The circuit class $\mathsf{QAC}^0$ was introduced by Moore (1999) as a model for constant depth quantum circuits where the gate set includes many-qubit Toffoli gates. Proving lower bounds against such circuits is a longstanding challenge in quantum circuit complexity; in particular, showing that polynomial-size $\mathsf{QAC}^0$ cannot compute the parity function has remained an open question for over 20 years. In this work, we identify a notion of the Pauli spectrum of $\mathsf{QAC}^0$ circuits, which can be viewed as the quantum analogue of the Fourier spectrum of classical $\mathsf{AC}^0$ circuits. We conjecture that the Pauli spectrum of $\mathsf{QAC}^0$ circuits satisfies low-degree concentration, in analogy to the famous Linial, Nisan, Mansour theorem on the low-degree Fourier concentration of $\mathsf{AC}^0$ circuits. If true, this conjecture immediately implies that polynomial-size $\mathsf{QAC}^0$ circuits cannot compute parity. We prove this conjecture for the class of depth-$d$, polynomial-size $\mathsf{QAC}^0$ circuits with at most $n^{O(1/d)}$ auxiliary qubits. We obtain new circuit lower bounds and learning results as applications: this class of circuits cannot correctly compute - the $n$-bit parity function on more than $(\frac{1}{2} + 2^{-\Omega(n^{1/d})})$-fraction of inputs, and - the $n$-bit majority function on more than $(\frac{1}{2} + O(n^{-1/4}))$-fraction of inputs. Additionally we show that this class of $\mathsf{QAC}^0$ circuits with limited auxiliary qubits can be learned with quasipolynomial sample complexity, giving the first learning result for $\mathsf{QAC}^0$ circuits. More broadly, our results add evidence that "Pauli-analytic" techniques can be a powerful tool in studying quantum circuits.

翻译：电路类$\mathsf{QAC}^0$由Moore(1999)引入，作为常量深度量子电路的模型，其门集合包含多量子比特Toffoli门。证明此类电路的下界是量子电路复杂性中的一个长期挑战；特别是，证明多项式规模$\mathsf{QAC}^0$无法计算奇偶函数已悬而未决超过20年。在这项工作中，我们提出了$\mathsf{QAC}^0$电路泡利谱的概念，可视为经典$\mathsf{AC}^0$电路傅里叶谱的量子类比。我们推测$\mathsf{QAC}^0$电路的泡利谱满足低度集中性质，这类似于关于$\mathsf{AC}^0$电路傅里叶低度集中的著名Linial-Nisan-Mansour定理。若该猜想成立，将直接意味着多项式规模$\mathsf{QAC}^0$电路无法计算奇偶函数。我们针对深度为$d$、多项式规模且辅助量子比特数不超过$n^{O(1/d)}$的$\mathsf{QAC}^0$电路证明了该猜想。作为应用，我们获得了新的电路下界和学习结果：此类电路无法在超过$(\frac{1}{2} + 2^{-\Omega(n^{1/d})})$比例的输入上正确计算$n$比特奇偶函数，也无法在超过$(\frac{1}{2} + O(n^{-1/4}))$比例的输入上正确计算$n$比特多数函数。此外，我们证明具有有限辅助量子比特的这些$\mathsf{QAC}^0$电路可在拟多项式样本复杂度下被学习，这是首个关于$\mathsf{QAC}^0$电路的学习结果。更广泛地，我们的结果为“泡利分析”技术可作为研究量子电路的有力工具提供了进一步证据。