Designing an Asymmetric Zero-Pi Qubit for Insensitivity to Charge Noise and Quasiparticle Tunneling

Abstract

Superconducting qubits are a promising platform for quantum computers, but they are vulnerable to decoherence from energy relaxation ((T_1)) and dephasing ((T_2)). The (0-\pi) qubit is a superconducting qubit that offers intrinsic protection against decoherence. To obtain (T_1) protection, the (0-\pi) has disjoint support: deep wells in its potential that exponentially suppress tunneling between its logical states. To obtain (T_2) protection, the (0-\pi) parameters can be tuned such that the logical states are first-order insensitive to flux noise and exponentially insensitive to charge noise. However, driving between the logical states of the (0-\pi) requires passing through an ancillary level that is sensitive to charge noise and quasiparticle tunneling events. This thesis introduces unequal Josephson junctions ((\Delta E_J\ne0)) in the (0-\pi) circuit to create an "asymmetric (0-\pi)" qubit. Using Python scripts, the asymmetric (0-\pi) parameter space was searched for regimes that reduce the ancillary level's sensitivities to charge noise and quasiparticles, but also retain the symmetric (0-\pi)'s (T_1) and (T_2) protection. These simulations revealed three qualitatively different regimes, corresponding to small, mid, and large (\Delta E_J) values. While the large (\Delta E_J) regime has clearly reduced sensitivity to both charge noise and quasiparticle tunneling, it loses the disjoint support necessary for (T_1) protection. Meanwhile, the small (\Delta E_J) regime displays partial protection against charge noise and quasiparticle tunneling, and it also successfully replicates the strengths of the symmetric (0-\pi), making it the most promising regime in the long term. Unfortunately, realizing a device in this regime is difficult given current fabrication error margins. This observation motivates further investigation of the mid (\Delta E_J) regime, which in general shows neither protection against quasiparticles nor disjoint support. However, its reliably large charge dispersions allow greater room for fabrication error and thus make it ideal for a proof-of-concept device to check that the asymmetric (0-\pi) theory can be reproduced in experiment. An initial device in the mid (\Delta E_J) regime has been fabricated, and the measurement of this device is currently in preparation.