Coogee '26 Talks - Friederike Butt (Aachen)

Friederike Butt presents a talk on code switching for measurement-free quantum computing. The talk begins by motivating the need for universal fault-tolerant gate sets, noting the Eastin-Knill theorem that no single code can have a universal transversal gate set. She proposes using two color codes: a 2D triangular color code (Steane code) and a 3D tetrahedral color code (15-qubit code), which have complementary transversal gates (H and CNOT for 2D, T and CNOT for 3D). By switching between these codes, one can implement a universal transversal gate set. She explains the switching procedure, which involves measuring stabilizers of the target code and applying gauge operators to fix the state. She outlines three conditions for fault-tolerant switching: independence of input state, no uncorrectable errors from stabilizer measurements, and distinguishability of data errors from random projections. She then discusses experimental implementation on a trapped-ion processor, using a morphed 10-qubit code due to qubit limitations, and shows state fidelities for logical states and Bell states. The second half of the talk focuses on measurement-free protocols, where mid-circuit measurements are replaced by coherent operations using auxiliary registers. She describes a circuit for measurement-free code switching, which uses Toffoli gates for syndrome update and coherent feedback. She explains how to scale to higher distances via concatenation, and presents simulation results showing that measurement-free protocols can outperform measurement-based ones for certain noise parameters. She concludes that measurement-free code switching is a promising approach for fault-tolerant quantum computing.