Quantum physics allows us to make computers whose computing power overcomes by large that of classical computers, whatever their future progress. This recent discovery has triggered a huge research effort for making quantum processors. Nowadays, the most advanced implementations are based on trapped ions and superconducting qubit circuits, and elementary instances of quantum algorithms were demonstrated with these systems and with some other ones. The Quantronics group at CEA-Paris-Saclay led by the applicant, Daniel Esteve, has played a key role in the development of superconducting qubits since its beginning. Despite the unsolved challenges raised by maintaining quantum coherence during processor evolution, and by the scalability issue, major players in information processing such as IBM, Microsoft, Intel or Google have developed since a few years in-house research and/or strong academic partnerships in order to mitigate the detrimental effect that a quantum breakthrough would have in their activities. Atos, a leading company in high performance computing, has similarly developed its own quantum computing activity. Since 2016, its teams develop quantum software and a powerful FPGA-based emulator of a quantum computer. Besides, Atos has initiated a collaboration with the Quantronics group of the applicant, and supports a first forthcoming CIFRE PhD thesis research for detecting a new type of quantum bit in the applicant laboratory. The work programme for this industrial chair first aims at providing to Atos the high level scientific watch in the field of quantum computing that the applicant and his team, who are well-recognized and well-connected to leading teams worldwide, can deliver. A second goal consists in providing to Atos physical models of different qubits embedding noise models, processing time, communication models in order to simulate them efficiently with the emulator Atos is presently developing. The goal is to enable Atos to get numerical metrics to be used for algorithm optimization in a given qubit platform. This analysis will be carried-out in-depth for all quantum bits developed by CEA. The main research objective is to develop new quantum bits with better quantum coherence than superconducting quantum bits. Given their limitations, there is no operational architecture for solving the quantum error correction issue in a superconducting processor when scaling the size. Indeed, the fault-tolerant architecture compatible with superconducting quantum bit error thresholds, namely the surface-code architecture, requires a prohibitive overhead in terms of physical qubit resources. In order to mitigate and solve these challenges, The applicant and his team propose to use nuclear spins as quantum bits, for which quantum error correction would be much less of a problem. When these nuclear spins are coupled to electronic spins by the hyperfine interaction, and these electronic spins are coupled to superconducting microwave resonators that transmit photons that can be measured, the combination of all these quantum systems provides an original attractive route towards a new quantum computing platform based on very coherent quantum bits. The potential route such a platform would have is of great interest for Atos, and applicant team has already obtained significant preliminary results in this direction. Last but not least, the collaboration between Atos and CEA in the field of quantum computing would even more connect Atos with the microfabrication capacity of CEA if an industrial development is foreseen.

Anyons are entities that evade the standard classification of particles into bosons and fermions, as exchanging two of them leads to an arbitrary exchange phase for the two-body wavefunction that is neither 0 (bosons) or pi (fermions). They can exist in strongly interacting two-dimensional systems like two dimensional electron gases in the Fractional Quantum Hall regime. Indeed, low-energy excitations of this regime carry a fractional charge and possess fractional exchange statistics, i.e., are anyons. While fractional charges have been measured twenty-five years ago, a direct proof of anyonic statistics has been missing, until 2020 where two teams reported such evidence. However, the demanding experimental conditions, as well as the fragility against disorder of these fractional states, impedes a further quantitative understanding of anyons and possible extensions to more exotic, non-Abelian states that are relevant for topological quantum computing. I propose a novel approach where strong correlations are engineered to make anyonic statistics emerge in the much more accessible and robust Integer Quantum Hall regime. The central ingredient is a small Ohmic contact with high charging energy connected to N > 1 ballistic Quantum Hall channels. Any electron injected in such an island will be fractionalized, with pulses of charges e/N that leave the island through the outgoing ballistic channels. I plan to unravel the predicted fractional exchange phase of such excitations using electronic Mach-Zehnder interferometry in one of the outgoing channels. The quantum simulation of anyons in relaxed experimental conditions (lower magnetic field, robustness against sample-dependent disorder) will constitute a first, and will enable quantitative comparisons between theory and experiment at an unprecedented level for anyonic quasiparticles. It will constitute an advance towards the generation and manipulation of topologically protected exotic states.

Single photon detection is a key resource for sensing at the quantum limit and the enabling technology for measurement-based quantum computing. Photon detection at optical frequencies relies on irreversible photo-assisted ionization of various materials (semiconductors, superconductors). However, microwave photons have energies 5 orders of magnitude lower than optical photons, and are therefore ineffective at triggering measurable phenomena at macroscopic scales. Here, we propose the exploration of a new type of interaction between a single two level system (qubit) and a microwave resonator. These two quantum systems do not interact coherently, instead, they share a common dissipative mechanism to a cold bath: the qubit irreversibly switches to its excited state if and only if a photon enters the resonator. We will exploit this highly correlated dissipation mechanism to detect itinerant photons impinging on the resonator. This scheme does not require any prior knowledge of the photon waveform nor its arrival time, and dominant decoherence mechanisms do not trigger spurious detection events (dark counts). Here, we propose the development of a practical photon detector based on dissipation engineering with performances approaching state-of-the-art optical photon detector both in terms of efficiency and dark counts. We will explore three directions in order to bridge the gap between optical and microwave technologies: First, we will develop cutting edge design for microwave circuitry and bring in-house circuit nanofabrication at their best level. Second, we will explore a key feature of the dissipative scheme, namely the ability to continuously monitor the detector state (click/no click) while operating. Associated with real-time feedback, it will allow for precise timing of photon arrivals and optimal detection efficiency enabled by the fast initialization of the detector in its measurement ready state. Finally, we will explore ideas introduced by quantum error correcting codes, we propose to incorporate quantum error mitigation against dark counts within the dissipative scheme itself. This would enable a dramatic reduction of false detection events, indeed this key figure of merit for photon detectors captures the overall noise and sensitivity performances of the device. This proposal establishes engineered non-linear dissipation as a key-enabling resource for a new class of low-noise microwave detectors, paving the way to cutting edge applications such as ultra-sensitive electron spin resonance, axion search in the microwave domain or modular quantum computing architectures.

The aim of the project is to develop a novel characterization tool of the magnetic properties of the material surface at nano-scale by combining very sensitive magnetoresistive sensors and atomic force microscopy. This tool will have three main innovative applications: magnetic nanometrology, magnetic susceptibility imaging and micro imaging of Nuclear Magnetic Resonance (NMR) spectroscopy.