arXiv subjects

Ethan Q. Williams

Publications and source records attributed to Ethan Q. Williams.

3 recordsLinked to original sources

arXiv2026

Spectral diffusion of phosphorus donors in silicon at high magnetic field

We study the central spin physics of a phosphorus donor electron in silicon interacting with a silicon-29 bath at high magnetic field (8.59 T). We find that the spectral diffusion time is shorter and exhibits a larger anisotropy with respect to crystal orientation in the magnetic field than in previous measurements at 0.35 T. The increased anisotropy suggests a modification of the hyperfine interactions at high field. The 1.2 THz cyclotron energy is a significant fraction of the 10.8 THz Rydberg energy of the bound donor, which can result in a non-trivial magnetic perturbation of the hydrogenic donor wavefunction. Low-power, above-bandgap optical excitation is seen to increase the spectral diffusion time, recovering the low-field spectral diffusion time at most crystal orientations. Understanding such perturbations to the spatial wavefunction of donor electron spins could be key to engineering their high-fidelity control.

quant-ph↗

arXiv2024

Characterizing the magnetic noise power spectrum of dark spins in diamond

Coherence times of spin qubits in solid-state platforms are often limited by the presence of a spin bath. While some properties of these typically dark bath spins can be indirectly characterized via the central qubit, it is important to characterize their properties by direct measurement. Here we use pulsed electron paramagnetic resonance (pEPR) based Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling to measure the magnetic noise power spectra for ensembles of P1 (substitutional nitrogen) centers in diamond that typically form the bath for NV (nitrogen-vacancy) centers. The experiments on the P1 centers were performed on a low [N] CVD (chemical vapor deposition) sample and a high [N] HPHT (high-temperature, high-pressure) sample at 89 mT. We characterize the NV centers of the latter sample using the same 2.5 GHz pEPR spectrometer. All power spectra show two distinct features, a broad component that is observed to scale as approximately $1/ω^{0.7-1.0}$, and a prominent peak at the $^{13}$C Larmor frequency. The behavior of the broad component is consistent with an inhomogeneous distribution of Lorentzian spectra due to clustering of P1 centers, which has recently been shown to be prevalent in HPHT diamond. It is unknown if such clustering occurs in CVD diamond. We develop techniques utilizing harmonics of the CPMG filter function to improve characterization of high-frequency signals, which we demonstrate on the $^{13}$C nuclear Larmor frequency. At 190 mT this is 2.04 MHz, 5.7 times higher than the CPMG modulation frequency ($<357$ kHz, hardware-limited). We assess the robustness of our methods in the presence of finite pulse widths and flip angle errors. Understanding the interactions of dark spins will inform methods of diamond fabrication for quantum technology. These techniques are applicable to ac magnetometry for nanoscale nuclear magnetic resonance and chemical sensing.

quant-ph↗

arXiv2022

Large Room Temperature Bulk DNP of $^{13}$C via P1 Centers in Diamond

We use microwave-induced dynamic nuclear polarization (DNP) of the substitutional nitrogen defects (P1 centers) in diamond to hyperpolarize bulk $^{13}$C nuclei in both single crystal and powder samples at room temperature at 3.34 T. The large ($>100$-fold) enhancements demonstrated correspond to a greater than 10,000 fold improvement in terms of signal averaging of the 1\% abundant $^{13}$C spins. The DNP was performed using low-power solid state sources under static (non-spinning) conditions. The DNP spectrum (DNP enhancement as a function of microwave frequency) of diamond powder shows features that broadly correlate with the EPR spectrum. A well-defined negative Overhauser peak and two solid effect peaks are observed for the central ($m_I=0$) manifold of the $^{14}$N spins. Previous low temperature measurements in diamond had measured a positive Overhauser enhancement in this manifold. Frequency-chirped millimeter-wave excitation of the electron spins is seen to significantly improve the enhancements for the two outer nuclear spin manifolds ($m_I = \pm 1$) and to blur some of the sharper features associated with the central manifolds. The outer lines are best fit using a combination of the cross effect and a truncated cross effect -- which is known to mimic features of an Overhauser effect. Similar features are also observed in experiments on single crystal samples. The observation of all of these mechanisms in a single material system under the same experimental conditions is likely due to the significant heterogeneity of the high pressure, high temperature (HPHT) type Ib diamond samples used. Large room temperature DNP enhancements at fields above a few Tesla enable spectroscopic studies with better chemical shift resolution under ambient conditions.

cond-mat.mtrl-sci↗