Composite pulses for robust universal control of singlet-triplet qubits

TL;DR

This paper develops composite pulse sequences to achieve robust, high-precision control of singlet-triplet qubits in semiconductor quantum dots, effectively correcting magnetic field gradient errors up to sixth order.

Contribution

It introduces the first theoretical composite pulse sequences that correct gradient fluctuation errors in singlet-triplet qubits under experimental constraints.

Findings

Sequences cancel errors up to sixth order for small gradients

Sequences enable arbitrary rotations with leading order error correction for large gradients

Demonstrates robustness of control sequences against magnetic field fluctuations

Abstract

Precise qubit manipulation is fundamental to quantum computing, yet experimental systems generally have stray coupling between the qubit and the environment, which hinders the necessary high-precision control. We report here the first theoretical progress in correcting an important class of errors stemming from fluctuations in the magnetic field gradient, in the context of the singlet-triplet spin qubit in a semiconductor double quantum dot. These errors are not amenable to correction via control techniques developed in other contexts, since here the experimenter has precise control only over the rotation rate about the z-axis of the Bloch sphere, and this rate is furthermore restricted to be positive and bounded. Despite these strong constraints, we construct simple electrical pulse sequences that, for small gradients, carry out z-axis rotations while canceling errors up to the sixth order in gradient fluctuations, and for large gradients, carry out arbitrary rotations while canceling the leading order error.