arXiv subjects

Javier Cerrillo

Publications and source records attributed to Javier Cerrillo.

At least 19 recordsLinked to original sources

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 $\eta$ 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 $\eta\approx0.5$ 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

Electron Shuttle Waiting Times for Electric Field Sensing

We explore the use of waiting-time statistics in a quantum electron shuttle for electric-field sensing. Electron shuttles convert nanomechanical motion into charge transport, showing a noise-broadened crossover between stochastic tunneling and mechanically assisted charge transfer. This allows investigation of how transport fluctuations encode electromechanical parameters. Using a single-level quantum shuttle in strong-Coulomb-blockade and high-bias regimes with a Markovian quantum master equation, we analyze stationary dynamics in phase space and waiting time distributions. By estimating the electromechanical coupling, proportional to the electric field, we evaluate the classical Fisher information in waiting times and compare it with the quantum Fisher information of the stationary state. We relate the metrological response to mean waiting time, variance, and Fano factor. Our results show that the crossover from tunneling to shuttling is characterized by enhanced fluctuations and increased parameter sensitivity, leading to a pronounced enhancement of the Fisher information.

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

Reversible photo-switching optical functionality in two-dimensional mixed-halide hybrid perovskites

Ion migration in halide perovskites is often associated with defects, irreversible processes, and structural instability, making them largely impractical for photo-switching applications. Here, we demonstrate a physical mechanism for reversible, defect-free light-induced halide-ion swapping in two-dimensional mixed-halide perovskites. We find that the halide-ion swapping process arises from strong light-lattice coupling rather than defects. By combining nudged elastic band and photo-force calculations, we show that photo-induced forces perform non-equilibrium work that drives halide ions along the halide-exchange path without reaching the fully swapped configuration. Thus, the cumulative light-induced work can only partially overcome the ground-state activation energy barrier in the presence of light. Analysis of lattice dynamics identifies a few soft phonon modes, two of which are IR-active with oscillator strength $\approx -0.14$ e/(amu)$^{1/2}$, which may be considered the microscopic origin of light-induced halide-ion swapping. This microscopic origin is further supported by band-edge-selective electron-phonon coupling, which amplifies interactions among excited carriers under illumination and with halide-ion motion without inducing a uniform dynamical instability. Using GW (G-Green's function and W-screened Coulomb interaction) calculations, we accurately reproduce the experimentally observed optical absorption spectra in the absence of light, enabling us to describe the light-induced excited state reliably. We demonstrate a clear redshift in the optical spectra in the presence of light, which is due to light-induced bandgap renormalization. Overall, these findings not only establish an intrinsic, defect-free mechanism for photoswitchable optical functionality in 2D mixed-halide perovskites but also demonstrate an intrinsic self-resetting feature in the absence of light.

cond-mat.mtrl-sci↗

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↗

arXiv2026

Nearly ballistic transport and high magnetic-field sensitivity in a $\text{Bi}_4\text{Br}_4$ topological Josephson weak link

Superconducting weak links with high transparency offer an appealing approach for designing compact magnetic-field sensors, as their phase-dependent Andreev bound-state (ABS) spectrum produces a strong flux-to-signal response with minimal dissipation. One way to achieve ballistic transport in the weak link is to use the edge states of topological insulators, since these states resist backscattering and provide a unique path to developing topological insulator-based weak-link devices for highly efficient magnetic-field sensing. Building on this, we propose a weak-link device using superconducting Nb electrodes and a nanoribbon of $\text{Bi}_4\text{Br}_4$ as the normal region, forming a Nb-1D (one-dimensional) $\text{Bi}_4\text{Br}_4$-Nb Josephson junction. We develop first-principles tight-binding Hamiltonians and orbital-resolved interface couplings in the Wannier basis, including spin-orbit coupling, based on density functional theory (DFT) calculations. The Eliashberg spectral function of bulk Nb, obtained via density functional perturbation theory (DFPT), indicates an electron-phonon coupling strength of 1.19 and a transition temperature of about 9 K, aligning well with conventional superconductivity in Nb. The subgap conductance is primarily influenced by Andreev processes. The ABS spectrum leads to a non-sinusoidal current-phase relation (CPR) with high forward skewness ($+$1.74) and phase sensitivity to the magnetic field. Overall, our findings suggest that the Nb-1D $\text{Bi}_4\text{Br}_4$-Nb weak link is a promising platform for on-chip superconducting magnetic sensors, compatible with scalable nanofabrication and broader development of topological-superconductor hybrid electronics.

cond-mat.supr-con↗

arXiv2026

Fast Arbitrary Qutrit Gates for NV Centers in the Low-Field Regime

The ground state of the negatively charged NV center forms a spin-1 manifold providing a versatile platform for sensing and information processing. Here we present a scheme for implementing fast arbitrary qutrit gates in the low-field regime using monochromatic microwave pulses of constant intensity tuned to the zero-field transition. By concatenating pulses with appropriate phases and durations, the NV-ERC scheme is extended from SU(2) operations in the double-quantum subspace to the full three-level structure. We show that arbitrary SU(3) operations can be decomposed into rotations in the double-quantum subspace together with effective implementations of the generators related to $\hat{\lambda}_5$ and $\hat{\lambda}_8$. We illustrate this decomposition with a use case: performing quantum state tomography of the complete three-level density matrix.

quant-ph↗

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

Simulating a quantum sensor: quantum state tomography of NV-spin systems

We employ a quantum computer to simulate the effect of spin impurities on nitrogen-vacancy (NV) centers in diamond. As these defects operate as nanoscale quantum sensors, modeling quantum noise is crucial to identify limitations in precision. The analysis is performed by means of quantum state tomography on two transmon qubits, representing respectively the NV center and a single spin impurity, modeling either a nuclear spin or an additional NV center. We demonstrate a versatile platform to simulate benchmark protocols such as Ramsey or Hahn-echo. Although we focus on a two-spin system, the same approach opens the door to using quantum processors as scalable simulators of many-spin environments, intractable in classical simulation due to the rapid exponential growth of the Hilbert space. The results reveal the effect different spin-sensor coupling regimes have on coherence, helping to identify detection schemes that maximize the sensitivity under the effect of impurities. Moreover, the role of entanglement generation is analyzed using the Peres-Horodecki criterion and CHSH inequalities. Although no violation of the latter is observed, the presence of entanglement is confirmed.

quant-ph↗

arXiv2025

Models of liquid samples confinement for nanoscale NMR

Diffusion is a prominent source of noise affecting nuclear magnetic resonance at the nanometer scale (nano-NMR), preventing high resolution studies of unpolarized liquid samples. Actively managing diffusion noise through, for example, sample confinement, is likely to unveil alternative noise sources which so far have been disregarded, as they occur in longer time-scales and are, consequently, masked by diffusion. These secondary noise sources could diminish the advantages provided by sample confinement but, on the other hand, they can provide with valuable information about the behavior of the sample and its interactions. In this article, we study for the first time and in detail two noise models for confined nano-NMR, namely, surface interactions and porosity, and discuss their implications for typical nano-NMR experiments.

quant-ph↗

arXiv2025

Entanglement Generation on the Double Quantum Transition of NV Ground State Via Globally Addressing Microwave Pulse

Entanglement is a key quantum feature that enables quantum sensors to improve their sensitivity up to the Heisenberg limit. In the NV center platform, the Heisenberg limit can only be achieved when the axes of the NV centers are parallel. Nevertheless, parallel NV centers are spectrally indistinguishable and no mechanisms to directly prepare Heisenberg--limit--grade entanglement in such configurations are known to date. In this work we propose for the first time a viable mechanism to prepare entangled states in the double quantum transition of two dipolarly coupled NV centers whose axes are parallel without populating intermediate states, so as to reach the Heisenberg limit in sensing. Our approach is based on the NV effective Raman coupling (NV-ERC) protocol and makes use of global addressing of both NV centers with a single monochromatic microwave pulse. Supported by an adiabatic elimination analysis, several mechanisms for the preparation of different entangled states are identified, all of which avoid the involvement of intermediate states. This not only minimizes the impact of additional noise sources, but also enables the state generation process itself to serve as effective sensing time--an advantage over conventional approaches where such preparation typically constitutes a separate, non--contributory stage. We consider the generation of different entangled states belonging to the double quantum transition, sensitive to either transverse electric fields or longitudinal magnetic fields, all with a fourfold improved sensitivity compared to conventional single NV settings.

quant-ph↗

arXiv2024

Full Qubit Control in the NV$^-$ Ground State for Low Field or High Frequency Sensing

We present a scheme for the implementation of fast arbitrary qubit gates in the ground state of the negatively charged nitrogen-vacancy (NV$^-$) defect in diamond. The protocol is especially useful in the low-field regime and for high-frequency sensing applications. It constitutes an extension to the NV-ERC technique, which has demonstrated efficient initialization and readout of the double quantum transition with no leakage to any third level thanks to an effective Raman coupling. Here we derive a full theoretical framework of the scheme, identifying the complete unitary associated to the approach, and more specifically the relevant basis change for each of two characteristic pulse durations. Based on this insight, we propose a scheme to perform fast qubit transformations in the double quantum transition. We study its robustness with respect to pulse-timing errors resulting from faulty identification of system parameters or phase-control limitations. We finally demonstrate that the technique can also be implemented in the presence of unknown electric or strain fields.

quant-ph↗

arXiv2024

Extracting Kinetic Information from Short-Time Trajectories: Relaxation and Disorder of Lossy Cavity Polaritons

The emerging field of molecular cavity polaritons has stimulated a surge of experimental and theoretical activities and presents a unique opportunity to develop the many-body simulation methodology. This paper presents a numerical scheme for the extraction of key kinetic information of lossy cavity polaritons based on the transfer tensor method (TTM). Steady state, relaxation timescales and oscillatory phenomena can all be deduced directly from a set of transfer tensors without the need for long-time simulation. Moreover, we generalize TTM to disordered systems by sampling dynamical maps and achieve fast convergence to disordered-averaged dynamics using a small set of realizations. Together, these techniques provide a toolbox for characterizing the interplay of cavity loss, disorder, and cooperativity in polariton relaxation and allow us to predict unusual dependences on the initial excitation state, photon decay rate, strength of disorder, and the type of cavity models. Thus, we have demonstrated significant potential in the use of the TTM towards both the efficient computation of long-time polariton dynamics and the extraction of crucial kinetic information about polariton relaxation from a small set of short-time trajectories.

quant-ph↗

arXiv2022

Low frequency signal detection via correlated Ramsey measurements

The low frequency region of the spectrum is a challenging regime for quantum probes. We support the idea that, in this regime, performing Ramsey measurements carefully controlling the time at which each measurement is initiated is an excellent signal detection strategy. We use the Fisher information to demonstrate a high quality performance in the low frequency regime, compared to more elaborated measurement sequences, and to optimise the correlated Ramsey sequence according to any given experimental parameters, showing that correlated Ramsey rivals with state-of-the-art protocols, and can even outperform commonly employed sequences such as dynamical decoupling in the detection of low frequency signals. Contrary to typical quantum detection protocols for oscillating signals, which require adjusting the time separation between pulses to match the half period of the target signal, and consequently see their scope limited to signals whose period is shorter than the characteristic decoherence time of the probe, or to those protocols whose target is primarily static signals, the time-tagged correlated Ramsey sequence simultaneously tracks the amplitude and the phase information of the target signal, regardless of its frequency, which crucially permits correlating measurements in post-processing, leading to efficient spectral reconstruction.

quant-ph↗

arXiv2022

Formation of Two-Ion Crystals by Injection from a Paul-Trap Source into a High-Magnetic-Field Penning Trap

Two-ion crystals constitute a platform for investigations of quantum nature that can be extended to any ion species or charged particle provided one of the ions in the crystal can be directly laser-cooled and manipulated with laser radiation. This paper presents the formation of two-ion crystals for quantum metrology in a 7-tesla open-ring Penning trap. $^{40}$Ca$^+$ ions are produced either internally by photoionization or externally in a (Paul-trap) source, transported through the strong magnetic field gradient of the superconducting solenoid, and captured in-flight with a mean kinetic energy of a few electronvolts with respect to the minimum of the Penning-trap potential well. Laser cooling of the two-ion crystal in a strong magnetic field towards reaching the quantum regime is also presented with particular emphasis on the cooling of the radial modes.

quant-ph↗

arXiv2022

Electron shuttle as an autonomous single-electron source

The majority of experimental realizations of single-electron sources rely on the periodic manipulation of the tunnel junctions through their gate voltages, and thus require a high level of control over the system. To circumvent the necessity of external driving, we utilize the self-oscillatory behavior of the electron shuttle. By means of waiting time distributions, which had not been applied to this autonomous system before, we extensively assess the performance of the shuttle as a single-electron source. We unveil a smooth transition between three regimes, whereas previous studies at the same mean field level of description only predict a sharp bifurcation based on the time-averaged electron current. Over the parameter range of already existing experimental realizations the electron shuttle can perform as a single-electron source, albeit with moderate precision. We propose an alternative design of the position-dependent tunneling rates, which significantly decreases the relative error of charge transmission, and thus paves the way for the realization of autonomous single-electron sources.

cond-mat.mes-hall↗

arXiv2022

Production of Fock Mixtures in Trapped Ions for Motional Metrology

We present a protocol to produce a class of non-thermal Fock state mixtures in trapped ions. This class of states features a clear metrological advantage with respect to the ground state, thus overcoming the standard quantum limit without the need for full sideband cooling and Fock-state preparation on a narrow electronic transition. The protocol consists in the cyclic repetition of red-sideband, measurement and preparation laser pulses. By means of the Kraus map representation of the protocol, it is possible to relate the length of the red sideband pulses to the specific class of states that can be generated. With the help of numerical simulations, we analyze the parametric regime where these states can be reliably reproduced.

quant-ph↗

arXiv2021

Zero- and Low-Field Sensing with Nitrogen Vacancy Centers

Over the years, an enormous effort has been made to establish nitrogen vacancy (NV) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin $S=1$ nature of the NV center, allowing us to detect nuclear spin signals at zero- and low-field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop new robust pulse sequences and optimized pulse pairs, which allow us to sense temperature and weak AC magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of NV centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.

quant-ph↗