Scaling quantum measurement

The Problem

As quantum processors scale up, readout becomes a bottleneck. Today’s multiplexed measurement schemes rely on deliberately mismatched impedances to steer measurement photons toward the detectors. This trick works for small systems, but it creates standing waves that lead to resonator linewidth variations. Those variations worsen with mismatch strength, making uniform fabrication harder and harder as processors get larger.

The Approach

I designed an “all-pass” readout architecture that achieves directional photon emission directly through resonator design, rather than impedance engineering. The idea is to use two λ/4 resonators symmetrically coupled to a transmon qubit and creating an “all-pass resonator,” which preferentially emits photons toward the output while keeping the measurement chain well-matched to 50 Ω.

Results & Impact

• 99% single-shot readout fidelity in 300 ns in experiment
• This approach enables a square-root reduction in linewidth-variation sensitivity compared to standard approaches
• Aims to preserve a well impedance-matched environment, a cornerstone of robust microwave engineering
• Supports modular processor layouts with flexible qubit placement

Why This Matters

This approach aims to tackle a real manufacturing bottleneck in quantum hardware. By improving uniformity and reducing extra infrastructure, it can make large-scale processor fabrication more reliable.