arXiv subjects

Gonzalo Reina Rivero

Publications and source records attributed to Gonzalo Reina Rivero.

6 recordsLinked to original sources

arXiv2026

The Transfer Tensor Method: an Analytical Study Case

The transfer tensor method is a versatile tool for analyzing and propagating general open quantum systems. It captures in a compact manner all memory effects in a non-Markovian system through a straightforward transformation of a set of dynamical maps. Transfer tensors provide the exact convolutional propagator associated with a given time discretization over the past evolution of an open quantum system. Here we show that, for any finite time discretization, the memory kernel of the Nakajima Zwanzig equation deviates from the exact transfer tensors, although both converge in the continuous-time limit, as expected. We examine this behaviour in the context of an analytically solvable model: a two level atom resonant with a lossy cavity in the Jaynes Cummings limit. The atomic dynamics separate into two decoupled degrees of freedom -- the coherence and the population inversion. We derive exact expressions for the dynamical map, the transfer tensors and the memory kernel governing the coherence, and we relate them to their counterparts for the population inversion. As a function of the ratio between the cavity loss rate and the atom-cavity coupling strength, we identify regions of enhanced non-Markovianity in which the system can be described as fully Markovian for certain time-step choices.

quant-ph↗

arXiv2026

Fast Generation of Metrologically Relevant Fock State Mixtures

We propose a fast laser pulse sequence for the generation of non-thermal Fock state mixtures of the motion of a trapped ion, targeted at displacement metrology beyond the standard quantum limit. Using a polaron-frame description of the ion-laser interaction, we identify a resonant operating point-zero detuning and a Rabi frequency matching the trap frequency-at which selective population trapping survives strong driving, enabling preparation speeds beyond the weak-driving limit of previous protocols without requiring ground-state cooling. We trace the residual infidelity at large Lamb-Dicke parameter $η$ to a single coherent process, the counter-rotating blue-sideband term neglected in the rotating-wave approximation, and show that it is suppressed by two routine calibrations: a percent-level refocusing of the pulse duration and a small compensating Bloch-Siegert detuning. Numerical simulations of the full sequence show that this refinement keeps the preparation error at or below the $10\%$ level up to $η\approx0.5$ and restores the displacement-sensing Fisher information that the uncorrected protocol loses at strong coupling, recovering up to 9 dB relative to the nominal sequence.

quant-ph↗

arXiv2026

Dark Polaron Theory for High Intensity Laser Cooling

Conventional laser control schemes for cooling and gate operation of trapped ions are limited to the regime of weak laser intensities and small Lamb-Dicke parameters. To overcome this limitation, we present the concept of dark polarons: spatially extended states of pseudospin polarization that are fully decoupled from a lambda laser configuration. In this picture, all high-order Lamb-Dicke terms collapse into a single linear coupling independent of laser intensity. We apply it to definitively elucidate the reasons behind cooling rate limitations observed in recent experimental implementations of electromagnetically induced transparency with high-intensity lasers.

quant-ph↗

arXiv2026

Critical Sensing with Autonomous Devices: The Self-Oscillation Threshold of a Frequency-Locked NV-Centre Magnetometer

Feedback locking of a probe frequency to a spin resonance is the standard operating mode of precision quantum sensors. Here we deliberately operate such a lock outside its stable regime: a continuous-wave nitrogen-vacancy (NV) ensemble magnetometer, frequency-modulation (FM) locked to one flank of its optically detected magnetic resonance (ODMR), is driven through the flip (period-doubling) bifurcation of its discrete feedback map by raising the software loop gain $G$. Beyond a critical gain $\Gc$ the lock becomes a self-sustained oscillator whose limit cycle is generated by the loop itself. We derive the threshold condition $\Gc = 2\,\Dcal/\Dtrue$, which identifies the measurable content of the threshold: the ratio of the transduction slope of the ODMR lock-in signal at calibration time $D_{cal}$ to its value at present $D_{true}$. We present an identifiability analysis showing which physical parameters this single scalar can and cannot distinguish, characterize the estimators of $\Gc$ under realistic noise, and report measurements on our current setup: an experimental bifurcation diagram with onset at $\Gc \approx 2$ as predicted for a self-calibrated loop, sub-threshold critical fluctuations following the predicted $\sqrt{G/(2-G)}$ divergence.

quant-ph↗

arXiv2025

Towards a 384-channel magnetoencephalography system based on optically pumped magnetometers

Magnetoencephalography using optically pumped magnetometers (OPM-MEG) is gaining significant traction as a neuroimaging tool, with the potential for improved performance and practicality compared to conventional instrumentation. However, OPM-MEG has so far lagged conventional-MEG in terms of the number of independent measurements of magnetic field that can be made across the scalp (i.e. the number of channels). This is important since increasing channel count offers improvements in sensitivity, spatial resolution and coverage. Unfortunately, increasing channel count also poses significant technical and practical challenges. Here, we describe a new OPM-MEG array which exploits triaxial sensors and integrated miniaturised electronic control units to measure MEG data from up to 384 channels. We also introduce a high-speed calibration method to ensure the that the fields measured by this array are high fidelity. The system was validated using a phantom, with results showing that dipole localisation accuracy is better than 1 mm, and correlation between the measured magnetic fields and a dipole model is >0.998. We then demonstrate utility in human MEG acquisition: via measurement of visual gamma oscillations we demonstrate the improvements in sensitivity that are afforded by high channel density, and via a moviewatching paradigm we quantify improvements in spatial resolution. In sum, we show the first OPM-MEG system with a channel count larger than that of typical conventional MEG devices. This represents a significant step on the path towards OPMs becoming the sensor of choice for MEG measurement.

physics.med-ph↗

arXiv2024

Determining sensor geometry and gain in a wearable MEG system

Optically pumped magnetometers (OPMs) are compact and lightweight sensors that can measure magnetic fields generated by current flow in neuronal assemblies in the brain. Such sensors enable construction of magnetoencephalography (MEG) instrumentation, with significant advantages over conventional MEG devices including adaptability to head size, enhanced movement tolerance, lower complexity and improved data quality. However, realising the potential of OPMs depends on our ability to perform system calibration, which means finding sensor locations, orientations, and the relationship between the sensor output and magnetic field (termed sensor gain). Such calibration is complex in OPMMEG since, for example, OPM placement can change from subject to subject (unlike in conventional MEG where sensor locations or orientations are fixed). Here, we present two methods for calibration, both based on generating well-characterised magnetic fields across a sensor array. Our first device (the HALO) is a head mounted system that generates dipole like fields from a set of coils. Our second (the matrix coil (MC)) generates fields using coils embedded in the walls of a magnetically shielded room. Our results show that both methods offer an accurate means to calibrate an OPM array (e.g. sensor locations within 2 mm of the ground truth) and that the calibrations produced by the two methods agree strongly with each other. When applied to data from human MEG experiments, both methods offer improved signal to noise ratio after beamforming suggesting that they give calibration parameters closer to the ground truth than factory settings and presumed physical sensor coordinates and orientations. Both techniques are practical and easy to integrate into real world MEG applications. This advances the field significantly closer to the routine use of OPMs for MEG recording.

physics.med-ph↗