Singlet-only Always-on Gapless Exchange Qubits with Baseband Control

TL;DR

The paper introduces a new singlet-only always-on gapless exchange (SAGE) spin qubit that offers enhanced coherence and simplified control for quantum computing, overcoming limitations of conventional exchange-only qubits.

Contribution

It presents the SAGE qubit design that is protected from magnetic gradient errors and enables universal control with simplified two-qubit gate operations.

Findings

Coherence times improve by an order of magnitude under magnetic gradient noise.

Single-qubit gate infidelity is significantly reduced compared to conventional qubits.

Two-qubit gates can be performed with a single exchange pulse, simplifying control sequences.

Abstract

We propose a singlet-only always-on gapless exchange (SAGE) spin qubit that encodes a single qubit in the spins of four electrons while allowing universal baseband control. While conventional exchange-only qubits suffer from magnetic-field-gradient-induced leakage and coherent errors, for instance due to local nuclear environments and variations in the $g$-factor, the SAGE qubit subspace is protected from coherent errors due to local magnetic field gradients and leakage out of the computational subspace is energetically suppressed due to the exchange interactions between electrons being always-on. Consequently, we find that when magnetic gradient noise dominates over charge noise, coherence times and single-qubit gate infidelities of the SAGE qubit improve by an order of magnitude compared to conventional exchange-only qubits. Moreover, using realistic parameters, two-qubit gates can be performed with a single interqubit exchange pulse with times comparable in duration to conventional exchange-only qubits but with a significantly simplified pulse sequence.