Authors:
Márcio M. Taddei, Jaime Cariñe, Daniel Martínez, Tania García, Nayda Guerrero, Alastair A. Abbott, Mateus Araújo, Cyril Branciard, Esteban S. Gómez, Stephen P. Walborn, Leandro Aolita, and Gustavo Lima
Abstract:
Models for quantum computation with circuit connections subject to the quantum superposition principle have recently been proposed. In them, 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, has been identified as 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)exponentially large in N. Here, we introduce a promise problem for which the quantum N-switch gives an equivalent computational speedup with target-system dimension as small as 2 regardless of N. We use state-of-the-art multicore 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, and it also demonstrates its usefulness for efficient phase estimation.
Journal: Physical Review X Quantum