Physicists Push Hole Spin Qubit Readout Past 97 Percent by Hunting Down Every Error

Quantum computers built from spins in semiconductor quantum dots have long promised a scalable route to quantum information processing, but one of the most deceptively difficult steps is simply reading out the answer. In a study published in Nature Electronics, a team at IBM Research Europe in Zurich reports a systematic dissection of every error process that occurs during the readout of hole spin qubits in germanium, and by carefully mitigating each one, they achieved a single-shot state preparation and measurement (SPAM) fidelity of 97.0 percent for single-qubit operation. The result marks a significant advance for a platform that has lagged behind its silicon electron-spin counterpart in readout performance, and it provides a detailed roadmap that other laboratories working on germanium and hole-based qubits can follow.

Hole spins in germanium quantum dots are attractive for several reasons. The Ge/SiGe heterostructures used to confine them exhibit low charge noise and disorder, and the strong spin-orbit interaction of holes allows all-electrical manipulation of the qubit, eliminating the need for bulky microwave antennas. The same anisotropic physics that enables fast electrical driving, however, also complicates readout. Standard spin readout relies on Pauli spin blockade, in which a two-spin state is converted into a charge state that a nearby sensor can detect. In hole systems, site-dependent spin anisotropies and short relaxation times conspire to corrupt this conversion, and the highest readout fidelity previously reported for holes in germanium hovered around 94 percent, well short of the 99 percent threshold that fault-tolerant quantum computing demands.