We explore the potential of levitating solids embedded with non-permanent, optically controllable electron spins, which can be used to hyperpolarize their nuclear spin environment with exceptionally long lifetimes. For example, pentacene-doped naphthalene, which will also serve as our prime example, can achieve bulk polarization exceeding 80 % at cryogenic temperatures with polarization lifetimes extending over weeks. These materials make a compelling case for applications such as matter-wave interferometry and novel uses of established NMR techniques. In that spirit, we design a multi-spin Stern-Gerlach-type interferometry protocol which, thanks to the homogeneous spin distribution and the absence of a preferential nuclear-spin quantization axis in such materials, avoids many of the limitations associated with solid state crystals hosting electronic spin defects, such as nanodiamonds containing NV centers. We assess the potential of our interferometer to enhance existing bounds on the free parameters of objective collapse models. Beyond matter-wave interferometry, we analyze the prospects for implementing magic angle spinning at frequencies surpassing the current standard in NMR, capitalizing on the exceptional rotational capabilities offered by levitation. Additionally, we outline a novel protocol for measuring spin ensemble polarization via the position of the nanoparticle and conduct an analysis of dominant noise sources, benchmarking the required isolation levels for various applications.
We explore the potential of levitating solids embedded with non-permanent, optically controllable electron spins, which can be used to hyperpolarize their nuclear spin environment with exceptionally long lifetimes. For example, pentacene-doped naphthalene, which will also serve as our prime example, can achieve bulk polarization exceeding 80 % at cryogenic temperatures with polarization lifetimes extending over weeks. These materials make a compelling case for applications such as matter-wave interferometry and novel uses of established NMR techniques. In that spirit, we design a multi-spin Stern-Gerlach-type interferometry protocol which, thanks to the homogeneous spin distribution and the absence of a preferential nuclear-spin quantization axis in such materials, avoids many of the limitations associated with solid state crystals hosting electronic spin defects, such as nanodiamonds containing NV centers. We assess the potential of our interferometer to enhance existing bounds on the free parameters of objective collapse models. Beyond matter-wave interferometry, we analyze the prospects for implementing magic angle spinning at frequencies surpassing the current standard in NMR, capitalizing on the exceptional rotational capabilities offered by levitation. Additionally, we outline a novel protocol for measuring spin ensemble polarization via the position of the nanoparticle and conduct an analysis of dominant noise sources, benchmarking the required isolation levels for various applications.
Levitated nanoparticles, free from the clamping losses and decoherence channels imposed by mechanical support, offer a growing versatile platform for precision sensing and tests of fundamental physics at increasingly larger masses. This thesis follows complementary directions based on this platform: the coupling of optically controllable nuclear spin ensembles to the mechanical and rotational degrees of freedom of levitated solids, and the identification of decoherence mechanisms that limit levitated systems in tests of macroscopic quantum superpositions. In the first part, we study levitated solids doped with non-permanent, optically addressable electron spins, with pentacene-doped naphthalene as a prime exam ple, whose nuclear spins can be hyperpolarised using dynamic nuclear polarisation techniques to over 80% at cryogenic temperatures, with lifetimes extending over weeks. Exploiting the homogeneous spin distribution and absence of a preferred nuclear quantisation axis in such materials, we design a multi-spin Stern-Gerlach interferometry protocol that circumvents key limitations of colour centres in nan odiamonds like NV centres, and use it to place improved bounds on objective collapse models. We further show that the unconstrained rotation available to levitated particles enables magic angle spinning at frequencies beyond current NMRstandards, and propose a protocol for reading out spin ensemble polarisation directly from the particle’s mechanical motion. Building on the same hyperpo larisation mechanism, we propose experiments to observe the nuclear Einstein-de Haas effect, using the hyperpolarised bath of nuclei as a reservoir for angular mo mentum. Furthermore, we propose a protocol to investigate the nuclear Barnett effect in the solid state, using the MHz-scale rotation rates accessible to levitated particles. We find that the proposed Stern-Gerlach interferometer could improve upon existing bounds on collapse-model parameters and that the nuclear Einstein de Haas and Barnett effects could be observable using near-term experimental technology, within parameter ranges already accessible to the community. In the second part, we turn to a central obstacle facing proposals for gravitationally induced entanglement (GIE) between levitated masses, namely the conducting and magnetic shields required to suppress direct electromagnetic coupling. We show that residual Casimir and magnetic-dipole interactions between the particles and the shield, averaged over run-to-run positional and orientational fluctuations of the experimental set-up, generate decoherence, and that the shield can itself mediate spurious entanglement between the particles that might mimic a genuine gravitational-entanglement signature. From this, we derive quantitative stability thresholds considerably stricter than previously assumed.
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 (T2) 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.
Microwave-Free 13C Hyperpolarization of Diamond Particles Enabled by Magic Angle Spinning and NV Centers
Nuclear hyperpolarization from optically pumped color centers in solids offers an alternative to conventional microwave-driven dynamic nuclear polarization (DNP). Diamond can host the nitrogen vacancy (NV) center, whose ground spin state can be readily polarized by light at room temperature, making diamond a candidate platform for nuclear hyperpolarization. We report 13C nuclear hyperpolarization in randomly oriented diamond particles with sizes ranging from 0.2 to 2 μm, both at natural 13C abundance (1.1 %) and at 20 % isotopic enrichment, at magnetic fields of 7.1 T and 9.4 T. The protocol combines optical illumination with magic angle spinning (MAS) and does not require microwave irradiation. By investigating the nuclear polarization as a function of the MAS frequency between 0 and 6 kHz at the magnetic field of 7.1 T, we find maximum light-induced polarization enhancements of 280-fold for the isotopically enriched sample and 411-fold for the natural abundance sample. Under continuous illumination, steady-state absolute 13C polarization levels above 0.1 % are reached. A model involving optical pumping of NV centers and spin dynamics near level anticrossings (LACs) in three-spin clusters formed by NV, a substitutional nitrogen (P1) and 13C is used to describe these findings. The protocol strongly mitigates the effect of the anisotropy of the NV spin Hamiltonian, allowing more than 99.9% of NV orientations to participate in the polarization transfer process. These results represent a first step toward transferring nuclear polarization from diamond particles to external nuclei, with potential applications in sensitive and high-resolution NMR at room temperature.
Abstract Hyperpolarization of nuclear spins is crucial for advancing nuclear magnetic resonance and quantum information technologies, as nuclear spins typically exhibit extremely low polarization at room temperature due to their small gyromagnetic ratios. A promising approach to achieving high nuclear spin polarization is transferring the polarization of electrons to nuclear spins. The nitrogen-vacancy (NV) center in diamond has emerged as a highly effective medium for this purpose, and various hyperpolarization protocols have been developed. Among these, the pulsed polarization (PulsePol) method has been extensively studied due to its robustness against static energy shifts of the electron spin. In this work, we present a novel polarization protocol and uncover a family of magic sequences for hyperpolarizing nuclear spins, with PulsePol emerging as a special case of our general approach. Notably, we demonstrate that some of these magic sequences exhibit significantly greater robustness compared to the PulsePol protocol in the presence of finite half π pulse duration of the protocol, Rabi and detuning errors. This enhanced robustness positions our protocol as a more suitable candidate for hyper-polarizing nuclear spins species with large gyromagnetic ratios and also ensures better compatibility with high-efficiency readout techniques at high magnetic fields. Additionally, the generality of our protocol allows for its direct application to other solid-state quantum systems beyond the NV center.
Gyroscopes find wide applications in everyday life from navigation and inertial sensing to rotation sensors in hand-held devices and automobiles. Current devices, based on either atomic or solid-state systems, typically impose a choice between long-time stability and high sensitivity in a miniaturized system. Thanks to their optical properties, nuclear spins associated with NV centers in diamond have been proposed to overcome this challenge. While optical polarization improves these devices' sensitivities, further improvement is needed. Here, we propose a gyroscope protocol based on a two-spin system that includes a spin intrinsically tied to the host material, while the other spin is effectively in an inertial frame. The rotation rate is then extracted by measuring the relative rotation angle between the two spins starting from their population states, which are robust against spin dephasing. Importantly, the relative rotation rate between the two spins is enhanced by their hyperfine coupling by more than an order of magnitude, further boosting the achievable sensitivity. The ultimate sensitivity of the gyroscope is limited by the lifetime of the spin system and is compatible with a broad dynamic range, even in the presence of magnetic noises or control errors due to initialization and qubit manipulations. Our result enables precise measurement of slow rotations and exploration of fundamental physics.
The goal of this project was to improve our control over nuclear spins in the solid state, and to extend this control to coupled electron-nuclear spins. Our key results are summarized below. 1. We extended the control techniques developed in liquid state NMR quantum information processing to the control of a three-qubit solid state NMR quantum information processor. 2. To explore multi-body spin dynamics and their sensitivity to decoherence, we have measured the decay of NMR multiple quantum coherence intensities in the presence and absence of the dipolar Hamiltonian, in a cubic and a one-dimensional spin system. 3. We also studied the transport of polarization in the one-dimensional fluoroapatite system, both experimentally and theoretically, and created special states in which the polarization is localized to the ends of the chain. This enables both universal Quantum Computing and Quantum Simulation in 1-D systems. 4. We devised a novel scheme for electron-nuclear quantum information processing that exploits the anisotropic hyperfine coupling. This scheme enables universal control over a 1-electron, N-nuclear spin system, addressing only a single electron spin transition. Not having to address the nuclear spins directly significantly speeds up the control. We designed and fabricated a pulsed electron spin resonance spectrometer, along with a cryogenic probe which we used to experimentally implement this scheme on a single crystal sample of irradiated malonic acid. 5. We demonstrated the role of nuclear spin dipolar diffusion in dynamic nuclear polarization (DNP) experiments, in dielectric samples with abundant nuclear spins. We achieved a 29Si polarization of 8.3% at 66 GHz and 1.1 K in single crystal P-doped, the highest ever reported, using DNP.