Quantum Process Tomography with Coherent States

In this study, 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 previous coherent-state quantum process tomography scheme in that it naturally circumvents the need for the Glauber- Sudarshan representation for states and the incurred P-function regularization used in the former scheme. This technique relies on 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 not only introduces a substantial simplification in the process identification task but also naturally eliminates a potential source of estimation error associated with the P-function regularization. The new relation also enables straightforward extensions of coherent-state quantum process tomography 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.