Practical Tomography of Multi-Time Processes

TL;DR

This paper demonstrates that a single qubit ancilla can fully characterize multi-time quantum processes without mid-circuit measurements, offering a resource-efficient approach for process tomography.

Contribution

It proves that one qubit of ancillary memory is sufficient for complete multi-time process tomography, avoiding complex intermediate operations.

Findings

Sequential interactions with one qubit ancilla generate informationally complete probes.

No mid-circuit measurements or reset are needed for full process characterization.

One qubit of ancillary memory suffices for arbitrary multi-time dynamics.

Abstract

Characterising multi-time quantum processes is essential for analysing temporally correlated noise and for designing effective control and mitigation strategies. A complete operational description through multi-time process tomography requires an informationally complete set of probes, which necessarily includes non-deterministic intermediate operations. On present-day quantum devices, such operations are commonly implemented using mid-circuit measurements and reset, which are technologically limited and can introduce noise and overhead in terms of ancilla requirement. In this work, we study the minimal ancillary dimension required for complete characterisation of multi-time processes. We show that sequential interactions with a single qubit ancilla can generate an informationally complete family of correlated probes for processes of arbitrary length, without requiring mid-circuit measurements or reset. Our result provides a resource-efficient route for complete multi-time process tomography and establishes that one qubit of coherent ancillary memory suffices for full reconstruction of arbitrary multi-time dynamics.