here, researchers achieve a strong coupling regime, where the strength of the interaction surpasses the decay rates of both the NV centres and the cavity itself. This strong coupling is essential for observing collective phenomena like superradiance. Further research focuses on optimising the spin sub-ensemble grating and the resulting echoes through the implementation of dynamical decoupling techniques, which aim to mitigate decoherence – the loss of quantum information – and preserve quantum coherence for extended periods. This optimisation holds considerable promise for advancing applications in precision sensing, where maintaining coherence is paramount, and opens avenues for creating coherent microwave sources and exploring new paradigms in quantum information processing.

Scientists demonstrate that optical cooling significantly enhances coherence and facilitates the observation of multiple echoes. Optical cooling utilises laser light to reduce the thermal motion of the NV centres, thereby extending the duration of quantum coherence. While coupling strength increases with optical cooling, it eventually saturates, indicating a limit to this enhancement technique. Theoretical modelling and Monte Carlo simulations corroborate the experimental findings, explaining the observed photon blockade effect – a phenomenon where only one photon can occupy the cavity at a time – in the strong coupling regime. This effect arises from the formation of spin-photon dressed modes, which suppress multiple echoes. This work builds upon previous investigations into echo trains in pulsed electron spin resonance, self-stimulated pulse echoes, frequency measurements of superradiance, and cavity quantum electrodynamics with NV centres, expanding the understanding of collective spin dynamics and paving the way for further exploration of quantum phenomena in solid-state systems.