Identifying and ameliorating dominant sources of decoherence are important steps in understanding and improving quantum systems. Here we show that the free induction decay time (T * 2 ) and the Rabi decay rate (Γ Rabi ) of the quantum dot hybrid qubit can be increased by more than an order of magnitude by appropriate tuning of the qubit parameters and operating points. By operating in the spin-like regime of this qubit, and choosing parameters that increase the qubit's resilience to charge noise (which we show is presently the limiting noise source for this qubit), we achieve a Ramsey decay time T * 2 of 177 ns and a Rabi decay time 1/Γ Rabi exceeding 1 µs. We find that the slowest Γ Rabi is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.

Universal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with highfidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of double quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. We further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau-Zener-Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots. INTRODUCTIONSince being proposed theoretically, 1,2 much experimental and theoretical progress has been made towards the development of a scalable quantum-computing architecture using electrically gated semiconductor quantum dot-based spin qubits.3-23 Twoqubit gates are essential, and capacitive coupling has been used in GaAs quantum dot-based spin qubits to demonstrate both conditional singlet-triplet exchange oscillations, 24 and the generation of the entanglement of two neighbouring singlet-triplet qubits. 25 Recently, one-and two-qubit gate operations have been demonstrated in 28 Si-based quantum dot spin qubits, 26,27 harnessing the substantial improvement in coherence time achievable through isotopic purification and the corresponding reduction in nuclear spin density. Improving gate speeds provides an alternative route to realise high fidelity single-and multi-qubit gates, and intensive efforts have been made to realise fast manipulation of semiconductor spin qubits by mixing the spin degrees of freedom with charge degrees of freedom through spin-orbit coupling or the introduction of micromagnets.

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Atomic-scale disorder at the top interface of a Si quantum well is known to suppress the valley splitting. Such disorder may be inherited from the underlying substrate and relaxed buffer growth, but can also arise at the top quantum well interface due to the random SiGe alloy. Here, we perform activation energy (transport) measurements in the quantum Hall regime to determine the source of the disorder affecting the valley splitting. We consider three Si/SiGe heterostructures with nominally identical substrates but different barriers at the top of the quantum well, including two samples with pure-Ge interfaces. For all three samples, we observe a surprisingly strong and universal dependence of the valley splitting on the electron density (Ev∼n 2.7 ) over the entire experimental range (Ev=30-200 µeV). We interpret these results via tight binding theory, arguing that the underlying valley physics is determined mainly by disorder arising from the substrate and relaxed buffer growth. arXiv:1804.01914v2 [cond-mat.mes-hall]

We report anomalous behavior in the energy dispersion of a three-electron doublequantum-dot hybrid qubit and argue that it is caused by atomic-scale disorder at the quantum-well interface. By employing tight-binding simulations, we identify potential disorder profiles that induce behavior consistent with the experiments. The results indicate that disorder can give rise to "sweet spots" where the decoherence caused by charge noise is suppressed, even in a parameter regime where true sweet spots are unexpected. Conversely, "hot spots" where the decoherence is enhanced can also occur. Our results suggest that, under appropriate conditions, interfacial atomic structure can be used as a tool to enhance the fidelity of Si double-dot qubits.