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Random unitaries in extremely low depth

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arxiv 2407.07754 v2 pith:FNXMRN66 submitted 2024-07-10 quant-ph cond-mat.str-elcs.CCcs.ITmath.IT

Random unitaries in extremely low depth

classification quant-ph cond-mat.str-elcs.CCcs.ITmath.IT
keywords circuitsunitariesdepthquantumrandomdesignslocalpoly
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We prove that random quantum circuits on any geometry, including a 1D line, can form approximate unitary designs over $n$ qubits in $\log n$ depth. In a similar manner, we construct pseudorandom unitaries (PRUs) in 1D circuits in $\text{poly}(\log n)$ depth, and in all-to-all-connected circuits in $\text{poly}(\log \log n)$ depth. In all three cases, the $n$ dependence is optimal and improves exponentially over known results. These shallow quantum circuits have low complexity and create only short-range entanglement, yet are indistinguishable from unitaries with exponential complexity. Our construction glues local random unitaries on $\log n$-sized or $\text{poly}(\log n)$-sized patches of qubits to form a global random unitary on all $n$ qubits. In the case of designs, the local unitaries are drawn from existing constructions of approximate unitary $k$-designs, and hence also inherit an optimal scaling in $k$. In the case of PRUs, the local unitaries are drawn from existing PRU constructions. Applications of our results include proving that classical shadows with 1D log-depth Clifford circuits are as powerful as those with deep circuits, demonstrating superpolynomial quantum advantage in learning low-complexity physical systems, and establishing quantum hardness for recognizing phases of matter with topological order.

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