Enhancing Spin Coherence of Optically-Addressed Molecular Qubit by Nuclear Spin Hyperpolarization

Abstract

Optically addressable molecular triplet spins provide a chemically tunable platform for quantum application, but their coherence is often limited by interactions with surrounding spin baths. Here we demonstrate controlled suppression of nuclear-bath-induced decoherence in photoexcited triplet spins of pentacene co-crystallized in high-purity naphthalene single crystals. By hyperpolarizing the proton spin bath through triplet dynamic nuclear polarization (triplet-DNP), magnetic noise generated by the nuclear spins is suppressed, leading to an extension of the electron spin transverse coherence time. Experimentally, we observe a 25\% enhancement of the spin-echo decay time with $60\%$ polarization of the proton spin bath. The measured scaling of the spin-echo decay time ($T_2$) with nuclear polarization quantitatively follows the predicted dependence derived from the polarization-controlled nuclear second moment. Both the enhancement and the absolute value of the coherence time are quantitatively reproduced by cluster correlation expansion (CCE) simulations. These results establish nuclear spin hyperpolarization as a general and actively tunable approach to engineering coherence in molecular qubits. This work provides a broadly applicable design framework for high-coherence molecular and solid-state spin systems.

Figures (3)

• Figure 1: Pentacene triplet spin system and proton spin bath hyperpolarization. (a) Lattice structure of naphthalene (light red) with a pentacene molecule substitution (red). The nuclear spin bath is dominated by protons in the protonated naphthalene host. The pentacene molecular electron spin can be optically excited and interacts with the surrounding nuclear spins through magnetic dipolar coupling. (b) Molecular structure of pentacene and spin energy levels. The experiment is performed with an external magnetic field applied along the long molecular axis of pentacene ($\sim$0.355 T). Optical excitation by a laser pulse and spin-selective intersystem crossing (ISC) populates the photo-induced triplet manifold ($T_+,T_0,T_-$), generating a highly spin-polarized electron ensemble. (c) Field-swept integrated solid effect (ISE) sequence used for triplet dynamic nuclear polarization (triplet-DNP). During the transient triplet lifetime ($\sim 100~\mu$s), the initialized electron spin polarization can be transferred to nearby proton spins via microwave irradiation ($\sim7\,\mu$s) while sweeping the magnetic field over a range of $\sim 2$ mT. This sequence is repeated at a kHz repetition rate to accumulate polarization in the nuclear spin bath. (d) Experimental build-up of nuclear spin polarization as a function of the DNP accumulation time. The polarization is obtained from the measured proton nuclear magnetic resonance (NMR) signal.(e,f) Schematic illustration of the proton nuclear spin bath before and after DNP. In thermal equilibrium the nuclear spins are nearly unpolarized, producing fluctuating magnetic fields at the electron spin. Partial polarization of the nuclear spins reduces these magnetic field variance and fluctuations, thereby suppressing electron spin decoherence.
• Figure 2: Triplet decoherence measurements. (a) Normalized echo signal intensity as a function of the total evolution time $2\tau$ for different nuclear polarizations $P_I$. The decay is fitted to a stretched exponential function, $E(2\tau)=\exp\!\left[-(2\tau/T_2)^\mu\right]$. Increasing nuclear polarization leads to a slower decay of the echo signal, demonstrating that nuclear hyperpolarization protects the coherence. (b) Extracted coherence time $T_2$ as a function of nuclear polarization $P_I$. A 25% enhanced electron coherence was measured with $P_I=60\%$. A linear function is fitted to $\log T_2$ versus $\log(1-P_I^2)$ (shown as the solid line) and the fitted slope is $-0.46$.
• Figure 3: CCE-calculated results. (a) Hahn-echo decay simulated using CCE at different nuclear spin polarizations, assuming the electron spin is localized at the center of the pentacene molecule. Oscillations arise from strongly coupled proton spins in the naphthalene lattice (Fig. \ref{['fig:scheme_triplet']}(a)). (b) Extracted $T_2$ from stretched-exponential fits. Inset: $T_2$ versus $(1-P_I^2)$ on a log-log scale; for $p_I<60\%$, a linear fit yields a slope of $\sim 0.4$, consistent with experiment.