Simone Fasciati

Abstract:

We report high qubit coherence as well as low cross-talk and single-qubit gate errors in a superconducting circuit architecture that promises to be tileable to two-dimensional (2D) lattices of qubits. The architecture integrates an inductively shunted cavity enclosure into a design featuring nongalvanic out-of-plane control wiring and qubits and resonators fabricated on opposing sides of a substrate. The proof-of-principle device features four uncoupled transmon qubits and exhibits average energy relaxation times T₁ = 149(38) μs, pure echoed dephasing times T_ϕ,e = 189(34) μs, and single-qubit gate fidelities F = 99.982(4)% as measured by simultaneous randomized benchmarking. The 3D integrated nature of the control wiring means that qubits will remain addressable as the architecture is tiled to form larger qubit lattices. Band structure simulations are used to predict that the tiled enclosure will still provide a clean electromagnetic environment to enclosed qubits at arbitrary scale.

Abstract:

Superconducting circuits are a leading platform for quantum information processing, partly due to the great freedom of tailoring circuit parameters which enables the implementation of a wide variety of Hamiltonians. Many simple superconducting circuits commonly employed as qubits contain a variation of superconducting quantum interference device (SQUID) that adds in-situ magnetic flux tunability to the system and further increases its flexibility. At the same time, this control parameter requires additional dedicated circuitry and can introduce flux noise which is detrimental to qubit performance. This increases hardware complexity and hinders the scaling to large qubit numbers. In this thesis, we develop and test a simple 3D-integrated architecture for individual flux control of tileable, coaxial, gradiometric superconducting qubits, achieving highly selective flux bias (low crosstalk) and incorporating both charge and flux control into a single off-chip element. The addition of flux tunability fully retains the simplicity of the fabrication and packaging process of the original, fixed-frequency coaxial architecture. We then use this experimental platform to study the inductively shunted transmon (IST), an interesting qubit species based on a radio-frequency (RF) SQUID loop with a small linear shunting inductance. Its qualitative behavior is similar to a transmon qubit but with a positive instead of negative anharmonicity. We design, simulate, fabricate and experimentally characterize gradiometric IST qubits, showing that they can be easily integrated into an existing transmon-based quantum processor architecture. Further, by directly coupling an IST to a transmon via a mutual capacitance, we demonstrate how the opposite signs of anharmonicity can be exploited to effectively reduce the undesired static longitudinal coupling (ZZ interaction) to zero. We also investigate microwave sideband transitions in this two-qubit system and benchmark a controlled-Z (CZ) entangling gate. This work paves the way towards hardware-efficient, crosstalk-suppressed superconducting quantum processors based on multi-species qubit lattices.