Spurious Antenna Modes of the Transmon Qubit

TL;DR

This paper investigates unintended antenna modes in superconducting transmon qubits, analyzing their impact on quasiparticle poisoning and energy relaxation, and aims to inform designs that mitigate these effects for improved qubit performance.

Contribution

It provides a detailed analysis of spurious antenna modes in various qubit types and their effects on quasiparticle poisoning and radiative losses, guiding better qubit design.

Findings

Antenna modes can cause quasiparticle poisoning via blackbody radiation absorption.

Radiative losses at qubit frequencies set a fundamental limit to energy relaxation.

Understanding these modes enables design improvements for more robust superconducting qubits.

Abstract

Superconducting qubits are resonant absorbers of pair-breaking radiation. The metal pads that form the qubit capacitance support standing wave modes at frequencies of order 100~GHz; these modes are strongly coupled to free-space impedance through their electric dipole moment. While the antenna mode of the 3D transmon qubit is easily seen to be a resonant dipole, other 2D qubit types can be understood as the aperture duals of wire loop antennas or folded dipoles. For typical Josephson junction parameters, the junction provides a reasonable conjugate match to the fundamental antenna mode. We calculate the contribution to quasiparticle poisoning from resonant absorption of blackbody radiation. We extend our analysis to dissipation at the qubit frequency, where radiative losses provide an ultimate limit to qubit energy relaxation time. A clear understanding of the spurious antenna modes of qubits will allow designs that are insensitive to pair-breaking radiation and that display reduced radiative losses at the qubit frequency.