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arXiv:2601.19976

A Surface-Scaffolded Molecular Qubit

Tian-Xing Zheng, M. Iqbal Bakti Utama, Xingyu Gao, Moumita Kar, Xiaofei Yu, Sungsu Kang, Hanyan Cai, Tengyang Ruan, David Ovetsky, Uri Zvi, Guanming Lao, Yu-Xin Wang, Omri Raz, Sanskriti Chitransh, Grant T. Smith, Leah R. Weiss, Magdalena H. Czyz, Shengsong Yang, Alex J. Fairhall, Kenji Watanabe, Takashi Taniguchi, David D. Awschalom, A. Paul Alivisatos, Randall H. Goldsmith, George C. Schatz, Mark C. Hersam, Peter C. Maurer

quant-phcond-mat.mes-hallcond-mat.mtrl-sciSubmitted Jan 27, 2026• Announced Jan 29, 2026
arXivHTMLPDF

Abstract

Fluorescent spin qubits are central building blocks of quantum technologies. Placing these qubits at surfaces maximizes coupling to nearby spins and fields, enabling nanoscale sensing and facilitating integration with photonic and superconducting devices. However, reducing the dimensions or size of established qubit systems without sacrificing the qubit performance or degrading the coherence lifetime remains challenging. Here, we introduce a surface molecular qubit formed by pentacene molecules scaffolded on a two-dimensional (2D) material, hexagonal boron nitride (hBN). The qubit exhibits stable fluorescence and optically detected magnetic resonance (ODMR) from cryogenic to ambient conditions. With fully deuterated pentacene, the Hahn-echo coherence reaches 22 $μ$s and further extends to 214 $μ$s under dynamical decoupling, outperforming state-of-the-art shallow NV centers in diamond, despite being positioned directly on the surface. We map the local spin environment, resolving couplings to nearby nuclear and electron spins that can serve as auxiliary quantum resources. This platform combines true surface integration, long qubit coherence, and scalable fabrication, opening routes to quantum sensing, quantum simulation, and hybrid quantum devices. It also paves the way for a broader family of 2D material-supported molecular qubits.

Comments
Main text, 10 pages, 4 figures

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Fluorescent spin qubits are central building blocks of quantum technologies. Placing these qubits at surfaces maximizes coupling to nearby spins and fields, enabling nanoscale sensing and facilitating integration with photonic and superconducting devices. However, reducing the dimensions or size of established qubit systems without sacrificing the qubit performance or degrading the coherence lifetime remains challenging. Here, we introduce a surface molecular qubit formed by pentacene molecules scaffolded on a two-dimensional (2D) material, hexagonal boron nitride (hBN). The qubit exhibits stable fluorescence and optically detected magnetic resonance (ODMR) from cryogenic to ambient conditions. With fully deuterated pentacene, the Hahn-echo coherence reaches 22 μμμs and further extends to 214 μμμs under dynamical decoupling, outperforming state-of-the-art shallow NV centers in diamond, despite being positioned directly on the surface. We map the local spin environment, resolving couplings to nearby nuclear and electron spins that can serve as auxiliary quantum resources. This platform combines true surface integration, long qubit coherence, and scalable fabrication, opening routes to quantum sensing, quantum simulation, and hybrid quantum devices. It also paves the way for a broader family of 2D material-supported molecular qubits.
Optically addressable spins at material surfaces have represented a long-standing ambition in quantum sensing, providing atomic resolution and quantum-limited sensitivity. However, they are constrained by a finite depth at which the quantum spins can be stabilized. Here, we demonstrate a hybrid molecular-2D architecture that realizes quantum spin sensors directly on top of the surface. By anchoring spin-active molecules onto hexagonal boron nitride (hBN), we eliminate the depth of the quantum sensor while also exhibiting robust spin properties from 4~K to room temperature (RT). The Hahn-echo spin coherence time exceeds \(T_2 = 3.4~\upmu\text{s}\) at 4~K, outperforming values in bulk organic crystals and overturning the prevailing expectation that spin inevitably deteriorates upon approaching the surface. By chemically tuning the molecule through deuteration, T2T_2T2​ improves by more than 10-fold, and under dynamic decoupling, coherence is prolonged to the intrinsic lifetime limit, exceeding 300~\(\upmu\text{s}\). Proximal proton spins and the magnetic response of two-dimensional magnets beneath the hBN layer have been detected at RT. These molecular spins form surface quantum sensors with long coherence, optical addressability, and interfacial versatility, enabling a scalable, adaptable architecture beyond what conventional solid-state platforms offer.
Abstract Advances in hybrid quantum systems and their precise control are pivotal for developing advanced quantum technologies. Two-dimensional (2D) materials with optically accessible spin defects have emerged as a promising platform for building integrated quantum spin systems due to their exceptional flexibility and scalability. However, experimentally realizing such systems and demonstrating their superiority remains challenging. Here, we present a hybrid spin system operating under ambient conditions, integrating boron vacancy ( VB−{{\rm{V}}}_{{\rm{B}}}^{-}VB−​ V B − ) spins in 2D hexagonal boron nitride flakes with a single nitrogen vacancy (NV) center in 3D single-crystal diamonds. This combined system achieves full controllability and exhibits enhanced performance for nanoscale magnetic sensing, including an improved dynamic range. Moreover, we investigate the rich many-body spin dynamics within the hybrid system, which enables us to estimate the concentration of VB−{{\rm{V}}}_{{\rm{B}}}^{-}VB−​ V B − spins. This work provides a critical foundation for advancing the development of 2D-3D integrated quantum spin systems.
Entanglement is central to quantum science and technology. Atomic defects in two-dimensional (2D) van der Waals (vdW) materials offer exciting prospects for quantum sensing, with spatial resolution reaching 1 nm demonstrated using scanning probe techniques. However, entangling qubits in vdW materials remains elusive. Here we report room-temperature quantum entanglement between an optically addressable electron spin and a strongly coupled 13^{13}13C nuclear spin in hexagonal boron nitride (hBN). We extend the electron spin coherence to 38 μμμs with dynamical decoupling, and create maximally entangled Bell states with a fidelity up to 0.89. We further use the nuclear spin as a long-lived quantum memory to enhance AC magnetic field sensing via correlation spectroscopy. These results establish entangled spin qubits in hBN as a robust platform for advanced quantum technologies based on 2D materials.
The negatively charged boron vacancy ($\VB$) in hexagonal boron nitride (\hBN{}) was the first optically addressable spin qubit identified inside a van der Waals crystal, allowing atomically defined placement relative to a target. Its coherence in bulk \hBN{} is limited by the boron nuclei of the layers flanking the defect plane, which a thin flake removes. Here, we use a generalized cluster-correlation expansion with an extended central-spin block to calculate the Hahn-echo coherence time of $\VB$ in h11^{11}11B15^{15}15N as a function of layer number. Our results show that T2T_2T2​ rises from $199\ns$ in the bulk limit to $653\ns$ in a monolayer, a factor of 3.33.33.3 that is already saturated at three layers and that a sublattice decomposition attributes entirely to boron. The enhancement is confined to low field, is insensitive to stacking registry and twist angle, and requires alignment to within about two degrees, an onset we reproduce with no free parameters. Under dynamical decoupling, a 1/e1/e1/e threshold returns $206\ns$ for every thickness, but this is a zero of the first-shell modulation rather than a decay: measured without a threshold, the layer contrast survives and reaches a factor of 170170170. Layer number thus emerges as a design parameter for $\VB$-based sensing.
Optical interfaces that connect long-lived spin qubits to photons are a central requirement for quantum networking and distributed quantum information processing. Currently, solid-state atomic defects are leading candidates due to their inherent spin and optical coherence. Building on these advancements, synthetically tailored molecular systems represent a fundamental change in the field, utilizing precise atomic control and consistent bottom-up assembly. However, the lack of a robust spin-photon interface combining bright fluorescence, high spectral stability, and the persistent spin lifetimes inherent to ground-state systems has prohibited the detection of individual molecular qubits. Here we show that a triplet ground state carbene molecule, embedded within a structurally matched host crystal, functions as a robust spin-photon interface with single-molecule addressability. The system exhibits narrow zero-phonon lines, spectral stability over more than an hour, spin-selective optical transitions and single-molecule optically detected magnetic resonance. Coherent control yields millisecond-scale dynamical-decoupling coherence and tens-of-milliseconds spin relaxation at a temperature of 4.5 K. These results establish molecular qubits as a viable platform for single-emitter quantum optics while preserving the advantages of bottom-up chemical design and processable materials.
Molecular quantum sensors represent a promising frontier for the detection of nuclear magnetic resonance signals and alternating current magnetic fields at the nanoscale, potentially reaching single-proton sensitivity. Although the triplet states of molecular pentacene provide a viable sensing architecture, the triplet pair states produced by singlet fission of pentacene dimers could enable more flexible quantum manipulations through entanglement. In this work, we model the quantum sensing efficacy of a spin-polarized quintet manifold in a photoexcited pentacene dimer generated via intramolecular singlet fission. Using a Lindblad master equation approach, we simulate the evolution of the triplet pair state under standard dynamical decoupling sequences, including spin echo, XY4, and XY8 and provide a direct performance comparison to the traditional pentacene monomer benchmark. While both architectures exhibit comparable sensitivity for isolated single-spin detection, our findings indicate that the dimer architecture provides a superior interaction cross-section for detecting small ensembles of nuclear spins. Analytical expressions derived for fluorescence modulation demonstrate that sensitivity is optimized in the low-magnetic field regime and scales with the number of pulses in the sensing protocol. This study establishes a theoretical baseline for utilizing high-spin multi-excitonic states as chemically tunable, high-sensitivity quantum probes.