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Room-Temperature Quantum Sensing based on Endohedral Fullerene Spin Qudits

Bing-Cai Chen, Peng‐Xiang Fu, Jiayue Yuan, Yu Zhang, Shanshan Wang, Shen Lin Zhou

Published Jul 27, 2026• 0 citations
Publisher

Abstract

Quantum sensors harnessing multi-quantum coherence offer a promising route to precision metrology beyond the standard quantum limit. Although molecular spin qudits provide engineerable high-dimensional Hilbert spaces, the rapid decoherence of high-order spin states typically restricts their operation to cryogenic environments. Here, we synthesize a high-spin molecular magnet N@C60-C9H8O (electron spin S = 3/2), via a single-step gas-phase ion implantation technique. Leveraging its ultralong room-temperature coherence time (~38 μs), N@C60-C9H8O achieves an AC magnetic sensing sensitivity of 4⋅10-8 T Hz-1/2. Based on full coherent control over the four-level electron spin manifold, we demonstrate the sensing enhancement beyond the standard quantum limit utilizing molecular spin cat states at room temperature. These results exhibit the immense application potential of high-spin molecular qudits in ambient quantum metrology.

DOI
10.26434/chemrxiv.15006604/v1

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Quantum sensors harnessing multi-quantum coherence offer a promising route to precision metrology beyond the standard quantum limit. Although molecular spin qudits provide engineerable high-dimensional Hilbert spaces, the rapid decoherence of high-order spin states typically restricts their operation to cryogenic environments. Here, we synthesize a high-spin molecular magnet N@C60-C9H8O (electron spin S = 3/2), via a single-step gas-phase ion implantation technique. Leveraging its ultralong room-temperature coherence time (~38 μs), N@C60-C9H8O achieves an AC magnetic sensing sensitivity of 4⋅10-8 T Hz-1/2. Based on full coherent control over the four-level electron spin manifold, we demonstrate the sensing enhancement beyond the standard quantum limit utilizing molecular spin cat states at room temperature. These results exhibit the immense application potential of high-spin molecular qudits in ambient quantum metrology.
Paramagnetic molecules with high-spin centers can accommodate multilevel spin manipulations. In the cocrystal PdF 20 TPP·C 60 /N@C 60 ·8Benzene (ENF-PdTPP), 14 N@C 60 ( S = 3/2) exhibits distinct transition addressability in the frequency domain and a long quantum coherence time ( T 2 ≈ 5.47 μs at 250 K), making it a promising candidate for multilevel qu d it applications. A three-level superposition state was prepared at near-room temperature (250 K) and validated by quantum phase interference. By varying the evolution time (Δ τ ) of the three-level superposition state, a controllable quantum phase effect has been observed. We found that such a phenomenon can be harnessed as a resource for coherent manipulation for the realization of a fast controlled-phase (C-phase) gate (23.5 MHz), enriching the potential of this high-spin system for quantum algorithm implementations.
Quantum sensors based on electronic spins have emerged as powerful probes of microwave-frequency fields. Among other solid-state platforms, spins in molecular crystals offer a range of advantages, from high spin density to functionalization via chemical tunability. Here, we demonstrate microwave vector magnetometry using the photoexcited spin triplet of deuterated pentacene molecules, operating at zero external magnetic field and room temperature. We achieve full three-dimensional microwave field reconstruction by detecting the Rabi frequencies of anisotropic spin-triplet transitions associated with two crystallographic orientations of pentacene in naphthalene crystals. We further introduce a phase alternated protocol that extends the rotating-frame coherence time by an order of magnitude and enables sensitivities of 1 μT/Hz1~μ\mathrm{T}/\sqrt{\mathrm{Hz}}1 μT/Hz​ with sub-micrometer spatial resolution. These results establish pentacene-based molecular spins as a practical and high-performance platform for microwave quantum sensing, and the control techniques are broadly applicable to other molecular and solid-state spin systems.
Quantum coherence serves as a crucial quantum resource for achieving high-sensitivity quantum sensing. Because of its long coherence time at room temperature, the nitrogen-vacancy (NV) center has emerged as a quantum sensor in various fields in recent years. While nanoscale quantum sensing at room temperature has been demonstrated for NV centers, noise on the diamond surface severely limits its further development at a higher sensitivity. Here, we utilize the hybridization between graphene and diamond surfaces to directly deplete surface unpaired electron spins, thereby achieving roughly two-fold enhancement in coherence. Through the combination of electron spin resonance spectra and first-principle calculations, we explain that this phenomenon arises from a significant reduction in electron spin density on the diamond surface due to interface electron orbital hybridization. Our research presents a new approach for solid-state quantum sensors to reach the desired sensitivity level and offers a new pathway for future studies on material interfaces.
We experimentally demonstrate the protection of a room-temperature hybrid spin register against environmental decoherence by performing repeated quantum error correction whilst maintaining sensitivity to signal fields. We use a long-lived nuclear spin to correct multiple phase errors on a sensitive electron spin in diamond and realize magnetic field sensing beyond the time scales set by natural decoherence. The universal extension of sensing time, robust to noise at any frequency, demonstrates the definitive advantage entangled multiqubit systems provide for quantum sensing and offers an important complement to quantum control techniques.
Quantum sensing has witnessed rapid development and transition from laboratories to practical applications in the past decade. Applications of quantum sensors, ranging from nanotechnologies to biosensing, are expected to benefit from quantum sensors' unprecedented spatial resolution and sensitivity. Solid-state spin systems are particularly attractive platforms for quantum sensing technologies because room temperature operation is viable while reaching the quantum limits of sensitivity. Among various solid-state spin systems, nitrogen-vacancy (NV) centers in diamond demonstrated high-fidelity initialization, coherent control, and high contrast readout of the electron spin state. However, electron spin coherence due to noise from the surrounding spin bath and this environment effect limits the sensitivity of NV centers. Thus, a critical task in NV center-based quantum sensing is sensitivity enhancement through coherence protection. Several strategies, such as dynamical decoupling techniques, feedback control, and nuclear spin-assisted sensing protocols, have been developed and realized for this task. Among these strategies, nuclear spin-assisted protocols have demonstrated greater enhancement of electron spin coherence. In addition, the electron and nuclear spin pair of an NV center in diamond naturally allows the application of the nuclear spin-assisted sensitivity enhancement protocol. Owing to long nuclear coherence times, further enhancement of sensitivity can be achieved by exploiting active nuclear spins (e.g., 14N, 13C) in the proximity of an NV center as memory ancillas when coupled with the NV center. Here, we review the spin properties of NV centers, mechanisms of the nuclear spin-assisted protocol and its gate variation, and the status of quantum sensing applications in high-resolution nuclear spin spectroscopy, atomic imaging, and magnetic field sensing. We discuss the potential for commercialization, current challenges in sensitivity enhancement, and conclude with future research directions for promoting the development of nuclear spin-assisted protocol and its integration into industrial applications.
Controlling the dynamics of targeted quantum correlations is a central goal across many disciplines of quantum science. For applications in nanoscale quantum sensing, nitrogen-vacancy (NV) centers in diamond have recently emerged as a leading platform due to their atom-like nature, exquisite magnetic sensitivity and well-developed coherent control techniques under ambient conditions. However, harnessing the entanglement structure required to improve this sensitivity beyond the standard quantum limit (SQL) from the native dipolar interactions between NV centers is challenging due to their strong positional disorder, three-dimensional geometry and low-fidelity readout. Coupled tightly to proof of principle experimental demonstrations, this thesis provides a theoretical blueprint to realize practical, entanglement-enhanced quantum sensing in the solid-state via far-from-equilibrium dynamics of dense ensembles of NV centers in diamond. Our most fundamental contribution is to identify a universal mechanism exploiting the thermalization of long-wavelength spin textures to generate scalable spin squeezing in the absence of finite temperature symmetry breaking. We further develop a novel sensing protocol based on asymmetric time-reversal capable of rapidly breaking the SQL without high-fidelity readout. Challenging long-standing beliefs in the field, these results open the door to a new era of entanglement-enhanced sensing under ambient conditions, with applications ranging from nanoscale imaging of biological structures to covariance magnetometry of exotic quantum materials.