Ultrafast quantum measurement

The Problem

Quantum measurement is a major bottleneck for fault-tolerant quantum computing. In recent below-threshold error-correction experiment, measurement alone accounted for over 30% of the error budget and took an order of magnitude longer time than gate operations.

The Approach

We designed a multi-mode “arm qubit” architecture that enables strong, non-perturbative cross-Kerr interactions for ultrafast measurement. The system combines:

• A fluxonium-like data mode that stores quantum information
• A transmon-like arm mode that couples to other elements

The arm mode mediates strong interaction with a measurement resonator while keeping the data mode computationally isolated. The breakthrough is achieving large cross-Kerr shifts (χ₁₂/2π > 1 GHz). When the data mode is in |1⟩, the arm and resonator modes hybridize, splitting their degeneracy. This enables amplitude-based readout: the resonator conditionally fills with photons only when the qubit is in the excited state.

Results & Impact

• Assignment error <10⁻⁴ in 30 ns with realistic measurement efficiency (η = 0.5)
• QND fidelity >99.7%, preserving computational state integrity
• 167 ms Purcell-limited lifetime without external filter components

Why This Matters

This proposal would deliver a substantial speedup in quantum measurement, cutting error-correction cycle times and reducing system overhead. By combining ultrafast readout with high QND fidelity and long lifetimes, the arm qubit directly addresses the fundamental bottleneck to scalable, fault-tolerant quantum computing.