Quantum Sensing of Copper-Phthalocyanine Electron Spins via NV Relaxometry

TL;DR

This work tackles the challenge of characterizing room-temperature molecular spin systems by coupling shallow NV centers to a CuPc spin bath and using $T_1$ relaxometry. A quantitative model incorporates the CuPc hyperfine structure, spin-bath correlation time $\tau_e$, and dipolar coupling to the NV, enabling extraction of spin-bath properties and local lattice orientation from field-dependent depolarization data. The key findings show electron–electron spin interactions dominate CuPc decoherence at room temperature, and the method yields $\tau_e$ in the few-nanosecond range and nanometer-precision NV depth estimates, with broader implications for molecular qubits and spin networks. Overall, the approach establishes NV centers as powerful nanoscale probes of molecular spins and offers a path toward molecular-scale quantum information processing and sensing.

Abstract

Molecular spin systems are promising candidates for quantum information processing and nanoscale sensing, yet their characterization at room temperature remains challenging due to fast spin decoherence. In this work, we use $T_1$ relaxometry of shallow nitrogen-vacancy (NV) centers in diamond to probe the electron spin ensemble of a polycrystalline copper phthalocyanine (CuPc) thin film. In addition to unequivocally identifying the NV-CuPc interaction thanks to its hyperfine spectrum, we further extract key parameters of the CuPc spin ensemble, including its correlation time and local lattice orientation, that cannot be measured in bulk electron resonance experiments. The analysis of our experimental results confirms that electron-electron interactions dominate the decoherence dynamics of CuPc at room temperature. Additionally, we demonstrate that the CuPc-enhanced NV relaxometry can serve as a robust method to estimate the NV depth with $\sim1$~nm precision. Our results establish NV centers as powerful probes for molecular spin systems, providing insights into molecular qubits, spin bath engineering, and hybrid quantum materials, and offering a potential pathway toward their applications such as molecular-scale quantum processors and spin-based quantum networks.

Figures (4)

• Figure 1: Experimental Setup. (a) Schematic of the experiment, where a shallow NV center in diamond is initialized and read out using a 532 nm laser. The NV center couples to the electron spin of CuPc molecules via magnetic dipolar interaction. (b) Illustration of the CuPc thin film deposited on the diamond surface. The red arrow represents the NV center in a (100)-oriented diamond, while the blue shaded arrows indicate the non-polarized electron spins of CuPc. An external magnetic field is applied along the NV axis.
• Figure 2: NV Relaxometry. (a) Simulated CuPc spectral density $S_e$ (broad line), NV resonance frequency ($\ket{0} \rightarrow \ket{\pm1}$ transition, red solid line) and free electron spin resonance frequency (red dashed line) as a function of external magnetic field strength. The four split peaks correspond to the hyperfine states of the CuPc electron spin, arising from coupling to the copper nuclear spin. They are broaden due to hyperfine with nitrogen nuclear spins. Vertical gray dashed lines indicate the four magnetic fields selected for experimental measurements. (b-e) $T_1$ of individual NV centers at the four magnetic fields, both in the presence (blue) and absence of the CuPc film. (f-i) Change in NV center depolarization rate, $\Delta\Gamma_1$ due to the presence of CuPc. The two fitting results correspond to different assumptions about the source of the electron spin bath, with both assuming that the electron spatial density equals the CuPc molecular density.
• Figure 3: CuPc Crystal Orientation Estimation. (a–d) Simulated CuPc spectral density with the NV center's resonance frequency overlaid as a red solid line, as a function of the relative orientation between the CuPc molecule and the external magnetic field. (a)–(d) correspond to the four magnetic fields studied. (e–h) Estimated CuPc molecular orientations on top of individual NV centers at the corresponding magnetic fields. At 231 G (a, e), there is no spectral overlap between the CuPc and NV transitions across all $\theta_e$, resulting in a large uncertainty in the estimated orientation. A similar situation occurs at 721 G (d, h). We leverage this minor effect of $\theta_e$ on $S_e(\omega_{NV})$ for accurate estimation of $\tau_e$ and $d_{NV}$. In contrast, at 461 G (c, g), the degree of spectral overlap depends more sensitively on $\theta_e$, allowing for a relatively precise estimation of the orientation angle. At 372 G (b, f), a crossing between the NV center resonance and one of the CuPc hyperfine transitions occurs, leading to two possible estimated orientations, shown in blue and red.
• Figure 4: NV center depth estimation. Data were collected at 231G and 721G. $d_{T_1}$ represents the depth extracted from the $T_1$ relaxometry measurements. $d_H$ denotes the depth measured after removing the CuPc layer using high-power laser illumination and exploiting the NV coupling to the hydrogen (H) in the objective oil pham2016nmr. These depths are used fixed parameters when estimating $\tau_e$ and $\theta_e$.