Nuclear Spin Polarization Research Articles

Initializing 214 Pure 14-Qubit Entangled Nuclear Spin States in a Hyperpolarized Molecular Solid.

Quantum entanglement has been realized on a variety of physical platforms such as quantum dots, trapped atomic ions, and superconductors. Here we introduce specific molecular solids as promising alternative platforms. Our model system is triplet pentacene in a host single crystal at level anticrossing (LAC) conditions. First, a laser pulse generates the triplet state and initiates entanglement between an electron spin and 14 hyperfine coupled proton spins (quantum bits or qubits). This gives rise to large nuclear spin polarization. Subsequently, a resonant high-power microwave (mw) pulse disentangles the electron spin from the nuclear spins. Simultaneously, high-dimensional multiqubit entanglement is formed among the proton spins. We verified the initialization of 214 pure 14-qubit entangled nuclear spin states with an average degree of entanglement of Eav = 0.77 ± 0.03. These results pave the way for large-scale quantum information processing with more than 10 000 multiqubit entangled states corresponding to computational (Hilbert) space dimensions of dim >1053.

Porphyrins as Versatile, Aggregation-Tolerant, and Biocompatible Polarizing Agents for Triplet Dynamic Nuclear Polarization of Biomolecules.

Triplet dynamic nuclear polarization (triplet-DNP) achieves nuclear spin polarization at moderate temperatures by using spin polarization of photoexcited triplet electrons. The applications of triplet-DNP for biomolecules have been hampered because acenes, the only polarizing agents used so far, tend to aggregate and lose their polarization in biomolecular matrices. Here, we report for the first time use of porphyrins as polarizing agents of triplet-DNP and propose a new concept of aggregation-tolerant polarizing agents. Sodium salts of tetrakis(4-carboxyphenyl)porphyrin (TCPPNa) can be dispersed in amorphous as well as crystalline biomolecular matrices, and importantly, it can generate polarized triplet electrons even in a slightly aggregated state. Triplet-DNP of crystalline erythritol containing slightly aggregated TCPPNa can achieve more than 120-fold signal enhancement. Because TCPPNa is also the first biocompatible triplet-DNP polarizing agent, this work provides a crucial step forward for the biological and medical applications of triplet-DNP.

Dynamic Nuclear Polarization Enhanced Nuclear Spin Optical Rotation

AbstractNuclear spin optical rotation (NSOR) has been investigated as a magneto‐optical effect, which holds the potential for applications, including hybrid optical‐nuclear magnetic resonance (NMR) spectroscopy and gradientless imaging. The intrinsic nature of NSOR renders its detection relatively insensitive, which has prevented it moving from a proof of concept to a method supporting chemical characterizations. In this work, the dissolution dynamic nuclear polarization technique is introduced to provide nuclear spin polarization, increasing the signal‐to‐noise ratio by several thousand times. NSOR signals of 1H and 19F nuclei are observed in a single scan for diluted compounds, which has made this effect suitable for the determination of electronic transitions from a specific nucleus in a large molecule.

Monte Carlo study of the spin-spin interactions between radicals used for dynamic nuclear polarization

The spectrum of the electron spin-spin interactions largely determines which mechanism is responsible for the growth of the nuclear spin polarization in dynamic nuclear polarization (DNP). When electron spin-spin interactions are weak and their spectrum is narrow, the solid effect (SE) dominates the process. When they are stronger, the cross effect (CE) and thermal mixing (TM) come into play. Then a narrow spectrum favours the CE—that is an exchange of electron Zeeman energy with the nuclear spins—and a broad spectrum also TM—that is an exchange of electron spin-spin interaction energy with the nuclear spins. Moreover, the spectrum of the electron spin-spin interactions critically determines the rate of spectral diffusion of electron spin polarization across the electron spin resonance (ESR) line, and the associated conversion of electron Zeeman energy into electron spin-spin interaction energy. This way electron spin-spin interactions indirectly influence the DNP process. The present work describes Monte Carlo simulations of the spectrum of these interactions for approximately spherical radicals in glasses and analytical approximations of the simulation results. As an example application expressions for the relative strengths of the energy flows due to the CE and TM are derived.

Double nuclear spin switching in single quantum dots

Hyperfine interaction in a quantum dot produces a spin-coupled system between an injected electron and a few tens of thousands of nuclei in the electron wavefunction. We report here that the double bistable phenomenon of nuclear spin polarization (NSP) can be observed by sweeping three control parameters. All the observations indicate that the NSP has three stable branches under a longitudinal magnetic field, whereas the number of the stable branches of NSP has been believed to be two at most. The incorporation of the electron spin relaxation due to the nuclear spin fluctuation enables reproduction of the observed features of the NSP and the electron spin polarization.

Nuclear polarization effects in cryptochrome-based magnetoreception.

The mechanism of the magnetic compass sense of migratory songbirds is thought to involve magnetically sensitive chemical reactions of light-induced radical pairs in cryptochrome proteins located in the birds' eyes. However, it is not yet clear whether this mechanism would be sensitive enough to form the basis of a viable compass. In the present work, we report spin dynamics simulations of models of cryptochrome-based radical pairs to assess whether accumulation of nuclear spin polarization in multiple photocycles could lead to significant enhancements in the sensitivity with which the proteins respond to the direction of the geomagnetic field. Although buildup of nuclear polarization appears to offer sensitivity advantages in the more idealized model systems studied, we find that these enhancements do not carry over to conditions that more closely resemble the situation thought to exist in vivo. On the basis of these simulations, we conclude that buildup of nuclear polarization seems unlikely to be a source of significant improvements in the performance of cryptochrome-based radical pair magnetoreceptors.

Direct high-resolution resonant Raman scattering measurements of dynamic nuclear spin polarization states of an InAs quantum dot

We report on the direct measurement of the electron spin splitting and the accompanying nuclear Overhauser (OH) field, and thus the underlying nuclear spin polarization (NSP) and fluctuation bandwidth, in a single InAs quantum dot under resonant excitation conditions with unprecedented spectral resolution. The electron spin splitting is measured directly via resonant spin-flip single photon Raman scattering detected by superconducting nanowires to generate excitation-emission energy maps. The observed two-dimensional maps reveal an OH field that has a non-linear dependence on excitation frequency. This study provides new insight into earlier reports of so-called avoidance and tracking, showing two distinct NSP responses directly by the addition of a emission energy axis. The data show that the polarization processes depend on which electron spin state is optically driven, with surprising differences in the polarization fluctuations for each case: in one case, a stabilized field characterized by a single-peaked distribution shifts monotonically with the laser excitation frequency resulting in a nearly constant optical interaction strength across a wide detuning range, while in the other case the previously reported avoidance behavior is actually the result of a nonlinear dependence on the laser excitation frequency near zero detuning leading to switching between two distinct mesoscopic nuclear spin states. The magnitude of the field, which is as large as 400 mT, is measured with sub-100 nuclear spin sensitivity. Stable/unstable points of the OH field distribution are observed, resulting from the non-linear feedback loop in the electron-trion-nuclear system. Nuclear spin polarization state switching occurs between fields differing by 160 mT at least as fast as 25 ms. Control experiments indicate that the strain-induced quadrupolar interaction may explain the measured OH fields.

Dynamic N14 nuclear spin polarization in nitrogen-vacancy centers in diamond

We studied the dynamic nuclear spin polarization of nitrogen in negatively charged nitrogen-vacancy (NV) centers in diamond both experimentally and theoretically over a wide range of magnetic fields from 0 to 1100 G covering both the excited-state level anti-crossing and the ground-state level anti-crossing magnetic field regions. Special attention was paid to the less studied ground-state level anti-crossing region. The nuclear spin polarization was inferred from measurements of the optically detected magnetic resonance signal. These measurements show that a very large (up to $96 \pm 2\%$) nuclear spin polarization of nitrogen can be achieved over a very broad range of magnetic field starting from around 400 G up to magnetic field values substantially exceeding the ground-state level anti-crossing at 1024 G. We measured the influence of angular deviations of the magnetic field from the NV axis on the nuclear spin polarization efficiency and found that, in the vicinity of the ground-state level anti-crossing, the nuclear spin polarization is more sensitive to this angle than in the vicinity of the excited-state level anti-crossing. Indeed, an angle as small as a tenth of a degree of arc can destroy almost completely the spin polarization of a nitrogen nucleus. In addition, we investigated theoretically the influence of strain and optical excitation power on the nuclear spin polarization.

Characterization of photonic band resonators for DNP NMR of thin film samples at 7 T magnetic field

Polarization of nuclear spins via Dynamic Nuclear Polarization (DNP) relies on generating sufficiently high mm-wave B1e fields over the sample, which could be achieved by developing suitable resonance structures. Recently, we have introduced one-dimensional photonic band gap (1D PBG) resonators for DNP and reported on prototype devices operating at ca. 200 GHz electron resonance frequency. Here we systematically compare the performance of five (5) PBG resonators constructed from various alternating dielectric layers by monitoring the DNP effect on natural-abundance 13C spins in synthetic diamond microparticles embedded into a commercial polyester-based lapping film of just 3 mil (76 μm) thickness. An odd-numbered configuration of dielectric layers for 1D PBG resonator was introduced to achieve further B1e enhancements. Among the PBG configurations tested, combinations of high-ε perovskite LiTaO3 together with AlN as well as AlN with optical quartz wafers have resulted in ca. 40 to over 50- fold gains in the average mm-wave power over the sample vs. the mirror-only configuration. The results are rationalized in terms of the electromagnetic energy distribution inside the resonators obtained analytically and from COMSOL simulations. It was found that average of B1e2 over the sample strongly depends on the arrangement of the dielectric layers that are the closest to the sample, which favors odd-numbered PBG resonator configurations for their use in DNP.

Heterogeneous Parahydrogen-Induced Polarization of Diethyl Ether for Magnetic Resonance Imaging Applications.

Magnetic resonance imaging (MRI) with the use of hyperpolarized gases as contrast agents provides valuable information on lungs structure and function. While the technology of 129 Xe hyperpolarization for clinical MRI research is well developed, it requires the expensive equipment for production and detection of hyperpolarized 129 Xe. Herein we present the 1 H hyperpolarization of diethyl ether vapor that can be imaged on any clinical MRI scanner. 1 H nuclear spin polarization of up to 1.3 % was achieved using heterogeneous hydrogenation of ethyl vinyl ether with parahydrogen over Rh/TiO2 catalyst. Liquefaction of diethyl ether vapor proceeds with partial preservation of hyperpolarization and prolongs its lifetime by ≈10 times. The proof-of-principle 2D 1 H MRI of hyperpolarized diethyl ether was demonstrated with 0.1×1.1 mm2 spatial and 120 ms temporal resolution. The long history of use of diethyl ether for anesthesia is expected to facilitate the clinical translation of the presented approach.

15 N NMR Hyperpolarization of Radiosensitizing Antibiotic Nimorazole by Reversible Parahydrogen Exchange in Microtesla Magnetic Fields.

Nimorazole belongs to the imidazole-based family of antibiotics to fight against anaerobic bacteria. Moreover, nimorazole is now in Phase 3 clinical trial in Europe for potential use as a hypoxia radiosensitizer for treatment of head and neck cancers. We envision the use of [15 N3 ]nimorazole as a theragnostic hypoxia contrast agent that can be potentially deployed in the next-generation MRI-LINAC systems. Herein, we report the first steps to create long-lasting (for tens of minutes) hyperpolarized state on three 15 N sites of [15 N3 ]nimorazole with T1 of up to ca. 6 minutes. The nuclear spin polarization was boosted by ca. 67000-fold at 1.4 T (corresponding to P15N of 3.2 %) by 15 N-15 N spin-relayed SABRE-SHEATH hyperpolarization technique, relying on simultaneous exchange of [15 N3 ]nimorazole and parahydrogen on polarization transfer Ir-IMes catalyst. The presented results pave the way to efficient spin-relayed SABRE-SHEATH hyperpolarization of a wide range of imidazole-based antibiotics and chemotherapeutics.

15N NMR Hyperpolarization of Radiosensitizing Antibiotic Nimorazole by Reversible Parahydrogen Exchange in Microtesla Magnetic Fields

AbstractNimorazole belongs to the imidazole‐based family of antibiotics to fight against anaerobic bacteria. Moreover, nimorazole is now in Phase 3 clinical trial in Europe for potential use as a hypoxia radiosensitizer for treatment of head and neck cancers. We envision the use of [15N3]nimorazole as a theragnostic hypoxia contrast agent that can be potentially deployed in the next‐generation MRI‐LINAC systems. Herein, we report the first steps to create long‐lasting (for tens of minutes) hyperpolarized state on three 15N sites of [15N3]nimorazole with T1 of up to ca. 6 minutes. The nuclear spin polarization was boosted by ca. 67000‐fold at 1.4 T (corresponding to P15N of 3.2 %) by 15N−15N spin‐relayed SABRE‐SHEATH hyperpolarization technique, relying on simultaneous exchange of [15N3]nimorazole and parahydrogen on polarization transfer Ir‐IMes catalyst. The presented results pave the way to efficient spin‐relayed SABRE‐SHEATH hyperpolarization of a wide range of imidazole‐based antibiotics and chemotherapeutics.

(Invited) Photon Upconversion and Triplet Dynamic Nuclear Polarization in MOFs

Photon upconversion (UC) is the key technology to overcome the thermodynamic efficiency limit of solar energy conversion devices. Triplet-triplet annihilation (TTA)-based UC has drawn increasing attention in recent years, primarily because of its occurrence under low-intensity, noncoherent light excitation. However, the application of TTA-UC has been significantly hampered by the slow diffusion of excited molecules in solid matrices. The use of emitter-based metal–organic frameworks (MOFs) allows the fast triplet exciton diffusion among the regularly aligned emitter chromophores. Besides, near-infrared (NIR)-to-visible (vis) TTA-UC is very important for various applications, however, it remains challenging mainly due to the energy loss during the S1-to-T1 intersystem crossing (ISC) of molecular sensitizers. We circumvent this energy loss by employing a sensitizer with direct singlet-to-triplet (S-T) absorption in the NIR region. We achieve the NIR-to-visible TTA-UC in the solid-state by introducing the S-T absorption sensitizer into emitter-based MOFs.Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) are powerful and versatile methods in modern chemistry and biology fields. Nevertheless, they suffer from intrinsically limited sensitivity due to the low nuclear spin polarization at ambient temperature. One of the promising methods to overcome this limitation is dynamic nuclear polarization based on photo-excited triplet (triplet-DNP) that allows the hyperpolarization at room temperature. Previous studies of triplet-DNP have been limited to dense crystalline and amorphous materials, and it remains difficult to hyperpolarize biology-relevant probes. To overcome this limitation, we introduce the chemistry of MOFs to the field of triplet-DNP. The nanoporous structure of MOFs allows the accommodation of polarizing agents as well as other guest molecules. This work paves the way towards the hyperpolarization of various probe molecules at room temperature for imaging applications.

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Summary Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins.

Resolving the discrepancy between theoretical and experimental polarization of hyperpolarized 129Xe using numerical simulations and in situ optical spectroscopy

For emerging biomedical applications of hyperpolarized xenon, the ability to obtain reliably high nuclear spin polarization levels is paramount. Yet, experimental nuclear spin polarization levels of xenon are highly variable and, more than often than not, well below what theory predicts. Despite rigorous and well-studied theoretical models for hyperpolarization and continuous-flow spin-exchange optical pumping (SEOP), there remains a substantial discrepancy between the theoretical and experimental polarization of 129Xe; inexplicably, seemingly similar experimental parameters can yield very different polarization values. In this paper, the validity of the assumptions typically made about the thermodynamic state of the Rb vapor inside the optical pumping cell and the gas dynamics are investigated through finite element analysis simulations of realistic optical pumping cell models, while in situ optical and nuclear magnetic resonance spectroscopy measurements are used to validate the results of the simulations. Our results show that shorter xenon gas residence times and lower Rb vapor densities than those predicted by empirical saturated vapor pressure curves, along with incorrect SEOP parameters, are the primary cause of the discrepancy between theoretical and experimental polarization values reported in the literature.

Oblique Nuclear Quadrupole Interaction in Self-Assembled Structures Based on Semiconductor Quantum Dots

Dynamic nuclear polarization (DNP) is well recognized as being important in spintronics and quantum-information processing. DNP gives rise to high nuclear spin polarization that not only can prolong electron-spin lifetime by generating an Overhauser field (OHF), but also has fertilized the idea to explore nuclear spin qubits. In strained quantum-dot structures (QDSs), a nuclear spin is coupled to a strain field via its quadrupole moment. It has been shown that such nuclear quadrupole interaction (NQI) can be used to achieve appreciable DNP and hence electron-spin polarization. Here, we uncover magneto-optical anomalies from a series of laterally arranged $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ QDSs that arise from the NQI and DNP in these nanostructures. We find that the principal axis of NQI in symmetry-lowered QDSs significantly deviates from the growth direction, resulting in tilting of OHF with an angle exceeding 37\ifmmode^\circ\else\textdegree\fi{}. The resulting transverse component of OHF is probed with respect to the crystallographic orientations and its influence on the DNP and ensemble spin dephasing is analyzed. We show that a high-symmetry electronic confinement potential for excitons does not guarantee a high-symmetry NQI for nuclei within the same nano-object, thereby calling for correlated optimization in the symmetry of the electronic confinement potential and that of the nuclear spin bath. Our results underline the role of oblique NQI in electron-spin decoherence and depolarization, which has so far largely been overlooked. This work thus sheds light on design rules for engineering the electronic and spin landscape of QDSs for better performance of DNP desirable for applications in spintronics and quantum computation.

Amplification of Nuclear Overhauser Effect Signals by Hyperpolarization for Screening of Ligand Binding to Immobilized Target Proteins.

Immobilization of a target protein enhances the cross-relaxation rates for transfer of nuclear spin polarization but reduces the accessible target concentration. Hyperpolarization of ligand spins by dissolution dynamic nuclear polarization (D-DNP) is shown to increase sensitivity for observing the intraligand nuclear Overhauser effect (NOE). This effect, also known as the transferred NOE (trNOE), can be used for detection of binding and for obtaining binding-related structural information. The measurement of hyperpolarized trNOE signals is demonstrated for the ligand 4'-hydroxyazobenzene-2-carboxylic acid interacting with avidin protein immobilized on polystyrene beads. In a sample containing 63.5 μM ligands and 0.83 μM accessible protein binding sites, the signal enhancement provided by D-DNP leads to single-scan detection of the NOE buildup, despite that this signal peaks at only 2% of the total ligand signal. These buildup curves allow the confirmation of binding through a change in the sign of the NOE and the quantitative determination of cross-relaxation rates. The combination of the D-DNP technique and protein immobilization may facilitate the identification of intraligand NOEs in ligand screening for drug discovery. The same method may be applied to in vivo characterization of ligand interactions with cell surface proteins.

Dynamic Nuclear Spin Polarization Induced by the Edelstein Effect at Bi(111) Surfaces.

Nuclear spin polarization induced by hyperfine interaction and mainly the Edelstein effect due to strong spin-orbit interaction, is investigated by quantum transport in Bi(111) thin film samples. The Bi(111) films are deposited on mica by van der Waals epitaxial growth. The Bi(111) films show micrometer-sized triangular islands with 0.39nm step height, corresponding to the Bi(111) bilayer height. At low temperatures a high current density is applied to generate a nonequilibrium carrier spin polarization by mainly the Edelstein effect at the Bi(111) surface, which then induces dynamic nuclear polarization by hyperfine interaction. Comparative quantum magnetotransport antilocalization measurements indicate a suppression of antilocalization by the in-plane Overhauser field from the nuclear polarization and allow a quantification of the Overhauser field. Hence nuclear polarization was both achieved and quantified by a purely electronic transport-based approach.

SABRE and PHIP pumped RASER and the route to chaos

In a RASER (Radio-frequency Amplification by Stimulated Emission of Radiation), the fast relaxing electromagnetic modes of an LC resonator are enslaved by the slow nuclear spin motion, whose coherence decays with the transverse relaxation rate γm=1/T2∗. Such a system obeys the slaving principle, mathematically identical with the adiabatic elimination procedure, leading to multi-mode RASER equations. If the pumping rate of nuclear spin polarization Γ>>γm, a second adiabatic elimination process applies and the spectral properties of the RASER can be predicted. The resulting model is similar to the model of two non-linear coupled oscillators and predicts the observed RASER phenomena, including frequency combs and mode collapse. If the second adiabatic elimination is not applicable, mode collapse is completely absent and successive period doubling processes and chaos occur at very high population inversions. We compare these theoretical predictions with experimental results from a PHIP (Para-Hydrogen Induced Polarization) pumped 1H RASER. Moreover, in SABRE (Signal Amplification By Reversible Exchange) pumped 1H experiments, RASER revivals are observed long after the parahydrogen pumping source has been switched off. All these findings shed light onto the links between NMR spectroscopy, RASER physics, synergetics and chaos theory. Several new applications are envisioned in the fields of quantum sensor technology, structure investigation or magnetic resonance imaging (MRI).

Parahydrogen-Induced Hyperpolarization of Gases.

Imaging of gases is a major challenge for any modality including MRI. NMR and MRI signals are directly proportional to the nuclear spin density and the degree of alignment of nuclear spins with applied static magnetic field, which is called nuclear spin polarization. The level of nuclear spin polarization is typically very low, i.e., one hundred thousandth of the potential maximum at 1.5 T and a physiologically relevant temperature. As a result, MRI typically focusses on imaging highly concentrated tissue water. Hyperpolarization methods transiently increase nuclear spin polarizations up to unity, yielding corresponding gains in MRI signal level of several orders of magnitude that enable the 3D imaging of dilute biomolecules including gases. Parahydrogen-induced polarization is a fast, highly scalable, and low-cost hyperpolarization technique. The focus of this Minireview is to highlight selected advances in the field of parahydrogen-induced polarization for the production of hyperpolarized compounds, which can be potentially employed as inhalable contrast agents.