Qubits supraconducteurs protégés basés sur des modes à haute impédance

Les circuits supraconducteurs quantiques constituent une plateforme de pointe pour le traitement de l’information quantique. L’utilisation de ce type de circuits en tant que qubits est en partie motivée par la grande flexibilité avec laquelle ces dispositifs peuvent être fabriqués. Cette flexibilité fait en sorte que les circuits supraconducteurs sont attractifs comme une architecture pour le design des qubits, des amplificateurs, des détecteurs de photons et d’autres dispositifs quantiques. Le domaine des qubits supraconducteurs est en rapide évolution depuis quelques an nées, ce qui a conduit à un certain nombre d’avancées majeures, dont la récente démonstration de la suprématie computationnelle quantique. Cela a été possible en partie grâce à l’introduction de l’architecture de l’électrodynamique quantique des circuits, et du qubit transmon. Le qubit transmon est protégé contre la source de bruit la plus nuisible dans les dispositifs mésoscopiques (bruit de charge), tout en possédant un design simple permettant sa mise en l’échelle. Cependant, malgré le succès retentissant du qubit transmon, d’autres qubits supraconducteurs, tels que le fluxonium et les circuits 0 − π, ont en principe le potentiel d’être plus performants. En particulier, le qubit 0 − π utilise des modes de circuit à haute impédance qui sont réalisés en utilisant de grandes inductances (ou superinductances) afin de rendre le système insensible au bruit de flux. Les superinductances, ainsi que les dispositifs de fluxonium et de 0 − π qubit, sont le principal objet de cette thèse.Abstract: Superconducting quantum circuits are a leading platform for quantum-information processing. Part of the motivation behind using superconducting circuits as qubits lies in the fact that these devices can be engineered with great flexibility. This also makes superconducting quantum circuits attractive as an architecture for building devices that go beyond qubits, such as amplifiers, photon detectors, among others, and for the exploration of the rich physics of quantum optics in new parameter regimes. The field of superconducting qubits has gone through a rapid development in the last fewyears, leading to a number of major breakthroughs including the recent quantum computational supremacy demonstration. This has been possible thanks in part to the introduction of the circuit quantum electrodynamics architecture and the transmon qubit. This qubit combines insensitivity to the most detrimental source of noise in mesoscopic devices (charge noise), with a simple design and scalable fabrication. However, despite the overwhelming success of the transmon qubit, other implementations of superconducting qubits, such as the fluxonium and the 0 p circuits, have the potential to perform better. In particular, the 0 p qubit makes use of high-impedance circuit modes, which are realized using large inductances (or superinductances), in order to render the system insensitive to flux noise. Superinductances, along with the fluxonium and 0 p qubit devices are the main focus of this thesis

Moving beyond the transmon: Noise-protected superconducting quantum circuits

Artificial atoms realized by superconducting circuits offer unique opportunities to store and process quantum information with high fidelity. Among them, implementations of circuits that harness intrinsic noise protection have been rapidly developed in recent years. These noise-protected devices constitute a new class of qubits in which the computational states are largely decoupled from local noise channels. The main challenges in engineering such systems are simultaneously guarding against both bit- and phase-flip errors, and also ensuring high-fidelity qubit control. Although partial noise protection is possible in superconducting circuits relying on a single quantum degree of freedom, the promise of complete protection can only be fulfilled by implementing multimode or hybrid circuits. This Perspective reviews the theoretical principles at the heart of these new qubits, describes recent experiments, and highlights the potential of robust encoding of quantum information in superconducting qubits

Accurate methods for the analysis of strong-drive effects in parametric gates

The ability to perform fast, high-fidelity entangling gates is an important requirement for a viable quantum processor. In practice, achieving fast gates often comes with the penalty of strong-drive effects that are not captured by the rotating-wave approximation. These effects can be analyzed in simulations of the gate protocol, but those are computationally costly and often hide the physics at play. Here, we show how to efficiently extract gate parameters by directly solving a Floquet eigenproblem using exact numerics and a perturbative analytical approach. As an example application of this toolkit, we study the space of parametric gates generated between two fixed-frequency transmon qubits connected by a parametrically driven coupler. Our analytical treatment, based on time-dependent Schrieffer-Wolff perturbation theory, yields closed-form expressions for gate frequencies and spurious interactions, and is valid for strong drives. From these calculations, we identify optimal regimes of operation for different types of gates including $i$SWAP, controlled-Z, and CNOT. These analytical results are supplemented by numerical Floquet computations from which we directly extract drive-dependent gate parameters. This approach has a considerable computational advantage over full simulations of time evolutions. More generally, our combined analytical and numerical strategy allows us to characterize two-qubit gates involving parametrically driven interactions, and can be applied to gate optimization and cross-talk mitigation such as the cancellation of unwanted ZZ interactions in multi-qubit architectures.Comment: 20 pages, 9 figures, 62 reference