New Hybrid Device Technology for Intrinsically Protected Superconducting Qubits
Abstract
Superconducting qubits, and particularly the transmon, are among the leading platforms for realizing computations outside the reach of classical supercomputers. Despite its success, it is still not clear if the superconducting transmon will be the qubit modality of the future. Here we propose to develop fundamentally new methods to fabricate the voltage-tunable variant of the transmon, the hybrid gatemon. We will use these next-generation gatemons to demonstrate a new form of flux insensitive parity protected qubit and a novel charge-phase qubit-qubit coupling, which will be used to realize a microwave gyrator, a non-reciprocal element recently shown to be central to the realization of on-chip cQED-compatible Gottesman-Kitaev-Preskill encoding of protected qubits. Gatemons are a variant of the transmon where the Josephson junction is formed by a semiconducting material proximitized by a superconductor. The material and qubit performance development for gatemons have not been a particularly active area of research, due to limitations imposed by the fabrication requirements. Here, we will demonstrate how a recently developed growth technique for nanowire-based Josephson junctions can be used to realize a new generation of gatemons, with high stability, high reproducibility and high coherence. The fabrication techniques rely on forming the Josephson junction on the nanowire in situ inside the molecular beam epitaxy growth chamber, by using a secondary nanowire to shadow a region of the proximitized nanowire. This so-called shadow approach to forming Josephson junctions results in a dramatic reduction of impurities, sharper definition of the junctions edges, higher reproducibility and improved charge stability. We will demonstrate how this technique also leads to vastly improved performance of gatemons. Furthermore, shadow-defined Josephson junctions are also compatible with more exotic superconductors such as tantalum, niobium and lead. We will explore the impact of such superconductors on the coherence times and performance of gatemons. There is currently a growing interest in the notion of protected qubits. Here, a Hamiltonian of the circuit is engineered such that its two lowest lying states (the computational basis) have an intrinsic property that protects against external perturbations. This is typically done by realizing (approximately) dispersionless energy bands or via degenerate levels. In a stark contrast to the physics of standard superconductor-insulator-superconductor Josephson junctions, the current-phase relation in gatemons is directly tunable via a gate voltage and can be made non-sinusoidal. This allows for the realization of exotic components such as the cos(2phi)-element and a direct charge-phase coupling. Such components are central elements in several of the proposed schemes for protected qubits. We will use the new modality of stable, high-coherence gatemons and their inherent non-sinusoidal current-phase relation to realize coupled single-mode cos(2phi)-qubits, resulting in the first experimental demonstration of a flux-insensitive parity protected qubit. Moreover, we will show how gatemons can be used to engineer a direct coupling between charge and phase degrees of freedom in a transmon-like qubit. Based on this coupling, we will implement a new family of two-qubit gates, and we will use this coupling to demonstrate an on-chip microwave gyrator, a non-reciprocal element central to realizing on-chip GKP-states.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 12, 2022
- Source ID
- W911NF2210042
Entities
People
- Morten Kjaergaard
Organizations
- Army Contracting Command
- National Security Agency
- University of Copenhagen