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arXiv:2512.01337

Long nuclear spin coherence times for molecules trapped in high-purity solid parahydrogen

Alexandar P. Rollings, Jonathan D. Weinstein

quant-phphysics.atom-phSubmitted Dec 1, 2025• Announced Dec 2, 2025
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Abstract

We measure the ensemble transverse relaxation time (T2*) and spin-echo coherence time (T2) of the proton spin of HD molecules trapped in solid parahydrogen. By using high-purity parahydrogen matrices, we are able to measure significantly longer T2 and T2* times than seen in prior work. We also measure the longitudinal spin relaxation time T1. We examine how these parameters scale with the matrix purity and find limits on the coherence time from the parahydrogen matrix itself.

DOI
10.1103/8cn8-7cyt

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We measure the ensemble transverse relaxation time (T2*) and spin-echo coherence time (T2) of the proton spin of HD molecules trapped in solid parahydrogen. By using high-purity parahydrogen matrices, we are able to measure significantly longer T2 and T2* times than seen in prior work. We also measure the longitudinal spin relaxation time T1. We examine how these parameters scale with the matrix purity and find limits on the coherence time from the parahydrogen matrix itself.
Coherence time is an essential parameter for quantum sensing, quantum information, and quantum computation. In this work, we demonstrate electron spin coherence times as long as 0.1 s for an ensemble of rubidium atoms trapped in a solid parahydrogen matrix. We explore the underlying physics limiting the coherence time. The properties of these matrix isolated atoms are very promising for future applications, including quantum sensing of nuclear spins. If combined with efficient single-atom readout, this would enable NMR and magnetic resonance imaging of single molecules cotrapped with alkali-metal atom quantum sensors within a parahydrogen matrix.
We measure the transverse relaxation of the spin state of an ensemble of ground-state rubidium atoms trapped in solid parahydrogen at cryogenic temperatures. We find the spin dephasing time of the ensemble (T2∗{T}_{2}^{*}T2∗​) is limited by inhomogeneous broadening. We determine that this broadening is dominated by electrostaticlike interactions with the host matrix, and can be reduced by preparing nonclassical spin superposition states. Driving these superposition states gives significantly narrower electron paramagnetic resonance lines and the longest reported electron spin T2∗{T}_{2}^{*}T2∗​ in any solid-phase system other than solid helium.
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%60\%60% polarization of the proton spin bath. The measured scaling of the spin-echo decay time (T2T_2T2​) 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.
We have grown crystals of solid parahydrogen using a single closed-cycle cryostat. We have doped the crystals with rubidium atoms at densities on the order of 10^{17} cm^{-3} and used optical pumping to polarize the spin state of the implanted atoms. The optical spectrum of the rubidium atoms shows larger broadening than previous work in which the rubidium was implanted in solid argon or neon. However, the optical pumping behavior is significantly improved, with both a larger optical pumping signal and a longer longitudinal relaxation time. The spin relaxation time shows a strong dependence on orthohydrogen impurity levels in the crystal, as well as the applied magnetic field. Current performance is comparable to state-of-the-art solid state systems at comparable spin densities, with potential for improvement at higher parahydrogen purities.
Long coherence times rank among the most important performance measures for many different types of quantum technology. In the electron-nuclear spin system of NV centers of diamond, the nuclear spins provide particularly long dephasing times. However, since initialization and readout require assistance from the electron spin, the apparent dephasing times can be reduced by the electron spin lifetime. Here we propose and implement schemes for measuring and extending the dephasing times of nuclear spins, resulting in dephasing times that are longer than the longitudinal relaxation time T 1 of the electron spin.