Direct characterization of arbitrary multi-particle states with ancilla-only measurements

The measurement of unknown quantum states is crucial in quantum information science. Standard quantum state tomography can retrieve information regarding unknown quantum states. However, as the number of qubits increases, the measurement basis required for standard tomography increases exponentially. When we are only interested in partial information of the quantum state, such as coherence and entanglement, global state reconstruction is not necessary. Therefore, direct characterization schemes are required to obtain the information of interest without complete state tomography. In this Letter, we present a circuit scheme with ancilla-only measurements for the direct characterization of multi-particle states that requires only four projection probabilities, independent of the system size. Ancilla-only measurements refer to a strategy where all readouts are on ancillas, never directly on the target qubits. Target state information is indirectly extracted via target–ancilla coupling. Moreover, our scheme can automatically reset the target qubits, thereby providing an immediately available quantum register for subsequent tasks and avoiding the target quantum system initialization required for the next task. Finally, numerical simulations and experimental verification were performed on a quantum cloud platform, confirming the validity of the theoretical scheme.