A new look at encoded-qubit quantum dot quantum computing in silicon

Teck Seng Koh

Niklas Rohling, Maximilian Rimbach-Russ, Guido Burkard

The valley degree of freedom in the electronic band structure of silicon, graphene, and other materials is often considered to be an obstacle for quantum computing (QC) based on electron spins in quantum dots. Here we show that control over the valley state opens new possibilities for quantum information processing. Combining qubits encoded in the singlet-triplet subspace of spin and valley states allows for universal QC using a universal two-qubit gate directly provided by the exchange interaction. We show how spin and valley qubits can be separated in order to allow for single-qubit rotations.

A Skinner, Bruce E. Kane, Michael Davenport

Nicholas E. Penthorn, Joshua S. Schoenfield, John Rooney +2 more

Abstract Quantum dots in silicon are a promising architecture for semiconductor quantum computing due to a high degree of electric control and compatibility with existing silicon fabrication processes. Although electron charge and spin are prominent methods for encoding the qubit state, valley states in silicon can also store quantum information via valley-orbit coupling with protection against charge noise. By observing coherent oscillations between valley states in a Si/SiGe double quantum dot device tuned to the two-electron charge configuration, we measure the valley energy splitting in both quantum dots individually. We further demonstrate two-axis quantum control of the valley qubit using gated pulse sequences with X and Z rotations occurring within a fast operation time of 300 ps. This control is used to completely map out the surface of the Bloch sphere in a single phase-space plot that is subsequently used for state and process tomography.

R. Mark Wilson

Aaron Weinstein, Matthew D. Reed, Aaron M. Jones +20 more

Abstract 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 28 Si/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% ± 0.5% 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.