arXiv subjects

Hélène le Sueur

Publications and source records attributed to Hélène le Sueur.

5 recordsLinked to original sources

arXiv2025

Absence of a dissipative quantum phase transition in Josephson junctions: Theory

We investigate the resistively shunted Josephson junction (RSJ) at equilibrium, using linear response, an exact path integral technique and symmetry considerations. All three approaches independently lead to conclude that the superconducting - insulating quantum phase transition long believed to occur in the RSJ, cannot exist. For all parameters, we find that shunting a junction makes it more superconducting. We reveal that the UV cutoff of the resistor plays an unforeseen key role in these systems, and show that the erroneous prediction of an insulating state resulted in part from assuming it would not. We also explain why the RSJ physics differs from that of 1D quantum impurity problems. Our results fully support and confirm the experimental invalidation of this quantum phase transition by Murani et al. in 2020.

cond-mat.supr-con↗

arXiv2020

Absence of a dissipative quantum phase transition in Josephson junctions

Half a century after its discovery, the Josephson junction has become the most important nonlinear quantum electronic component at our disposal. It has helped reshape the SI system around quantum effects and is used in scores of quantum devices. By itself, the use of Josephson junctions in the volt metrology seems to imply an exquisite understanding of the component in every aspect. Yet, surprisingly, there have been long-standing subtle issues regarding the modeling of the interaction of a junction with its electromagnetic environment. Here, we find that a Josephson junction connected to a resistor does not become insulating beyond a given value of the resistance due to a dissipative quantum phase transition, as is commonly believed. Our work clarifies how this key quantum component behaves in the presence of a dissipative environment and provides a comprehensive and consistent picture, notably regarding the treatment of its phase.

cond-mat.mes-hall↗

arXiv2018

Microscopic charged fluctuators as a limit to the coherence of disordered superconductor devices

By performing experiments with thin-film resonators of NbSi, we elucidate a decoherence mechanism at work in disordered superconductors. This decoherence is caused by charged Two Level Systems (TLS) which couple to the conduction electrons in the BCS ground state; it does not involve any out-of-equilibrium quasiparticles, vortices, etc. Standard theories of mesoscopic disordered conductors enable making predictions regarding this mechanism, notably that decoherence should increase as the superconductor cross section decreases. Given the omnipresence of charged TLS in solid-state systems, this decoherence mechanism affects, to some degree, all experiments involving disordered superconductors. In particular, we show it easily explains the poor coherence observed in quantum phase slip experiments and may contribute to lowering the quality factors in some disordered superconductor resonators.

cond-mat.supr-con↗

arXiv2017

Design of a Cooper pair box electrometer for application to solid-state and astroparticle physics

We describe the design and principle of operation of a fast and sensitive electrometer operated at millikelvin temperatures, which aims at replacing conventional semiconducting charge amplifiers in experiments needing low back-action or high sensitivity. The electrometer consists of a Cooper Pair box (CPB) coupled to a microwave resonator, which converts charge variations to resonance frequency shifts. We analyze the dependence of the sensitivity on the various parameters of the device, and derive their optimization. By exploiting the nonlinearities of this electrometer, and using conventional nanofabrication and measurement techniques, a charge sensitivity of a few $10^{- 7} e / \sqrt{Hz}$ can be achieved which outperforms existing single charge electrometers.

cond-mat.mes-hall↗

arXiv2016

Manipulating Fock states of a harmonic oscillator while preserving its linearity

We present a new scheme for controlling the quantum state of a harmonic oscillator by coupling it to an anharmonic multilevel system (MLS) with first to second excited state transition frequency on-resonance with the oscillator. In this scheme that we call "ef-resonant", the spurious oscillator Kerr non-linearity inherited from the MLS is very small, while its Fock states can still be selectively addressed via an MLS transition at a frequency that depends on the number of photons. We implement this concept in a circuit-QED setup with a microwave 3D cavity (the oscillator, with frequency 6.4 GHz and quality factor QO=2E-6) embedding a frequency tunable transmon qubit (the MLS). We characterize the system spectroscopically and demonstrate selective addressing of Fock states and a Kerr non-linearity below 350 Hz. At times much longer than the transmon coherence times, a non-linear cavity response with driving power is also observed and explained.

quant-ph↗