A quantum spectrum analyzer enhanced by a nuclear spin memory

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Quantum sensors based on nitrogen-vacancy (NV) centers in diamond show promise for a number of fascinating applications in condensed matter physics, materials science, and biology.1, 2 By embedding them in a variety of nanostructures, such as tips,3,4,5,6 nanocrystals,7 or surface layers,7, 8 local properties of samples can be investigated with high sensitivity and spatial resolution. In particular, diamond chips with near-surface NV centers have enabled pioneering experiments in nanoscale detection of nuclear magnetic resonance (NMR), potentially enabling structural analysis of individual molecules with atomic resolution.9,10,11
A key feature of many quantum sensing experiments is the ability to record time-dependent signals and to reconstruct their frequency spectra. The canonical approach uses dynamical decoupling sequences, which are sensitive to frequencies commensurate with the pulse spacing while efficiently rejecting most other frequencies.12,13,14,15,16 The spectral resolution of dynamical decoupling spectroscopy, however, is Fourier-limited by decoherence to ~(πT 2)−1 ∼ 10–100 kHz for shallow NV centers,17 where T 2 is the electronic decoherence time. It has recently been recognized that by correlating two consecutive decoupling sequences, separated by a variable waiting time t, the spectral resolution can be extended to the inverse state life time T 1, which can be 10–100× longer than T 2 (refs. 18, 19). Correlation spectroscopy has been applied to both generic ac magnetic fields and to nuclear spin detection, and spectral resolutions of a few 100 Hz have been demonstrated.20,21,22
Despite these impressive advances, there is a strong motivation to further extend the spectral resolution. For example, many proposed nanoscale NMR experiments9,10,11 require discrimination of fine spectral features, often in the few-Hz range. In addition, atomic-scale mapping of nuclear spin positions strongly relies on precise measurements of NMR frequencies and hyperfine coupling constants.22 Therefore, methods to acquire frequency spectra with even higher spectral resolution are highly desirable.
In this study, we implement a two-qubit sensor designed to further refine the spectral resolution by a factor of 10–100×. Our two-qubit sensor consists of an active sensing qubit and an auxiliary memory qubit, formed by the electronic spin and the 15N nuclear spin of the NV center in diamond. By intermittently storing the state information in the nuclear—rather than electronic—spin qubit, we extend the maximum waiting time t from T 1 ~ 1 ms to the nuclear T 1,n > 50 ms, with a corresponding gain in spectral resolution. In addition, we use the nuclear memory to enhance sensor readout efficiency through repeated readout,23, 24 which would otherwise result in untenably long acquisition times. The presented two-qubit system is particularly useful because it is intrinsic to the NV center, with no need for additional sensor engineering.

The advantages of one or more “auxiliary” qubits have been recognized in several different contexts, including the quantum logic clock for enhanced atomic clock performance25,26,27 and resettable auxiliary qubit(s) for emulating the environment in quantum simulation.28, 29 Multi-qubit probes may also assist the detection of cross-correlations in environmental noise30 or the study of non-classical dynamics.31 In recent experiments with spin qubits in diamond, auxiliary nuclear spins have been used to increase the effective coherence time of an electronic sensor spin by quantum error correction,32 quantum feedback,33, 34 or by exploiting double-quantum coherence.35 Moreover, ancillary nuclei have been used to enhance the readout efficiency.23, 24 In our study we utilize the auxiliary nuclear spin as a long-lived memory for the electron qubit’s state.
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