arXiv subjects

Matthieu M. Desjardins

Publications and source records attributed to Matthieu M. Desjardins.

4 recordsLinked to original sources

arXiv2026

Nanoscale stray fields from micromagnets for optimal spin qubit architecture

On-chip micromagnets generate local magnetic-field asymmetries, enabling electrical control of spin qubits via electric dipole spin resonance and their integration into circuit quantum electrodynamics (QED) architectures. Accurate prediction of spin-qubit performance requires modeling micromagnet stray fields beyond the saturated-magnet approximation, accounting for nonuniform magnetization. Here, we combine thin-film characterization of Co, Co/Ta multilayers, and CoFe films with nanoscale stray-field measurements using NV-center magnetometry in the unsaturated regime to establish a reliable micromagnetic simulation framework. We show that CoFe micromagnets generate antisymmetric fields in double quantum-dot geometries exceeding +/- 100mT, owing to their high saturation magnetization and favorable magnetocrystalline anisotropy. For spin qubits coupled to microwave resonators, the predicted spin-photon coupling reaches $\left| g_s/g_c \right| \approx 0.5$, where $g_c$ denotes the charge-photon coupling strength of the underlying charge qubit, highlighting the potential for high-fidelity operations in circuit QED architectures.

cond-mat.mes-hall↗

arXiv2025

Optimal Connectivity from Idle Qubit residual coupling Cross-Talks in a Cavity Mediated Entangling Gate

Quantum processors operated through long range interaction mediated by a microwave resonator have been envisioned to allow for high connectivity. The ability to selectively operate qubits rely on the possibility to dynamically suppress the coupling between each qubit and the resonator, however there always remains a residual coupling. In this article, we investigate the effect of high processor connectivity on average two qubit gate fidelity in a cavity based architecture with tunable coupling. Via a perturbative approach, we quantify the cross-talk errors from transverse residual couplings and show that they scale as $nm^2$ where $n$ is the number of idle qubits and $m$ is the ratio between the transverse residual and active couplings. Setting an error threshold $E_\mathrm{thr}$, we demonstrate that cross-talks restrict the hardware topology and prevent the full use of all-to-all connectivity. We predict that the maximum number of qubits allowed by $E_\mathrm{thr}$ scales as $n \propto E_\mathrm{thr}/m^2$.

quant-ph↗

arXiv2022

Optimal design of nanomagnets for on-chip field gradients

The generation of localized magnetic field gradients by on-chip nanomagnets is important for a variety of technological applications, in particular for spin qubits. To advance beyond the empirical design of these nanomagnets, we propose a systematic and general approach based on the micromagnetic formulation of an optimal field gradient source. We study the different field configurations that can be realized and find out quantitatively the most suitable ferromagnetic layer geometries. Using micromagnetic simulations, we then investigate the minimum requirements for reaching magnetic saturation in these nanomagnets. In terms of either longitudinal or transverse field gradient, the results provide an optimal solution for uniform, saturated nanomagnets, where the magnetic material can be selected according to the strength of the external fields that can be used.

cond-mat.mes-hall↗

arXiv2017

Cavity QED with hybrid nanocircuits: from atomic-like physics to condensed matter phenomena

Circuit QED techniques have been instrumental to manipulate and probe with exquisite sensitivity the quantum state of superconducting quantum bits coupled to microwave cavities. Recently, it has become possible to fabricate new devices where the superconducting quantum bits are replaced by hybrid mesoscopic circuits combining nanoconductors and metallic reservoirs. This mesoscopic QED provides a new experimental playground to study the light-matter interaction in electronic circuits. Here, we present the experimental state of the art of Mesoscopic QED and its theoretical description. A first class of experiments focuses on the artificial atom limit, where some quasiparticles are trapped in nanocircuit bound states. In this limit, the Circuit QED techniques can be used to manipulate and probe electronic degrees of freedom such as confined charges, spins, or Andreev pairs. A second class of experiments consists in using cavity photons to reveal the dynamics of electron tunneling between a nanoconductor and fermionic reservoirs. For instance, the Kondo effect, the charge relaxation caused by grounded metallic contacts, and the photo-emission caused by voltage-biased reservoirs have been studied. The tunnel coupling between nanoconductors and fermionic reservoirs also enable one to obtain split Cooper pairs, or Majorana bound states. Cavity photons represent a qualitatively new tool to study these exotic condensed matter states.

cond-mat.mes-hall↗