Universal logic with encoded spin qubits in silicon

Quantum computation features known examples of hardware acceleration for certain problems, but is challenging to realize because of its susceptibility to small errors from noise or imperfect control. The principles of fault tolerance may enable computational acceleration with imperfect hardware, but they place strict requirements on the character and correlation of errors(1). For many qubit technologies(2-21), some challenges to achieving fault tolerance can be traced to correlated errors arising from the need to control qubits by injecting microwave energy matching qubit resonances. Here we demonstrate an alternative approach to quantum computation that uses energy-degenerate encoded qubit states controlled by nearest-neighbour contact interactions that partially swap the spin states of electrons with those of their neighbours. Calibrated sequences of such partial swaps, implemented using only voltage pulses, allow universal quantum control while bypassing microwave-associated correlated error sources(1,22-28). We use an array of six Si-28/SiGe quantum dots, built using a platform that is capable of extending in two dimensions following processes used in conventional microelectronics(29). We quantify the operational fidelity of universal control of two encoded qubits using interleaved randomized benchmarking(30), finding a fidelity of 96.3% +/- 0.7% for encoded controlled NOT operations and 99.3% +/- .5% for encoded SWAP. The quantum coherence offered by enriched silicon(5-9,16,18,20,22,27,29,31-37), the all-electrical and low-crosstalk-control of partial swap operations(1,22-28) and the configurable insensitivity of our encoding to certain error sources(28,33,34,38) all combine to offer a strong pathway towards scalable fault tolerance and computational advantage.