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Suppose I have a state $|\psi\rangle = U|0\rangle$ where $U$ is a $d$-dimensional unitary sampled uniformly with respect the Haar measure. I'm interested in computing or approximating analytically an average quantity of the form

$$ \mathbb{E}_{U \sim U(d)} \left[ \frac{|\psi_i|^2}{|\psi_i|^2 + |\psi_j|^2} \right] = \int_{U(d)} \frac{|U_{i0}|^2}{|U_{i0}|^2 + |U_{j0}|^2} \mu(dU) \tag{1} $$ for some $0 \leq i,j \leq d-1$ and $\mu(dU)$ denotes the Haar measure. What are possible ways to compute or approximate this?

I figured one possibility is to take the limit $d \rightarrow \infty$ and then use a Gaussian approximation for $\psi_j$ (as explained in the answer to this question, for example). However I am a bit concerned about singularities in this polynomial making it an ill-defined computation somehow.

Are there other approaches? Alternatively a reference for explicit computation of rational polynomials of the form of (1) would be appreciated.

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  • $\begingroup$ Assuming $\sum_i|\psi_i|^2 = 1 \implies |\psi_i|^2>0 \forall i$. Where do you expect to observe a singularity? $\endgroup$
    – Cuhrazatee
    May 12 at 20:24
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    $\begingroup$ That implication is not necessarily true when $d > 1$ $\endgroup$
    – forky40
    May 12 at 21:35

1 Answer 1

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The integrand in the above equation corresponds to an observable. Representing that observable in the Pauli basis can, in general, be performed by $$ \mathcal{O} \mapsto \frac{1}{2^n}\sum_{i}Tr(\mathcal{O}P_i^n)P_i^n$$

Where $P_i^n \in \{I, X, Y, Z\}^n$. Once you've found the set of $P_{\mathcal{O}} = \{P_i^n | Tr(P_i^n\mathcal{O}) \neq 0\}$, then you need to find a set of unitary transformations that map commuting subgroups of $P_G \subset P_{\mathcal{O}}$ to a set of single qubit Z measurements, i.e. $UP_GU^{\dagger} = \{ZI\cdots I,IZI\cdots I,\cdots, I\cdots Z\}$.

For the measure, this corresponds to state preparation choice. You can find circuits that are prepared by a Haar random unitary in qiskit, to perform the expectation value, simple append the state prep circuit with the above unitary transformation and repeatedly measure and gather statistics!

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  • $\begingroup$ "The integrand in the above equation corresponds to an observable. " - it does? I thought that observables $\text{Tr}( O | \psi\rangle\langle \psi|^{\otimes k})$ were $k$-th order polynomial in $\psi_i,\bar{\psi}_i$, not rational. $\endgroup$
    – forky40
    May 12 at 18:12
  • $\begingroup$ Good question. Maybe my reply was too hasty. I have also a question about the form of the rational integrand. If $U_i$ are unitary, then wouldn't $|U_i|^2 = U_i^\dagger U_i = I$? $\endgroup$
    – Cuhrazatee
    May 12 at 20:26
  • $\begingroup$ the $U_{i0}$ is the $i$-th element of the first column of $U$, $U_{i0}$ could be zero for some choices of $i$ $\endgroup$
    – forky40
    May 12 at 21:37

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