This paper is joint work with Tim Ralph, who assisted with the theoretical component, and Jemery O’Brien and Geoff Pryde, who performed the experimental work. In fact, this paper isn’t so new. It’s been sitting on the arXiv for quite a while now (pre-print available at quant-ph/0411144), but I haven’t had the chance to write a post about it yet.

In my previous post on my paper “Frequency and temporal effects in linear optics quantum computing”, I discussed the importance of photon distinguishability on the operation of linear optics quantum gates (see my previous post for an introduction). In this previous work we considered the effects of input distinguishability, which arises during the production of photons. In reality, while this is a very significant problem worth considering, photon distinguishability can arise in more ways than just during preparation. Specifically, it can arise internally within gates, a phenomena referred to as mode-mismatch. This is one of the most significant problems facing the experimental realization of linear optics quantum computing. In this paper we consider this more general problem of mode-mismatch. We develop a means by which to model it theoretically, and demonstrate this by applying it to a model of the controlled-NOT gate which was experimentally demonstrated at the University of Queensland Quantum Technology Lab.

Using our model for mode-mismatch and experimental data obtained from the laboratory, we were able to use a fitting procedure to estimate the parameters characterizing the magnitude of the mode-mismatch at different locations in the circuit. This is very useful as an experimental diagnostic tool, since it allows us to non-intrusively gain insight into what’s going on inside experimental gates using data measured from the gate. This information, which gives an indication as to where things are going wrong, can then potentially be used to tweak experimental gates and improve their performance.