The Quantum Supremacy Tsirelson Inequality

William Kretschmer

Department of Computer Science, The University of Texas at Austin, Austin, TX 78712, USA

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A leading proposal for verifying near-term quantum supremacy experiments on noisy random quantum circuits is linear cross-entropy benchmarking. For a quantum circuit $C$ on $n$ qubits and a sample $z \in \{0,1\}^n$, the benchmark involves computing $|\langle z|C|0^n \rangle|^2$, i.e. the probability of measuring $z$ from the output distribution of $C$ on the all zeros input. Under a strong conjecture about the classical hardness of estimating output probabilities of quantum circuits, no polynomial-time classical algorithm given $C$ can output a string $z$ such that $|\langle z|C|0^n\rangle|^2$ is substantially larger than $\frac{1}{2^n}$ (Aaronson and Gunn, 2019). On the other hand, for a random quantum circuit $C$, sampling $z$ from the output distribution of $C$ achieves $|\langle z|C|0^n\rangle|^2 \approx \frac{2}{2^n}$ on average (Arute et al., 2019).
In analogy with the Tsirelson inequality from quantum nonlocal correlations, we ask: can a polynomial-time quantum algorithm do substantially better than $\frac{2}{2^n}$? We study this question in the query (or black box) model, where the quantum algorithm is given oracle access to $C$. We show that, for any $\varepsilon \ge \frac{1}{\mathrm{poly}(n)}$, outputting a sample $z$ such that $|\langle z|C|0^n\rangle|^2 \ge \frac{2 + \varepsilon}{2^n}$ on average requires at least $\Omega\left(\frac{2^{n/4}}{\mathrm{poly}(n)}\right)$ queries to $C$, but not more than $O\left(2^{n/3}\right)$ queries to $C$, if $C$ is either a Haar-random $n$-qubit unitary, or a canonical state preparation oracle for a Haar-random $n$-qubit state. We also show that when $C$ samples from the Fourier distribution of a random Boolean function, the naive algorithm that samples from $C$ is the optimal 1-query algorithm for maximizing $|\langle z|C|0^n\rangle|^2$ on average.

Recent quantum supremacy experiments were verified using a statistical test called the "Linear Cross-Entropy Benchmark" (or Linear XEB). This benchmark was chosen because of complexity-theoretic evidence that an efficient quantum algorithm can achieve a higher Linear XEB score than any possible efficient classical algorithm.

We argue that this upper bound on the power of classical algorithms for the Linear XEB is analogous to the Bell inequality in nonlocal correlations: both capture inherent limits on the power of classical information and computation that may be violated in the quantum setting. Motivated by this connection, we ask: what is the quantum supremacy analogue of the Tsirelson inequality? That is, what is the highest Linear XEB score achievable by an efficient quantum algorithm? We give evidence that the naive quantum algorithm for passing the benchmark is essentially optimal in this regard.

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Cited by

[1] Scott Aaronson, "Open Problems Related to Quantum Query Complexity", arXiv:2109.06917.

[2] Nicholas LaRacuente, "Quantum Oracle Separations from Complex but Easily Specified States", arXiv:2104.07247.

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