Quantum process tomography with coherent states

Assembling a complex quantum optical information processor requires precise knowledge of the properties of each of its components, i.e., the ability to predict the effect of the components on an arbitrary input state. This gives rise to a quantum version of the famous "black box problem", which is addressed by means of "quantum process tomography" (QPT). In this presentation, I introduce a new technique for characterizing quantum optical processes based on probing unknown quantum processes with coherent states. The original proposal [M. Lobino et al., Science 322, 563 (2008)] uses a filtered Glauber-Sudarshan decomposition to determine the effect of the process on an arbitrary state. A distinctive feature of our new method is that it obviates the need to filter the Glauber-Sudarshan representations for states. Thus, it significantly simplifies the procedure and enhances its application also to multi-mode and non-trace-preserving processes. We illustrate our findings with a set of examples, in which, by knowing the effect of some of the fundamental quantum optical processes on coherent states, we analytically derive their process tensors in the Fock basis. Moreover, to 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 (as a reasonable physical resource).