Experimental fault-tolerant code switching

Quantum error correction is essential for mitigating hardware errors in quantum computers by encoding logical information into several physical qubits. However, no single error-correcting code intrinsically supports a fault-tolerant implementation of all the gates needed for universal quantum computing. One approach for addressing this problem is to switch between two suitable error-correcting codes that in combination provide a fault-tolerant universal gate set. Here we present the experimental implementation of fault-tolerant code switching between two different codes in a trapped-ion processor. We switch between the 7-qubit colour code, which features fault-tolerant CNOT and H quantum gates, and the 10-qubit code, which allows for a fault-tolerant T gate implementation. Together, these codes form a complementary universal gate set. We construct logical circuits and prepare 12 different logical states that are not accessible natively in a fault-tolerant way within a single code. Finally, we use code switching to entangle two logical qubits using the full universal gate set in a single logical quantum circuit. Our results experimentally demonstrate a route towards deterministic control over logical qubits with low auxiliary qubit overhead and without relying on the probabilistic preparation of resource states. Quantum error correction is essential for reliable quantum computing, but no single code supports all required fault-tolerant gates. The demonstration of switching between two codes now enables universal quantum computation with reduced overhead.