Universal quantum computation will require qubit technology based on a scalable platform, together with quantum error correction protocols that place strict limits on the maximum infidelities for one-and two-qubit gate operations 1,2 . While a variety of qubit systems have shown high fidelities at the one-qubit level 3-9 , superconductor technologies have been the only solidstate qubits manufactured via standard lithographic techniques which have demonstrated twoqubit fidelities near the fault-tolerant threshold 5 . Silicon-based quantum dot qubits are also amenable to large-scale manufacture and can achieve high single-qubit gate fidelities (exceeding 99.9 %) using isotopically enriched silicon 10-12 . However, while two-qubit gates have been demonstrated in silicon 13-15 , it has not yet been possible to rigorously assess their fidelities using randomized benchmarking, since this requires sequences of significant numbers of qubit operations ( 20) to be completed with non-vanishing fidelity. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80 % to 89 %, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7 % and average Controlled-ROT (CROT) fidelity of 98.0 %. These fidelities are found to be limited by the relatively slow gate times employed here compared with the decoherence times T * 2 of the qubits. Silicon qubit designs employing fast gate operations based on high Rabi frequencies 16-18 , together with advanced pulsing techniques 19 , should therefore enable significantly higher fidelities in the near future.Silicon provides an ideal environment for spin qubits thanks to its compatibility with industrial manufacturing technologies and the near-perfect nuclear-spin vacuum that isotopically enriched 28 Si provides 10,11 . Qubits can be encoded directly on the spins of individual nuclei, donor-bound electrons, or electrons confined in gatedefined quantum dots, or they can be encoded in subspaces provided by two or more spins 12 . Electrostatic gate electrodes allow initialization, readout 23 and, in some cases, manipulation of qubits 24 to be implemented with local electrical pulses. For qubits encoded on single spins, one-qubit gates can be driven using an AC magnetic field to perform electron spin resonance (ESR) directly 8,25 , through an AC electric field produced by a gate electrode combined with the magnetic field gradient from an on-chip micro-magnet 16,17,26 , or with an AC electric field acting on the spin-orbit field 27-29 . In enriched 28 Si devices such one-qubit gates have attained fidelities of 99.9 % or above 18,30,31 .Two-qubit gates, required to complete the universal gate set, are commonly implemented in spin systems as the √ SW AP 24,32 , the C-Phase 13,14 or the CROT 13,15 . While the √ SW AP and the C-Phase gates require fast temporal control of the exchange interaction J, accurately synchronized with spin resonance pulses, the CROT can also be implemented wit...

Silicon quantum dots are considered an excellent platform for spin qubits, partly due to their weak spin-orbit interaction. However, the sharp interfaces in the heterostructures induce a small but significant spin-orbit interaction which degrade the performance of the qubits or, when understood and controlled, could be used as a powerful resource. To understand how to control this interaction we build a detailed profile of the spin-orbit interaction of a silicon metal-oxide-semiconductor double quantum dot system. We probe the derivative of the Stark shift, g-factor and g-factor difference for two single-electron quantum dot qubits as a function of external magnetic field and find that they are dominated by spin-orbit interactions originating from the vector potential, consistent with recent theoretical predictions. Conversely, by populating the double dot with two electrons we probe the mixing of singlet and spin-polarized triplet states during electron tunneling, which we conclude is dominated by momentum-term spin-orbit interactions that varies from 1.85 MHz up to 27.5 MHz depending on the magnetic field orientation. Finally, we exploit the tunability of the derivative of the Stark shift of one of the dots to reduce its sensitivity to electric noise and observe an 80 % increase in T * 2 . We conclude that the tuning of the spin-orbit interaction will be crucial for scalable quantum computing in silicon and that the optimal setting will depend on the exact mode of qubit operations used.Silicon-based spin qubits have attracted attention as candidates for large scale quantum computing thanks to their long coherence times, excellent controllability and fabrication techniques that are well established in the semiconductor industry [1][2][3][4][5][6][7][8][9][10][11][12][13]. Even though being weak compared, for instance, to GaAs, the spin-orbit interaction (SOI) significantly affects the behaviour of silicon spin qubits, especially through the dependency of the SOI on the valley state [12,14,15]. SOI is responsible for effects such as the Stark shift of the electron spin resonance (ESR) frequency, variation of Lande gfactors, and mixing between singlet (S) and polarized triplet (T − ) states [15][16][17][18]. These effects can be harnessed, for instance, to drive the ESR transition electrically via Stark shift or by exploiting the variation in the g-factors to address qubits individually with a global microwave (MW) field [14,[19][20][21]. In contrast, spin-orbit effects such as spin-flip tunneling and strong Stark shift can cause state leakage or increased sensitivity to electric noise [14,16,22]. Hence, understanding and controlling the SOI will be important for spin qubit control in larger arrays of dots in the future [23][24][25].Here, we fully characterize the SOI and demonstrate how we can tune it in a silicon metal-oxide-semiconductor * current address: Microsoft Quantum, (SiMOS) double quantum dot (QD) structure. The structure studied here is shown in Fig. 1b and described in Ref. [24]. We vary the directio...