then be connected to other neurons. This makes it concrete how experimentally complex the network would be to build and operate, including how it could be trained.

The design employs the Cerf-Adami-Kwiat (C-A-K) protocol [@CAK], where a single photon with polarisation and multiple possible spatial modes encodes the quantum state; the scheme falls into the category of hyper-entangling schemes, which entangle different degrees of freedom. One qubit is the polarisation; digital encodings of the spatial mode labels give rise to the others. With four spatial modes this implements 3 qubits, with basis $\ket{0/1}\ket{H/V}\ket{0/1}$, where $H/V$ are two different polarisation states, and the other bits label the four spatial modes. The first bit says whether it is in the top two or bottom two pairs of modes and the last bit whether it is the upper or lower one in one of those pairs. This scheme and related ones such as [@Reck; @Clements] are experimentally viable, theoretically clean and can implement any unitary on a single photon spread out over spatial modes. In such a single photon scenario they do not scale well however. The number of spatial modes grows exponentially in the number of qubits. Thus for larger networks our 