Magic and communication complexity

Abstract

We establish novel connections between magic in quantum circuits and communication complexity. In particular, we show that functions computable with low magic have low communication cost. Our first result shows that the $\mathsf{D}\|$ (deterministic simultaneous message passing) cost of a Boolean function $f$ is at most the number of single-qubit magic gates in a quantum circuit computing $f$ with any quantum advice state. If we allow mid-circuit measurements and adaptive circuits, we obtain an upper bound on the two-way communication complexity of $f$ in terms of the magic + measurement cost of the circuit for $f$. As an application, we obtain magic-count lower bounds of $Ω(n)$ for the $n$-qubit generalized Toffoli gate as well as the $n$-qubit quantum multiplexer. Our second result gives a general method to transform $\mathsf{Q}\|^*$ protocols (simultaneous quantum messages with shared entanglement) into $\mathsf{R}\|^*$ protocols (simultaneous classical messages with shared entanglement) which incurs only a polynomial blowup in the communication and entanglement complexity, provided the referee's action in the $\mathsf{Q}\|^*$ protocol is implementable in constant $T$-depth. The resulting $\mathsf{R}\|^*$ protocols satisfy strong privacy constraints and are $\mathsf{PSM}^*$ protocols (private simultaneous message passing with shared entanglement), where the referee learns almost nothing about the inputs other than the function value. As an application, we demonstrate $n$-bit partial Boolean functions whose $\mathsf{R}\|^*$ complexity is $\mathrm{polylog}(n)$ and whose $\mathsf{R}$ (interactive randomized) complexity is $n^{Ω(1)}$, establishing the first exponential separations between $\mathsf{R}\|^*$ and $\mathsf{R}$ for Boolean functions.