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John W. Blanchard

Publications and source records attributed to John W. Blanchard.

At least 19 recordsLinked to original sources

Principles and Techniques for Agentic Research in Experimental Physics

The use of artificial intelligence (AI) in physics research is increasingly common, yet few guidelines exist for its effective use. Starting with a principles-based approach, we outline a set of ten requirements for collaborative AI agents in experimental physics, spanning scientific norms, effective agent use, and domain expertise. We take a human-centered approach in which agents act as collaborators that amplify rather than replace the expertise of a human researcher (user) who directs the work and is responsible for its results. We then present a framework designed to enforce these requirements, detailing methods for improving off-the-shelf behavior of large language models (LLMs) with custom scaffolding and describing general-purpose scientific workflows and skills. In particular, we highlight the power of closed-loop campaigns where agents work iteratively toward a well-defined goal. We demonstrate the potential of this mode of research through six case studies in experimental physics, covering device design, hardware control, neural network optimization, physical model identification, and data analysis. All six case studies result in useful work products that add utility to our research, including a neural network architecture and solid-state quantum defect photodynamic model that substantially outperform our published work; a microwave waveguide design that extends a published design concept to a fabrication-ready device meeting our experimental constraints; a quantum control pulse sequence candidate that improves simulated performance by more than an order of magnitude; commissioning of an NMR spectrometer to $\sim$1~Hz frequency precision on live hardware; and finally a data-fitting pipeline that achieves, to our knowledge, the first reconstruction of the Ising microstate of an artificial spin-ice array from diffraction-limited magnetic imaging.

physics.comp-ph↗

Hierarchical Maximum Likelihood Estimation for Time-Resolved NMR Data

Metabolic monitoring and reaction rate estimation using hyperpolarized NMR technology requires accurate quantitative analysis of multidimensional data scenarios. Currently, this analysis is often performed in a two-stage procedure, which is prone to errors in uncertainty propagation and estimation. We propose an approach derived from a Bayesian hierarchical model that intrinsically propagates uncertainties and operates on the full data to maximize the precision at minimal uncertainty. In an analytic treatment, we reduce the estimation procedure to a least-squares optimization problem which can be understood as an extension of the Variable Projection (VarPro) approach for data scenarios with two predictors. We investigate the method's efficacy in two experiments with hyperpolarized metabolites recorded with conventional high-field NMR devices and a micronscale NMR setup using Nitrogen-Vacancy centers in diamond for detection, respectively. In both examples, the new approach improves estimates compared to Fourier methods and proves operational advantages over a two-stage procedure employing VarPro. While the approach presented is motivated by NMR analysis, it is straightforwardly applicable to further estimation scenarios with similar data structure, such as time-resolved photospectroscopy.

q-bio.QM↗

Eigenframe Synchronization in Disordered Driven Quantum Ensembles

Periodic driving underlies many forms of quantum control, including spin-based quantum sensing. However, in an ensemble sensor, one waveform must act on spins with different detunings, drive amplitudes, hyperfine environments, and local fields. Existing robust-control approaches to such disorder are usually framed in terms of coherence, effective Hamiltonians, filter functions, or refocusing. Here we identify a complementary geometric requirement for collective ensemble Floquet control: disorder realizations must share a common Floquet eigenframe. When this condition is met, initialization, protection, signal coupling, and readout are defined in one dressed basis, so the ensemble responds as a collective Floquet sensor rather than as an average over inequivalent driven members. We make this condition measurable with an eigenvector-based synchronization order parameter and a complementary fragmentation metric that quantify the alignment and spread of Floquet quantization axes across the ensemble. As one realization of this framework, we use a continuous counterdiabatic Floquet drive to derive synchronization criteria and predict resonance-governed breakdown at the first two low-order commensurabilities between the engineered Floquet gap and the drive modulation, with detuning and amplitude disorder producing distinct breakdown channels. Experiments on a nitrogen-vacancy (NV) ensemble in diamond verify the synchronized regime through long-lived collective oscillations, a two-dimensional disorder-robustness map, and breakdown resonances that shift with the modulation rate. Finally, we demonstrate a harmonic-free continuous-drive AC magnetometry protocol whose collective single-tone response is enabled by the synchronized Floquet eigenframe. These results establish Floquet eigenframe synchronization as a measurable condition for disorder-resilient collective control and quantum sensing.

quant-ph↗

Noise Suppression via Pulsed All-Optical Magnetometry with Nitrogen-Vacancy Ensembles

All-optical (AO) microwave-free magnetometry using nitrogen-vacancy (NV) centers in diamond simplifies experimental design and broadens sample compatibility. While continuous-wave (cw) detection of AO photoluminescence (PL) changes is commonly employed, its performance is susceptible to systematic fluctuations such as optical intensity noise. To address these challenges, we introduce a pulsed AO protocol that employs two PL measurements within an optical pulse to suppress common-mode noise. At near-zero magnetic field, we experimentally demonstrate that the pulsed AO protocol resolves AO-PL contrast features arising from NV-NV cross-relaxation, achieving up to 10$\times$ improvement in the low-frequency noise floor compared to conventional cw AO techniques. We further investigate the dependence of AO-PL contrast on PL readout timing and the dark time duration $τ$ between optical pulses, with the optimal $τ$ varying based on NV concentrations. These findings provide insights into optimizing NV-diamond samples for effective AO operation across diverse applications.

quant-ph↗

Direct Measurement of the Singlet Lifetime and Photoexcitation Behavior of the Boron Vacancy Center in Hexagonal Boron Nitride

Optically active spin defects in van der Waals (vdW) materials are a promising platform for quantum sensing, potentially enabling shorter standoff distances than defects in diamond and thus improved measurement signal-to-noise ratio (SNR) and spatial resolution. The most studied such defect is the negatively charged boron vacancy center ($V^{-}_{B}$) in hexagonal boron nitride (hBN), yet many of its electronic and spin transition rates and branching ratios remain unknown. Here, we use time-resolved photoluminescence (PL) measurements with a nanosecond rise-time 515 nm laser to directly measure the singlet state lifetime of a $V^{-}_{B}$ ensemble in neutron-irradiated, sub-micron flakes of hBN. We perform this measurement on 16 flakes at room temperature and obtain an average lifetime of 15(3) ns. Additionally, we probe the PL dynamics of thermal and optically polarized electronic spin distributions of the $V^{-}_{B}$ ensemble in a sub-micron hBN flake, and fit our results to a 9-level model to extract electronic transition rates. Lastly, we present PL measurements that potentially indicate optically-induced conversion of $V^{-}_{B}$ to another electronic state, or possibly the neutral charge state ($V^{0}_{B}$), in neutron-irradiated hBN flakes of size $>$ 1 $μ$m.

quant-ph↗

All-Optical Photoluminescence Response of Nitrogen-Vacancy Ensembles in Diamond at Low Magnetic Fields

All-optical (AO), microwave-free magnetometry using nitrogen-vacancy (NV) centers in diamond is attractive due to its broad sample compatibility and reduced experimental complexity. In this work, we investigate room-temperature AO photoluminescence (PL) at low magnetic fields (<2 mT) using diamonds with NV ensembles at ppm concentrations. Measured AO-PL contrast features as a function of applied magnetic field magnitude and direction are correlated with near-degenerate NV electronic spin and hyperfine transitions from different NV orientations within the diamond host. Reasonable agreement is found between low-field AO-PL measurements and model-based simulations of the effects of resonant dipolar interactions between NV centers. Maximum observed AO-PL contrast depends on both NV concentration and laser illumination intensity at 532 nm. These results imply different optimal conditions for low-field AO NV sensing compared to conventional optically detected magnetic resonance (ODMR) techniques, suggesting new research and application opportunities using AO measurements with lower system complexity, size, weight, and power.

quant-ph↗

AC magnetometry in the strong drive regime with NV centers in diamond

Magnetic response measurements in the presence of AC drive fields provide critical insight into the properties of magnetic and conductive materials, such as phase transitions in two-dimensional van der Waals magnets, the heating efficiency of magnetic nanoparticles in biological environments, and the integrity of metals in eddy current testing. Nitrogen-vacancy (NV) centers in diamond are a commonly-used platform for such studies, due to their high spatial resolution and sensitivity, but are typically limited to weak-drive conditions, i.e., AC drive fields well below the NV microwave (MW) pulse Rabi strength. Once the AC drive field grows comparable to or larger than the Rabi strength, the induced MW pulse detuning suppresses NV sensitivity to the out-of-phase magnetic response, which encodes dissipation and conductivity in materials of interest. Here, we introduce a phase modulation protocol that cancels MW pulse detuning to leading order, and extends NV AC magnetometry into the strong drive field regime. The protocol, termed SIPHT (Signal Isolation through PHase Tuning), is experimentally demonstrated using an NV ensemble. By directly comparing SIPHT to the conventional Hahn echo AC sensing protocol, we quantify the preservation of NV magnetometry contrast for an out-of-phase signal. We further showcase SIPHT by detecting eddy current-induced magnetic fields from Cu, Al, and Ti samples, with the measured response field phase delays reflecting their distinct conductivities. SIPHT extends NV AC magnetometry to regimes inaccessible to standard dynamical decoupling measurement protocols, unlocking novel utility, e.g., in the study of magnetic hyperthermia and nondestructive testing of conductors.

quant-ph↗

Prospects for Ultralow-Mass Nuclear Magnetic Resonance using Spin Defects in Hexagonal Boron Nitride

Optically active quantum defects in solids, such as the nitrogen vacancy (NV) center in diamond, are a leading modality for micron-scale and nanoscale (ultralow-mass) nuclear magnetic resonance (NMR) spectroscopy and imaging under ambient conditions. However, the spin and optical properties of NV centers degrade when closer than about 10 nm from the diamond surface, limiting NMR sensitivity as well as spectral and spatial resolution. Here we outline efforts to develop an alternative nanoscale NMR sensor using the negatively charged boron vacancy ($V_B^-$) in hexagonal boron nitride (hBN). As a van der Waals material, hBN's surface is free from dangling bonds and other sources of paramagnetic noise that degrade the performance of near surface NVs, allowing stable $V_B^-$ defects to exist $\sim1\,$nm from the material surface. We discuss the properties of boron vacancies as they apply to narrowband (AC) magnetic field sensing and outline experimental designs optimized for this system. We propose measurement protocols for $V_B^-$ NMR for both statistically and uniformly polarized samples at the nano- and micron-scales, including relevant pulse sequences, sensitivity calculations, and sample confinement strategies; and compare the expected performance to NV-NMR. We estimate back-action effects between the $V_B^-$ electronic spins and the sample nuclear spins at the nanoscale; and account for unconventional diffusion dynamics in the flow-restricted nanoscale regime, calculating its effects on the expected $V_B^-$ NMR signal. Lastly, we identify potential sample targets and operational regimes best suited for both nanoscale and micron-scale $V_B^-$ NMR.

quant-ph↗

Zero- to Ultralow-field Nuclear Magnetic Resonance

Zero and ultralow-field nuclear magnetic resonance (ZULF NMR) is an NMR modality where experiments are performed in fields at which spin-spin interactions within molecules and materials are stronger than Zeeman interactions. This typically occurs at external fields of microtesla strength or below, considerably smaller than Earth's field. In ZULF NMR, the measurement of spin-spin couplings and spin relaxation rates provides a nondestructive means for identifying chemicals and chemical fragments, and for conducting sample or process analyses. The absence of the symmetry imposed by a strong external magnetic field enables experiments that exploit terms in the nuclear spin Hamiltonian that are suppressed in high-field NMR, which in turn opens up new capabilities in a broad range of fields, from the search for dark matter to the preparation of hyperpolarized contrast agents for clinical imaging. Furthermore, as in ZULF NMR the Larmor frequencies are typically in the audio band, the nuclear spins can be manipulated with d.c. magnetic field pulses, and highly sensitive magnetometers are used for detection. In contrast to high-field NMR, the low-frequency signals readily pass through conductive materials such as metals, and heterogeneous samples do not lead to resonance line broadening, meaning that high-resolution spectroscopy is possible. Notable practical advantages of ZULF NMR spectroscopy are the low cost and relative simplicity and portability of the spectrometer system. In recent years ZULF NMR has become more accessible, thanks to improvements in magnetometer sensitivity and their commercial availability, and the development of hyperpolarization methods that provide a simple means to boost signal strengths by several orders of magnitude. These topics are reviewed and a perspective on potential future avenues of ZULF-NMR research is presented.

physics.chem-ph↗

Quantum Diamond Microscope for Narrowband Magnetic Imaging with High Spatial and Spectral Resolution

The quantum diamond microscope (QDM) is a recently developed technology for near-field imaging of magnetic fields with micron-scale spatial resolution. In the present work, we integrate a QDM with a narrowband measurement protocol and a lock-in camera; and demonstrate imaging of radiofrequency (RF) magnetic field patterns produced by microcoils, with spectral resolution $\approx1$\,Hz. This RF-QDM provides multi-frequency imaging with a central detection frequency that is easily tunable over the MHz-scale, allowing spatial discrimination of both crowded spectral peaks and spectrally well-separated signals. The present instrument has spatial resolution $\approx2\,\mathrm{μm}$, field-of-view $\approx300\times300\,\mathrm{μm^2}$, and per-pixel sensitivity to narrowband fields $\sim{1}\,$nT$\cdot$Hz$^{-1/2}$. Spatial noise can be reduced to the picotesla scale by signal averaging and/or spatial binning. The RF-QDM enables simultaneous imaging of the amplitude, frequency, and phase of narrowband magnetic field patterns at the micron-scale, with potential applications in real-space NMR imaging, AC susceptibility mapping, impedance tomography, analysis of electronic circuits, and spatial eddy-current-based inspection.

physics.optics↗

Nutation-Based Longitudinal Sensing Protocols for High-Field NMR With Nitrogen-Vacancy Centers in Diamond

Nitrogen vacancy (NV) centers in diamond enable nuclear magnetic resonance (NMR) spectroscopy of samples at the nano- and micron scales. However, at typical tesla-scale NMR magnetic field strengths, NV-NMR protocols become difficult to implement due to the challenge of driving fast NV pulse sequences sensitive to nuclear Larmor frequencies above a few megahertz. We perform simulations and theoretical analysis of the experimental viability of NV-NMR at tesla-scale magnetic fields using a new measurement protocol called DRACAERIS (Double Rewound ACquisition Amplitude Encoded Radio Induced Signal). DRACAERIS detects the NMR sample's longitudinal magnetization at a much lower driven Rabi frequency, more suitable technically for NV detection. We discuss how pulse errors, finite pulse lengths, and nuclear spin-spin couplings affect the resulting NMR spectra. We find that DRACAERIS is less susceptible to pulse imperfections and off-resonance effects than previous protocols for longitudinal magnetization detection. We also identify reasonable parameters for experimental implementation.

quant-ph↗

Zero-Field J-spectroscopy of Quadrupolar Nuclei

Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) is a powerful version of NMR that allows studying molecules and their transformations in the regime dominated by intrinsic spin-spin interactions. While spin dynamics at zero magnetic field can be probed indirectly - via shuttling a sample that underwent evolution at zero field to a high-field NMR spectrometer for detection - J-spectra can also be measured directly at zero field by using non-inductive sensors, for example, atomic magnetometers. To date, no zero-field J-spectra of molecules featuring the coupling to quadrupolar nuclei were reported. Here we show that zero-field J-spectra can be collected from molecules containing quadrupolar nuclei with I = 1 and demonstrate this for solutions containing various isotopologues of ammonium cations, namely, 14NH4+ and 15NDxH(4-x)+ (where x = 0, 1, 2, or 3). Lower ZULF NMR signals are observed for molecules containing larger numbers of deuterons compared to protons; this is attributed to less overall magnetization and not to the scalar relaxation of the second kind. Values for the 15N-1H and 14N-1H J-couplings of -73.416(3) Hz, 52.395(2) Hz, respectively, are extracted from the ZULF NMR spectra. Precision measurement of the J(15NH)/J(14NH) ratio resulted in the value within a range of 1.4009-1.4013; this is statistically different from {γ_{15N}}/{γ_{14N}}=1.4027 reported in the literature indicating the presence of the primary isotope effect. We analyze the energy structure for the studied molecular cations in detail and demonstrate that spectral line positions depend dramatically both on the sign of each J-coupling and on the magnetic pulse length.

physics.chem-ph↗

Robust Parahydrogen-Induced Polarization at High Concentrations

Parahydrogen-Induced Polarization (PHIP) is a potent technique for generating target molecules with high nuclear spin polarization. The PHIP process involves a chemical reaction between parahydrogen and a target molecule, followed by the transformation of nuclear singlet spin order into magnetization of a designated nucleus through magnetic field manipulations. Although the singlet-to-magnetization polarization transfer process works effectively at moderate concentrations, it is observed to become much less efficient at high molar polarization, defined as the product of polarization and concentration. This strong dependence on the molar polarization is attributed to interference from the field produced by the sample's magnetization during polarization transfer, which leads to complex dynamics and can severely impact the scalability of the technique. We address this challenge with a pulse sequence that negates the influence of the distant dipolar field, while simultaneously achieving singlet-to-magnetization polarization transfer to the desired target spins, free from restrictions on the molar polarization.

physics.chem-ph↗

Quantum Diamond Microscope for Dynamic Imaging of Magnetic Fields

Wide-field imaging of magnetic signals using ensembles of nitrogen-vacancy (NV) centers in diamond has garnered increasing interest due to its combination of micron-scale resolution, millimeter-scale field of view, and compatibility with diverse samples from across the physical and life sciences. Recently, wide-field NV magnetic imaging based on the Ramsey protocol has achieved uniform and enhanced sensitivity compared to conventional measurements. Here, we integrate the Ramsey-based protocol with spin-bath driving to extend the NV spin dephasing time and improve magnetic sensitivity. We also employ a high-speed camera to enable dynamic wide-field magnetic imaging. We benchmark the utility of this quantum diamond microscope (QDM) by imaging magnetic fields produced from a fabricated wire phantom. Over a $270\times270 \hspace{0.08333em} μ\mathrm{m}$$^2$ field of view, a median per-pixel magnetic sensitivity of $4.1(1)\hspace{0.08333em}\mathrm{nT}$$/\sqrt{\mathrm{Hz}}$ is realized with a spatial resolution $\lesssim\hspace{0.08333em}10\hspace{0.08333em}μ\mathrm{m}$ and sub-millisecond temporal resolution. Importantly, the spatial magnetic noise floor can be reduced to the picotesla scale by time-averaging and signal modulation, which enables imaging of a magnetic-field pattern with a peak-to-peak amplitude difference of about $300\hspace{0.08333em}\mathrm{pT}$. Finally, we discuss potential new applications of this dynamic QDM in studying biomineralization and electrically-active cells.

quant-ph↗

Possible Applications of Dissolution Dynamic Nuclear Polarization in Conjunction with Zero- to Ultralow-Field Nuclear Magnetic Resonance

The combination of a powerful and broadly applicable nuclear hyperpolarization technique with emerging (near-)zero-field modalities offer novel opportunities in a broad range of nuclear magnetic resonance spectroscopy and imaging applications, including biomedical diagnostics, monitoring catalytic reactions within metal reactors and many others. These are discussed along with a roadmap for future developments.

physics.chem-ph↗

Using parity-nonconserving spin-spin coupling to measure the Tl nuclear anapole moment in a TlF molecular beam

An experiment utilizing a TlF molecular beam is being developed by the CeNTREX collaboration to search for hadronic interactions that violate both time-reversal (T) and parity (P) invariance. Here we propose to use the same beam to look for a T-invariance conserving but P-nonconserving (PNC) effect induced by the anapole moment of the Tl nucleus, via a vector coupling of the two nuclear spins in TlF. To measure the nuclear anapole moment, the dc electric and magnetic fields in CeNTREX are replaced by rf fields resonant with a nuclear spin flip transition. We adapt the relativistic coupled cluster method in a combination with relativistic density functional theory for the calculation of the molecular PNC spin-spin vector coupling constant that links the experimental signal with the anapole moment. The value of the P-conserving spin-spin coupling constant calculated within the same approach is found to be in good agreement with available experimental data.

physics.atom-ph↗

Radio-Frequency Sweeps at μT Fields for Parahydrogen-Induced Polarization of Biomolecules

Magnetic resonance imaging of $^{13}$C-labeled metabolites enhanced by parahydrogen-induced polarization (PHIP) can enable real-time monitoring of processes within the body. We introduce a robust, easily implementable technique for transferring parahydrogen-derived singlet order into 13C magnetization using adiabatic radio-frequency sweeps at $μ$T fields. We experimentally demonstrate the applicability of this technique to several molecules, including some molecules relevant for metabolic imaging, where we show significant improvements in the achievable polarization, in some cases reaching above 60%. Furthermore, we introduce a site-selective deuteration scheme, where deuterium is included in the coupling network of a pyruvate ester to enhance the efficiency of the polarization transfer. These improvements are enabled by the fact that the transfer protocol avoids relaxation induced by strongly coupled quadrupolar nuclei.

physics.chem-ph↗

Metabolic Reactions Studied by Zero- and Low-Field Nuclear Magnetic Resonance

State-of-the-art magnetic resonance imaging uses hyperpolarized molecules to track metabolism in vivo, but large superconducting magnets are required, and the strong magnetic fields largely preclude measurement in the presence of conductive materials and magnify problems of magnetic susceptibility inhomogeneity. Operating at zero and low field circumvents these limitations, but until now has not been possible due to limited sensitivity. We show that zero- and low-field nuclear magnetic resonance can be used for probing two important metabolic reactions: the conversion of hyperpolarized fumarate to malate and pyruvate to lactate. This work paves the way to a heretofore unexplored class of biomedical imaging applications.

physics.chem-ph↗