Why do we use two optical modes per qubit in the KLM scheme?

Question

The original KLM paper says:
The basic states of a bosonic qubit encoded in modes $l_1$ and $l_2$ are $|0ranglemapsto|0rangle_{l_1}|1rangle_{l_2}$ and $|1ranglemapsto|1rangle_{l_1}|0rangle_{l_2}$.
Why couldn't we simply encode qubits as $|0ranglemapsto|0rangle_{l_1}$ and $|1ranglemapsto|1rangle_{l_1}$?

unknown · Answer

Mathematically, both options are ok. You can use "single-rail" or "dual-rail" encoding. Dual-rail has some implementation advantages. Look here for example :
https://arxiv.org/pdf/1103.6071.pdf

Why do we use two optical modes per qubit in the KLM scheme?

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