arXiv subjects

Guinevere Mathies

Publications and source records attributed to Guinevere Mathies.

4 recordsLinked to original sources

arXiv2026

Quantum optimal control of steady orbits

Periodically driven dissipative systems can settle into steady orbits - fixed loops on their dynamical manifolds. In quantum mechanics, steady orbits occur in cooling engines (used to initialise quantum devices), coherent oscillators (such as lasers and masers), precision metrology devices (atomic clocks, optical and spin magnetometers), and magnetic resonance (steady state free precession, dynamic nuclear polarisation). Steady orbits and stroboscopic steady states are a promising target for quantum optimal control, but the numerical complexity is prohibitive: the infinite loop defeats gradient ascent pulse engineering (GRAPE) which relies on explicit numerical propagation in the time domain. Here we propose an efficient quantum control strategy for stroboscopic steady states and limit cycles that are approached asymptotically when a control sequence is repeated infinitely many times. The formalism is different from Floquet-Lindblad state engineering and effective Hamiltonian theories: it finds control sequences that drive a dissipative quantum system towards a steady orbit passing through user-specified waypoints. The software implementation (same numerical complexity scaling as GRAPE) is done for the Spinach library.

quant-ph↗

arXiv2025

Simulation of pulsed dynamic nuclear polarization in the steady state

In pulsed dynamic nuclear polarization (DNP), enhancement of the polarization of bulk nuclei requires the repeated application of a microwave pulse sequence. So far, analysis of a one-time transfer of electron spin polarization to a dipolar-coupled nuclear spin has guided the design of DNP pulse sequences. This has obvious shortcomings, such as an inability to predict the optimal repetition time. In an actual pulsed DNP experiment, a balance is reached between the polarization arriving from the unpaired electrons and nuclear relaxation. In this article, we explore three algorithms to compute this stroboscopic steady state: (1) explicit time evolution by propagator squaring, (2) generation of an effective propagator using the matrix logarithm, and (3) direct calculation of the steady state with the Newton-Raphson method. Algorithm (2) is numerically unstable for this purpose. Algorithm (1) and (3) are both stable; algorithm (3) is the most efficient. We compare the steady-state simulations to existing experimental results at 0.34 T and 1.2 T and to the first experimental observation of X-inverse-X (XiX) DNP at 3.4 T: agreement is good, and improves further when electron-proton distance and electron Rabi frequency distributions are accounted for. We demonstrate that the trajectory of the spin system during one-time application of a microwave pulse sequence differs from the steady orbit. This has implications for DNP pulse sequence design.

physics.chem-ph↗

arXiv2024

Dynamic Nuclear Polarization Mechanisms using TEMPOL and trityl OX063 radicals at 1 T and 77 K

A sensitivity increase of two orders of magnitude in proton (1H) and carbon (13C) spins via dynamic nuclear polarization (DNP) has been accomplished recently using a compact benchtop DNP polarizer operating at 1 T and 77 K. However the DNP mechanisms at play at such low magnetic field and high operating temperature are still not elucidated. A deeper understanding of the dominant polarization transfer mechanisms between electrons and 1H and 13C spins at these unconventional benchtop conditions is therefore required if one wants to devise strategies to boost sensitivity further. In this study, we found that DNP is generally dominated by solid effect for narrow electron paramagnetic resonance (EPR) line radicals (15 mM trityl OX063) and cross effect for broad EPR line radicals (50 mM TEMPOL). For both radicals, the dominant DNP mechanisms were investigated varying the microwave frequency and measuring the 1H and 13C DNP enhancement factors to obtain 1H and 13C DNP spectra. The impact of varying the microwave power on the 1H DNP buildup times and the 1H nuclear spin relaxation times were important as well to distinguish between solid effect and cross effect DNP. Finally, time-resolved electron saturation simulations under continuous microwave irradiation could replicate the experimental 1H and 13C DNP spectra at 1 T and 77 K for both radicals considering their electron relaxation properties. Only for trityl OX063, the 13C DNP spectra showed additional DNP maxima compared to the simulations. This has been attributed to methyl rotor induced 1H-13C heteronuclear cross relaxation in [1-13C] acetate present at 1 T and 77 K.

physics.chem-ph↗

arXiv2023

Simulation and design of shaped pulses beyond the piecewise-constant approximation

Response functions of resonant circuits create ringing artefacts if their input changes rapidly. When physical limits of electromagnetic spectroscopies are explored, this creates two types of problems. Firstly, simulation: the system must be propagated accurately through every response transient, this may be computationally expensive. Secondly, optimal control: circuit response must be taken into account; it may be advantageous to design pulses that are resilient to such distortions. At the root of both problems is the popular piecewise-constant approximation for control sequences in the rotating frame; in magnetic resonance it has persisted since the earliest days and has become entrenched in the commercially available hardware. In this paper, we report an implementation and benchmarks of recent Lie-group methods that can efficiently simulate and optimise smooth control sequences.

eess.SY↗