Edge-Pass Purcell Filter Hits 99.57% Readout Fidelity While Unifying Protection and Reset for Superconducting Qubits

The team fabricated high-pass and low-pass versions of a single-edge transmission filter that removes the bandwidth limits of conventional bandpass designs. On-chip tests with superconducting qubits showed the filters extend T1 lifetimes beyond the bare Purcell limit, with protection improving at higher filter order. An intrinsic dissipation mode inside the filter simultaneously provides a fast qubit-reset channel without extra hardware.

This architecture directly addresses the readout bottleneck that has constrained scaling beyond a few dozen qubits. Prior bandpass filters required large footprints and restricted resonator frequencies; the edge-pass approach collapses readout, protection, and reset into one compact network. The reported fidelities approach the thresholds needed for surface-code error correction while preserving the short gate times essential for fault tolerance.

Related work on dispersive readout chains (Blais et al., Rev. Mod. Phys. 2021) and Purcell-limited T1 measurements (Barends et al., Phys. Rev. Lett. 2013) shows that filter-induced loss has repeatedly set the practical ceiling on qubit lifetime. By moving the cutoff edge rather than a full passband, the new design relaxes frequency crowding and footprint constraints that have slowed multi-chip module integration.

Next steps include embedding the filter in 50-plus qubit processors and verifying that reset crosstalk remains below the 0.1% threshold required for repeated syndrome extraction. If successful, the unified component could shorten the timeline to logical qubits by removing one layer of auxiliary circuitry.