arXiv subjects

Charlie J. Patrickson

Publications and source records attributed to Charlie J. Patrickson.

5 recordsLinked to original sources

arXiv2026

A Quantum Dynamics Tutorial: Visualising Dynamical Decoupling Sequences on the Bloch Sphere

Dynamical decoupling protocols provide a versatile toolbox for robust control of quantum systems, suppressing noise that would otherwise limit their performance. This tutorial aims to provide an intuitive interpretation of these protocols. Written from an experimentalist's perspective, we use the Bloch sphere picture to visualise quantum dynamics for a range of sequences. Here each dynamical decoupling operation can be viewed as an "effective field vector", whilst the phase and population of the two-level system is encoded into a "Bloch vector". The two-level dynamics are calculated by the cross product between these two vectors, such that the quantum state represented by the Bloch vector rotates around the effective field vector. Several prominent control schemes are simulated using this picture, starting with a Rabi oscillation, followed by the Ramsey sequence and a selection of pulsed and continuous dynamical decoupling techniques. These sequences are engineered to preserve control of the quantum system for as long as possible, with their efficacy often quantified by a coherence time. We clarify the definition of the longitudinal $T_1$, transverse $T_2$ and inhomogeneous $T_2^*$ coherence times, which are used across the literature. Each simulation has an accompanying animation to illustrate the protocol's two-level dynamics on the Bloch sphere. The custom simulation and Bloch sphere plotting scripts are also provided in an open access repository to allow the reader to reproduce, modify, and explore these results. Dynamical decoupling sequences have established themselves as an invaluable tool across the entire quantum technologies remit; by providing an open framework alongside this tutorial, we aim to make these protocols more accessible to new researchers discovering this thriving field.

quant-ph↗

arXiv2025

Coherent control of nitrogen nuclear spins via the V$_B^-$-center in hexagonal boron nitride

Charged boron vacancies (V$_\text{B}^-$) in hexagonal boron nitride (hBN) have emerged as a promising platform for quantum nanoscale sensing and imaging. While these primarily involve electron spins, nuclear spins provide an additional resource for quantum operations. This work presents a comprehensive experimental and theoretical study of the properties and coherent control of the nearest-neighbor $^{15}$N nuclear spins of V$_\text{B}^-$-ensembles in isotope-enriched h$^{10}$B$^{15}$N. Multi-nuclear spin states are selectively addressed, enabled by state-specific nuclear spin transitions arising from spin-state mixing. We perform Rabi driving between selected state pairs, define elementary quantum gates, and measure longer than 10~$\mu$s nuclear Rabi coherence times. We observe a two orders of magnitude nuclear g-factor enhancement that underpins fast nuclear spin gates. Accompanying numerical simulations provide a deep insight into the underlying mechanisms. These results establish the foundations for leveraging nuclear spins in V$_\text{B}^-$ center-based quantum applications, particularly for extending coherence times and enhancing the sensitivity of 2D quantum sensing foils.

cond-mat.mtrl-sci↗

arXiv2024

Continuous drive heterodyne microwave sensing with spin qubits in hexagonal boron nitride

Quantum sensors that use solid state spin defects have emerged as effective probes of weak alternating magnetic signals. By recording the phase of a signal relative to an external clock, these devices can resolve signal frequencies to a precision orders of magnitude longer than the spin state lifetime. However, these quantum heterodyne protocols suffer from sub-optimal sensitivity, as they are currently limited to pulsed spin control techniques, which are susceptible to cumulative pulse-area errors, or single continuous drives which offer no protection of the spin coherence. Here, we present a control scheme based on a continuous microwave drive that extends spin coherence towards the effective $T_2 \approx \frac{1}{2}T_1$ limit and can resolve the frequency, amplitude and phase of GHz magnetic fields. The scheme is demonstrated using an ensemble of boron vacancies in hexagonal boron nitride, and achieves an amplitude sensitivity of $\eta \approx 3-5 \:\mathrm{\mu T \sqrt{Hz}}$ and phase sensitivity of $\eta_{\phi} \approx 0.076 \:\mathrm{rads \sqrt{Hz}}$. By repeatedly referencing the phase of a resonant signal against the coherent continuous microwave drive in a quantum heterodyne demonstration, we measure a GHz signal with a resolution $<$1 Hz over a 10 s measurement. Achieving this level of performance in a two-dimensional material platform could have broad applications, from probing nanoscale condensed matter systems to integration into heterostructures for quantum networking.

quant-ph↗

arXiv2023

High Frequency Magnetometry with an Ensemble of Spin Qubits in Hexagonal Boron Nitride

Sensors based on spin qubits in 2D crystals offer the prospect of nanoscale sensing volumes, where the close proximity of the sensor and source could provide access to otherwise inaccessible signals. For AC magnetometry, the sensitivity and frequency range is typically limited by the noise spectrum, which determines the qubit coherence time. This poses a problem for III-V materials, as the non-zero spin of the host nuclei introduces a considerable source of magnetic noise. Here, we overcome this with a sensing protocol based on phase modulated continuous concatenated dynamic decoupling, which extends the coherence time towards the $T_1$ limit at room temperature and enables tuneable narrowband AC magnetometry. We demonstrate the protocol with an ensemble of negatively charged boron vacancies in hexagonal boron nitride, detecting in-plane AC fields within $\pm 150~\mathrm{MHz}$ of the electron spin resonance, and out-of-plane fields in the range of $\sim10-150~\mathrm{MHz}$. We measure an AC magnetic field sensitivity of $\sim1~\mathrm{μT/\sqrt{Hz}}$ at $\sim2.5~\mathrm{GHz}$, for a sensor volume of $\sim0.1~\mathrm{μm^3}$, and demonstrate that the sensor can reconstruct the AC magnetic field from a wire loop antenna. This work establishes the viability of spin defects in 2D materials for high frequency magnetometry, demonstrating sensitivities that are comparable to nitrogen vacancy centres in diamond for microscopic sensing volumes, and with wide-ranging applications across science and technology.

cond-mat.mtrl-sci↗

arXiv2022

Room temperature coherent control of protected qubit in hexagonal boron nitride

Spin defects in foils of hexagonal boron nitride are an attractive platform for magnetic field imaging, since the probe can be placed in close proximity to the target. However, as a III-V material the electron spin coherence is limited by the nuclear spin environment, with spin echo coherence time of $\sim100~\mathrm{ns}$ at room temperature accessible magnetic fields. We use a strong continuous microwave drive with a modulation in order to stabilize a Rabi oscillation, extending the coherence time up to $\sim4~\mathrm{μs}$, which is close to the 10-$\mathrm{μs}$ electron spin lifetime in our sample. We then define a protected qubit basis, and show full control of the protected qubit. The coherence times of a superposition of the protected qubit can be as high as $0.8~\mathrm{μs}$. This work establishes that boron vacancies in hexagonal boron nitride can have electron spin coherence times that are competitive with typical NV-centers in small nanodiamonds under ambient conditions.

cond-mat.mtrl-sci↗