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?

  • 2
    $\begingroup$ Might depend on the context. Generally, just any state which is relevant to the construction/discussion/... $\endgroup$ Feb 19, 2018 at 7:29

2 Answers 2


What a "fiduciary state" or "fiducial state" is in quantum information and related fields depends on the context. Generally, just any state which is relevant to the construction/discussion/..., such as a reference state chosen, or a particularly simple to prepare state.

  • $\begingroup$ what is the "construction/discussion/" ? $\endgroup$
    – Penguin9
    Aug 12 at 20:24
  • $\begingroup$ The context in which the word is used. $\endgroup$ Aug 13 at 15:53

Fiduciary state: A quantum state that can be reliably reproduced with low variability


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