Ancilla-Assisted Quantum Process Tomography

TLDR

Complete and precise characterization of a quantum dynamical process can be achieved via the method of quantum process tomography. We implemented several correlated‑photon methods—including ancilla‑assisted process tomography (AAPT)—to probe unitary, decohering, and polarizing processes, and theoretically identified the conditions under which AAPT is feasible. Our data show that entanglement is not required for process tomography, yet using entangled input states yields superior results compared to separable states.

Abstract

Complete and precise characterization of a quantum dynamical process can be achieved via the method of quantum process tomography. Using a source of correlated photons, we have implemented several methods, each investigating a wide range of processes, e.g., unitary, decohering, and polarizing. One of these methods, ancilla-assisted process tomography (AAPT), makes use of an additional "ancilla system," and we have theoretically determined the conditions when AAPT is possible. Surprisingly, entanglement is not required. We present data obtained using both separable and entangled input states. The use of entanglement yields superior results, however.