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.

Single-qubit measurements efficiently characterise multi-qubit Hamiltonians at the Heisenberg limit

Reconstructing quantum dynamics now requires to be reduced total query time, from eO(N) to widetilde{O}(N), representing a sharp acceleration in understanding N-qubit systems. Employing strong SWAP gates, operations which exchange qubit states, alongside coherent state transfer via a connected reference backbone enabled performance at the Heisenberg limit while matching fundamental lower bounds up to logarithmic factors.