2020/02/29 by Márcio M. Taddei, Jaime Cariñe, Daniel Martínez +9 · 1 citation
Physics and Astronomy · #quant-ph
paper · pdf · doi:10.1103/prxquantum.2.010320
published as PRX Quantum 2, 010320 (2021) · Main text: 9 pages, 3 figures; total 15 pages, 5 figures
arxiv created 2021/02/09 · arxiv updated 2021/03/17
Models for quantum computation with circuit connections subject to the quantum superposition principle have been recently proposed. There, a control quantum system can coherently determine the order in which a target quantum system undergoes N gate operations. This process, known as the quantum N-switch, is a resource for several information-processing tasks. In particular, it provides a computational advantage -- over fixed-gate-order quantum circuits -- for phase-estimation problems involving N unknown unitary gates. However, the corresponding algorithm requires an experimentally unfeasible target-system dimension (super)exponential in N. Here, we introduce a promise problem for which the quantum N-switch gives an equivalent computational speed-up with target-system dimension as small as 2 regardless of N. We use state-of-the-art multi-core optical-fiber technology to experimentally demonstrate the quantum N-switch with N=4 gates acting on a photonic-polarization qubit. This is the first observation of a quantum superposition of more than N=2 temporal orders, demonstrating its usefulness for efficient phase-estimation.