Efficient Quantum Sensing and High-Fidelity Electron Spin Gates for Scaling Diamond Quantum Registers

Quantum technologies are increasingly relevant for real-world applications nowadays, particularly in magnetometry. Nitrogen-vacancy (NV) centers in diamond are one of the most versatile solid-state platforms for such applications due to their favorable coherence properties at room temperature, the straightforward initialization and readout, the combination of nanoscale spatial resolution and biocompatibility.

Nevertheless, decoherence remains a major challenge. Pulsed dynamical decoupling addresses the problem by using fast, strong pulses to suppress the unwanted qubit-environment interactions. However, this can be challenging due to heating or with biological samples. We propose and demonstrate decoupling with a continuous field with discrete phase changes for quantum sensing and robust noise suppression. The technique is particularly suitable in experiments with limited driving power or at high magnetic fields as phase control is much more precise than the amplitude control, required in standard continuous sensing schemes.

Second, we develop and demonstrate experimentally a general noise protection strategy that utilizes the destructive interference of two cross-correlated noise sources to extend the coherence time of an NV electron spin by an order of magnitude, improve control fidelity, and surpass the state-of-the-art sensitivity for high frequency magnetic field sensing. We also demonstrate multipoint correlation spectroscopy that combines the advantages of correlation spectroscopy and quantum heterodyne detection for temporally efficient measurements of statistically polarized samples. We achieve single hertz frequency precision, highlighting the potential applications for nanoscale nuclear magnetic resonance with spin ensembles.

Diamond is also a promising platform for quantum information processing. A prerequisite for scalability are high fidelity interactions between the NV electron spins. Entanglement between dipolar-coupled NV spin pairs has been demonstrated previously but only with a limited fidelity, and the error sources have not been characterized. We demonstrate experimentally a record gate fidelity of 96 ± 2.5% under ambient conditions. Our identification of the dominant errors paves the way towards NV-NV gates beyond the error correction threshold.