Optically Addressable Molecular Spins at 2D Surfaces

Abstract

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, \(T_2\) 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.

Figures (4)

• Figure 1: Surface molecular spins hosted on hBN.a, Schematic illustration of Pc molecules coupled to hBN surface defects in the edge-on configuration, which enables optical addressing of spins. b, Spatial distributions of the highest occupied molecular orbitals (HOMO) of the triplet spins. c, Photoluminescence spectra of Pc molecules on hBN at 4 K and room temperature. $\omega_0$ denotes a characteristic molecular phonon mode. d, ODMR spectrum of Pc at ambient conditions, revealing the $\ket{\mathrm{T_x}} – \ket{\mathrm{T_y}}$ and $\ket{\mathrm{T_y}} – \ket{\mathrm{T_z}}$ transitions at 917 MHz and 1433 MHz, respectively. The $\ket{\mathrm{T_x}} – \ket{\mathrm{T_z}}$ transition is accessed via a double-resonance scheme, as illustrated in the right inset, where two microwave tones are applied. e, Hahn-echo spin-coherence measurements of Pc spins at room temperature and 4 K, yielding a characteristic $T_2$ time of 3.4 $\upmu$s at 4K.
• Figure 2: Pc configuration on hBN surfaces.a, Calculated spin-density distribution of Pc, showing a shift of spin density toward the hBN plane, which leads to an increased zero-field splitting with optical dipole orientation of the Pc aligned along the molecular Y axis. The molecular axis orientation is indicated according to the conventional definition. Right: schematic of the polarization-resolved measurement used to determine the Pc molecular Y-axis orientation. b, Out-of-plane Magnetic-field dependence of the $\ket{\mathrm{T_x}} – \ket{\mathrm{T_y}}$ and $\ket{\mathrm{T_y}} – \ket{\mathrm{T_z}}$ transitions, consistent with an edge-on configuration of Pc molecules. c, Representative polarization maps for Pc orientations at $0^{\circ}$, $60^{\circ}$, and $120^{\circ}$ with respect to the hBN edges show clear linear polarization behavior. d, Histogram of hundreds of Pc ODMR spots, revealing the threefold symmetry of Pc alignment relative to the underlying hBN lattice.
• Figure 3: Engineering of spin coherence toward the lifetime limit.a, Schematic illustration of molecular deuteration from Pc-H$_{14}$ to Pc-D$_{14}$, which suppresses nuclear-spin-induced decoherence. b, The Hahn-echo spin coherence time ($T_2$) in Pc-D$_{14}$ exhibits a tenfold enhancement relative to Pc-H$_{14}$. For comparison, the Pc-H$_{14}$ data are plotted as a semi-transparent reference curve. c, Schematic of dynamical decoupling pulse sequences (CPMG) used to protect spin coherence in the molecular $\ket{\mathrm{T_x}} – \ket{\mathrm{T_z}}$ basis with $\mathrm{T_y}$ for readout. d, Extended spin coherence of Pc-D$_{14}$ under dynamical decoupling, with spins protected in the $\ket{\mathrm{T_x}} – \ket{\mathrm{T_z}}$ manifold. e, The prolongation of the coherence time as a function of the number of applied $\pi$ pulses N. For both the $\ket{\mathrm{T_y}} – \ket{\mathrm{T_z}}$ and $\ket{\mathrm{T_x}} – \ket{\mathrm{T_z}}$ transitions, the spin coherence approaches the lifetime limit for their own spin manifolds.
• Figure 4: Exploiting Pc as a room temperature spin surface sensor.a, Hahn-echo measurements recorded for six different values of the magnetic field in order to detect Pc’s own proton spins. $\Delta$PL reflects the degree of spin polarization. The observed revivals in spin polarization indicate coupling to nearby nuclear spins. b, Field dependence of the nuclear precession frequency, showing an effective gyromagnetic ratio close to that of the protons. c, Illustration of the two-dimensional (2D) sensing architecture, where Pc molecules are deposited on an hBN layer that itself covers a 2D ferromagnet (Fe$_3$GaTe$_2$). d, ODMR spectrum of Pc molecules located in the hBN area covering the 2D magnet. Also shown is a reference spectrum on hBN far away from FGT. The arrow marks the observed shift induced by the magnetic layer.