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In Giovanetti et al.'s paper "Quantum Random Access Memory" (arXiv:0708.1879) they state:

If the qutrit is initially in the $|wait\rangle$ state, the unitary swaps the state of the qubit in the two $|\text{left}\rangle$-$|\text{right}\rangle$ levels of the qutrit (i.e. $U|0\rangle |\text{wait}\rangle=|f\rangle|\text{left}\rangle$ and $U|1\rangle |\text{wait}\rangle=|f\rangle|\text{right}\rangle$, where $|f\rangle$ is a fiduciary state of the qubit).

What do they mean by "fiduciary state"? The best I could find with Google is Hardy (2001) who states, in reference to a column vector $p=\begin{pmatrix} p_1 & p_2 & \cdots& p_K\end{pmatrix}^T$:

We will call the probability measurements labeled by k = 1 to K used in determining the state the fiducial measurements.

Is a fiducial state just a state in the computational basis? Is it an arbitrary state?

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    $\begingroup$ Might depend on the context. Generally, just any state which is relevant to the construction/discussion/... $\endgroup$ – Norbert Schuch Feb 19 '18 at 7:29

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