Exploring the Fidelity of Flux Qubit Measurement in Different Bases via the Quantum Flux Parametron

High-fidelity qubit readout is a fundamental requirement for practical quantum computing systems. In this work, we investigate methods to enhance the measurement fidelity of flux qubits via a quantum flux parametron-mediated readout scheme. Through theoretical modeling and numerical simulations, we analyze the impact of different measurement bases on fidelity in single-qubit and coupled two-qubit systems. For single-qubit systems, we show that energy bases consistently outperform flux bases in achieving higher fidelity. In coupled two-qubit systems, we explore two measurement models: sequential and simultaneous measurements, both aimed at reading out a single target qubit. Our results indicate that the highest fidelity can be achieved either by performing sequential measurement in a dressed basis over a longer duration or by conducting simultaneous measurement in a bare basis over a shorter duration. Importantly, the sequential measurement model consistently yields more robust and higher fidelity readouts compared to the simultaneous approach. These findings quantify achievable fidelities and provide valuable guidance for optimizing measurement protocols in emerging quantum computing architectures.

💡 Research Summary

This paper investigates how the choice of measurement basis and protocol influences the fidelity of flux‑qubit readout when a quantum flux parametron (QFP) is used as an intermediary amplifier and isolator. The authors first describe the QFP—a controllable rf‑SQUID with a large inductance, high critical current, and a small CJJ loop whose external flux can be adiabatically ramped. By slowly increasing the CJJ flux, the potential barrier of the QFP is raised, converting it from a monostable resonator into a bistable latch that captures the persistent‑current state of a coupled flux qubit. This “annealing” step entangles the qubit with the QFP’s displaced ground‑state wavepackets, after which the combined system can be described by an effective two‑level Hamiltonian
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