# What is a “fiduciary” quantum state?

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?

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