  The subtraction of a single photon from a multimode quantum field is analyzed
as the conditional evolution of an open quantum system. We develop a theory
describing different subtraction schemes in a unified approach and we introduce
the concept of subtraction modes intrinsic to the process. The matching between
the subtraction modes and the modes of the field defines different possible
scenarios for the photon subtraction. In particular, our framework identifies:
the conditions of pure photon subtraction, the quantum states of the field
modes conditioned to the photon subtraction, the mode with the highest fidelity
with a single-photon state when the subtraction is performed on multimode
squeezed light. We use our theory to analyze the photon subtraction from a
highly multimode quantum resource - a train of quantum squeezed or correlated
optical pulses. Performing the photon subtraction optimally on various
multimode light field has the potential to implement a number of quantum
information protocols in a multiplexed and scalable way.
