Researchers Learn Quantum System with Single Qubit Readout

Quantum dynamics are being reconstructed without requiring direct measurement access to the entire system. Fast single-qubit control and a connected reference backbone demonstrate that one measurable qubit is sufficient to learn all O(N) independent parameters of a bounded-degree two-body Hamiltonian on N qubits at the Heisenberg limit. Key to this process is the use of strong SWAP gates synthesised by quantum signal processing, which enable coherent transfer of states evolving under distant Hamiltonian parameters to the measurable qubit. This transfer requires no prior calibration of the Hamiltonian parameters of the intermediate links. A parallel learning architecture accelerates parameter estimation sharply. Reconstructing quantum…