system, [@GJN11a], [@GJNC11]. However, depending on the complexity of the non-classical state, it may possibly be difficult to determine suitable ancilla systems, and indeed the superposition case was not straightforward. In this paper we present an alternative approach to the embedding that also allows for the derivation of the quantum filter. The extended system forms a non-Markovian system, with the ancilla, system and field initialized in a superposition state. While standard filtering results do not apply, the quantum stochastic methods can nevertheless be applied to determine the quantum filters. In this way, we expand the range of methods that may be applied to derive quantum filters for non-classical states.

The paper is organized as follows. In section \[sec:problem\] the idealized filtering problem to be solved in this paper is formulated. The continuous mode single photon states are defined in Section \[sec:photon-state\]. The master equation for the single photon field state is derived in Section \[sec:master\] using the model presented in Section \[sec:problem\]. This leads naturally to Section \[sec:the-matrix\], where the system is embedded in a larger model, inspired by the approach used in [@HPB04] but differing in the details. This extended system provides a compact and transparent description of the 