Epsilon-nets, unitary designs and random quantum circuits

Abstract: Epsilon-nets and approximate unitary $t$-designs are natural notions that capture properties of unitary operations relevant for numerous applications in quantum information and quantum computing. The former constitute subsets of unitary channels that are epsilon-close to any unitary channel in the diamond norm. The latter are ensembles of unitaries that (approximately) recover Haar averages of polynomials in entries of unitary channels up to order $t$. In this work we establish quantitative connections between these two notions. Specifically, we prove that, for a fixed dimension $d$ of the Hilbert space, unitaries constituting $\delta$-approximate $t$-expanders form $\epsilon$-nets for $t\simeq\frac{d^{5/2}}{\epsilon}$ and $\delta=\left(\frac{\epsilon^{3/2}}{d}\right)^{d^2}$. We also show that $\epsilon$-nets can be used to construct $\delta$-approximate unitary $t$-designs for $\delta= \epsilon t$. Finally, we prove that the degree of an exact unitary $t$-design necessary to obtain an $\epsilon$-net must grow at least fast as $\frac1\epsilon$ (for fixed $d$) and not slower than $d^2$ (for fixed $\epsilon$). This shows near optimality of our result connecting $t$-designs and $\epsilon$-nets. We apply our findings in the context of quantum computing. First, we show that that approximate t-designs can be generated by shallow random circuits formed from a set of universal two-qudit gates in the parallel and sequential local architectures. Our gate sets need not to be symmetric (i.e. contain gates together with their inverses) or consist of gates with algebraic entries. We also show a non-constructive version of the Solovay-Kitaev theorem for general universal gate sets. Our main technical contribution is a new construction of efficient polynomial approximations to the Dirac delta in the space of quantum channels, which can be of independent interest.