Quantum process tomography with coherent states

TL;DR

This paper presents an improved method for quantum process tomography using only coherent states, simplifying the process and enabling extensions to complex quantum optical processes with quantifiable accuracy.

Contribution

The authors introduce a new relation that bypasses the Glauber-Sudarshan decomposition, simplifying quantum process characterization and extending applicability to multi-mode and non-trace-preserving processes.

Findings

Method accuracy scales inversely with the square root of photon-number cutoff

Analytic representations of fundamental quantum optical processes are derived

The new relation reduces errors and simplifies process identification

Abstract

We develop an enhanced technique for characterizing quantum optical processes based on probing unknown quantum processes only with coherent states. Our method substantially improves the original proposal [M. Lobino et al., Science 322, 563 (2008)], which uses a filtered Glauber-Sudarshan decomposition to determine the effect of the process on an arbitrary state. We introduce a new relation between the action of a general quantum process on coherent state inputs and its action on an arbitrary quantum state. This relation eliminates the need to invoke the Glauber-Sudarshan representation for states; hence it dramatically simplifies the task of process identification and removes a potential source of error. The new relation also enables straightforward extensions of the method to multi-mode and non-trace-preserving processes. We illustrate our formalism with several examples, in which we derive analytic representations of several fundamental quantum optical processes in the Fock basis. In particular, we introduce photon-number cutoff as a reasonable physical resource limitation and address resource vs accuracy trade-off in practical applications. We show that the accuracy of process estimation scales inversely with the square root of photon-number cutoff.