A Surface-Scaffolded Molecular Qubit

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.

Figures (4)

• Figure 1: Photophysics and ODMR of the hBN-scaffolded pentacene.a, Schematic of the pentacene molecular qubit on the surface of hBN. The qubit is excited and read out by an off-resonance green laser ($520$ nm), and its spin levels are controlled by microwaves through our home-built confocal microscopes (see Methods). b, The $T_2$ coherence time versus qubit size or depth for optically-addressable spin qubits including: shallow NV centers sangtawesin2019originswyatt2025creationknowles2014observingguo2024direct, molecules mena2024roomishiwata2025molecular, hBN defects rizzato2023extendingstern2024quantumbiswas2025quantum, protein qubits feder2025fluorescent, and hBN-scaffolded pentacene (this work). The arrow indicates the preferred qubit coherence and size for quantum applications. The longest demonstrated $T_2$ coherence times for different qubit platforms are plotted, regardless of measurement conditions. The $T_2$ reported in this work is obtained using a dynamical decoupling sequence (Fig. \ref{['fig3']}f), and the qubit size is defined by the longest molecular dimension of pentacene ($\sim$1.5 nm). c, ODMR spectrum measured at 4 K in vacuum. Purple: The signal at $2.38$ GHz emerges with $\pi$-pulse at the $1.43$ GHz transition during initialization and before the laser readout. The ODMR contrast is defined as $C = I_{\mathrm{MW,on}}/I_{\mathrm{MW,off}}$. Inset: Energy-level diagram of the pentacene molecule, containing a bright singlet manifold $\{S_0, S_1\}$ and a dark triplet manifold $\{T_x, T_y, T_z\}$. d, ODMR contrast monitored under continuous-wave laser excitation ($\sim13~$kW/cm$^2$). The fitted half-life of the hBN-scaffolded pentacene qubit at ambient conditions is 37 $\pm$ 3 min, and no bleaching has been observed for qubits at 4 K in vacuum after more than six months. The stability of the pentacene qubit in ambient conditions is improved significantly by hBN encapsulation with half-life of 58 $\pm$ 17 hours. e, Top: TEM image of the hBN surface obtained from a 100 nm slice cut by focused ion beam (FIB) from a hBN-encapsulated pentacene sample. Bottom: EDS line-scan results showing the distribution of carbon, boron, and nitrogen elements along the vertical direction of the hBN surface. The carbon signal is fitted with two Gaussian components having FWHMs of 2.28 nm and 1.21 nm, respectively.
• Figure 2: Orientation of the qubits on the hBN scaffold. Dependence of ODMR signals on the external magnetic field applied perpendicular to (a) and within (b & c) the hBN plane. Solid black curves represent the simulated pentacene molecules' ODMR spectrum based on the $D$ and $E$ values measured in Fig. \ref{['fig1']}c and applying the magnetic field along the $X$ (a), $Y$ (b) and $Z$ (c) directions of the spin triplet (Eq. \ref{['spin-1_hamiltonian']}). d, Fluorescence intensity (arbitrary unit) of the hBN-scaffolded pentacene versus the excitation laser's polarization.
• Figure 3: Coherent control of the qubits.a, Room temperature Rabi oscillation, with $\Omega_R = 2\pi\times(58.9\pm0.1)$ MHz, of the hBN-scaffolded pentacene qubit driven by microwave at the $T_y\leftrightarrow T_z$ transition. b, Spin echo $T_2$ of the pentacene-h$_{14}$ (purple) and d$_{14}$ (blue) qubits scaffolded by hBN at 4 K. The h$_{14}$'s data is fitted to $\exp[-(t/T_2)^\nu]$ with $\nu=1.05\pm0.05$, and the d$_{14}$ is fitted to a phenomenological model $\exp[-(t/T_2)^\nu][a-b\sin^2(\omega t/4)]$childress2006coherent with $\nu=1.10\pm0.05$ and $\omega=2\pi\times140.2$ kHz. Inset: structure of pentacene-h$_{14}$ and pentacene-d$_{14}$ molecules. c, Schematic of multi-level control. The most populated, short-lived spin level $T_y$ is used for readout, while two long-lived spin levels serve as the qubit states. d, The populations and lifetimes of the triplet states are measured by initializing the molecule into its steady state through laser excitation and subsequently monitoring its fluorescence ($S_0$ population) after a relaxation time $\tau$. The data is fitted to triple exponential decay with the $T_{x},T_{y},T_{z}$ states' population and lifetimes as parameters (see results in Tab. \ref{['tab:pentacene-kinetics']}). e, Multi-level coherent control sequence of the hBN-scaffolded pentacene qubit. f,$T_2$ coherence time of the long-lived $T_x$–$T_z$ transition measured using XY8 (4 K) and CPMG (295 K) dynamical decoupling sequences. The longest $T_2$ reaches $214 \pm 19~\mu$s at 4 K and $6.4 \pm 0.4~\mu$s at ambient conditions. Data fitted with $T_2(N)^{-1} = (T_2(1)\cdot N^\nu)^{-1}+(2T_{1\rho})^{-1}$, where $N$ denotes the number of $\pi$-pulses, $\nu = 0.53 \pm 0.02, 1.23 \pm 0.1$, $T_{1\rho} = 405 \pm 156, 3.2 \pm 0.1~\mu$s for 4 K and ambient conditions respectively. Inset: Coherence time data of the longest measured $T_2$ see Supplemental Fig. S7 for full data).
• Figure 4: Detecting and controlling the spin environmenta, An AC magnetic field signal (orange curve) can be detected when its period matches the duration ($2\tau$) of the spin-echo sequence. In this case, the qubit coherence is suppressed due to the random phase of the AC signal. b, Spin echo measured at 1500 G aligned to the molecule's Z-axis for hBN-scaffolded pentacene-h$_{14}$ and pentacene-d$_{14}$. The modulations appear at $(2k+1)/f_{\text{AC}} = 2\tau, k=0,1,2,3...$ Inset: modulation frequencies of the spin echo signal under different B-fields (see Supplemental Fig. S10 for full data). c, Pulse sequence of double electron-electron resonance (DEER). The $\pi$-pulse (red) flips the electron spins simultaneously with the refocus $\pi$-pulse (blue) in spin echo, and induces extra decoherence on the fluorescent qubit. d, DEER spectrum measured at 1900 G with $t_{\text{fix}}=500$ ns. Blue dashed line: Zeeman splitting of a $g=2$ electron. Inset: frequencies of the DEER signal versus the magnetic field (see Supplemental Fig. S11 for full data). e, Pulse sequence for coherently driving the dark electron spin that couples to the hBN-scaffolded pentacene qubit. f, Rabi oscillation, with $\Omega_R = 2\pi\times(35.4\pm0.8)$ MHz, of the dark electron spins measured by the fluorescent pentacene qubit with. The reduced contrast is induced by the local disorder of the dark spins.