Singlet-only always-on gapless exchange (SAGE) spin qubits: Charge noise effects and two-qubit gates

TL;DR

This paper analyzes the performance of SAGE spin qubits under charge noise, demonstrating that pulse sequences can extend coherence times and improve two-qubit gate fidelity despite noise challenges.

Contribution

It provides a detailed characterization of SAGE qubits' robustness to charge noise and proposes refocusing strategies to enhance coherence and gate fidelity.

Findings

CPMG-like sequences extend SAGE qubit coherence times.

Refocusing strategies mitigate charge and magnetic noise effects.

Increased ramp times reduce leakage during two-qubit gates.

Abstract

Singlet-only always-on gapless exchange (SAGE) spin qubits are an alternative type of exchange-only (EO) qubits that encode a single qubit in the spins of four electrons located in four tunnel-coupled quantum dots. While conventional EO qubits are susceptible to local magnetic field gradients caused by local nuclear environments and $g$-factor variations, the SAGE qubit subspace is inherently protected from magnetic-gradient-induced Pauli errors by virtue of the singlet-only encoding, which is invariant under magnetic field gradients, and the always-on exchange couplings, which provide energetic leakage protection. However, the always-on operation simultaneously increases the qubit's sensitivity to charge noise. Here, starting from a Hubbard model describing the underlying electronic structure of the coupled quantum dots, we characterize the performance of SAGE qubits in the presence of $1/f$ charge noise that induces fluctuations in both the dot chemical potentials and the interdot tunnel couplings. We calculate SAGE idle coherence times and show that realistic CPMG-like pulse sequences can be used to significantly extend SAGE single-qubit coherence times for experimentally relevant charge noise strengths. We likewise study the fidelity of SAGE two-qubit gates in the presence of charge and magnetic noise and again propose a simple refocusing strategy to mitigate the noise, while increased ramp times of the entangling pulse suppress leakage into noncomputational states.