Measurement of a superconducting qubit with a microwave photon counter

TLDR

Fast, high‑fidelity measurement is essential for quantum error correction, yet conventional linear‑amplification and heterodyne detection schemes do not scale to large systems. The authors propose a new measurement approach that uses a microwave photon counter. They implement the counter, exploiting its intrinsic damping to recover the energy released during measurement and thereby enable repeated high‑fidelity quantum‑non‑demolition readout. The technique achieves a raw single‑shot fidelity of 92 % and provides direct access to the classical measurement outcome at millikelvin temperatures. In future systems, counter‑based measurement could serve as a scalable quantum‑to‑classical interface.

Abstract

Fast, high-fidelity measurement is a key ingredient for quantum error correction. Conventional approaches to the measurement of superconducting qubits, involving linear amplification of a microwave probe tone followed by heterodyne detection at room temperature, do not scale well to large system sizes. Here we introduce an alternative approach to measurement based on a microwave photon counter. We demonstrate raw single-shot measurement fidelity of 92%. Moreover, we exploit the intrinsic damping of the counter to extract the energy released by the measurement process, allowing repeated high-fidelity quantum non-demolition measurements. Crucially, our scheme provides access to the classical outcome of projective quantum measurement at the millikelvin stage. In a future system, counter-based measurement could form the basis for a scalable quantum-to-classical interface.