We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmonlike device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (~0.8 μs) and dephasing times (~1 μs), exceeding gate operation times by 2 orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces cross talk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.

The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting circuits, from setting qubit transition frequencies and interqubit coupling strengths to the gain of parametric amplifiers for quantum-limited readout. The inductance is either set by tailoring the metal oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices with local current-biased flux lines. JJs based on superconductor-semiconductor hybrids represent a tantalizing all-electric alternative. The gatemon is a recently developed transmon variant that employs locally gated nanowire superconductor-semiconductor JJs for qubit control. Here we go beyond proof-of-concept and demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a suitable platform for building a scalable gatemon-based quantum computer. We show that 2DEG gatemons meet the requirements by performing voltage-controlled single qubit rotations and two-qubit swap operations. We measure qubit coherence times up to ~2 μs, limited by dielectric loss in the 2DEG substrate.

Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si nanowire double quantum dot using a fast pulsed-gate method and dispersive readout. An inhomogeneous dephasing time T2* 0.18 μs exceeds corresponding measurements in III–V semiconductors by more than an order of magnitude, as expected for predominately nuclear-spin-free materials. Dephasing is observed to be exponential in time, indicating the presence of a broadband noise source, rather than Gaussian, previously seen in systems with nuclear-spin-dominated dephasing.

Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic for semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltagecontrolled Z rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors. The scalability and ubiquity of semiconductor technology make it an attractive platform for a quantum processor. Semiconductor qubit devices offer simple and flexible control using voltages on high impedance gate electrodes that readily allow low-power operation at cryogenic temperatures. However, such field effect-based control also makes semiconductor qubits susceptible to electrical charge noise that can strongly degrade the fidelity of gate operations. In both semiconductor charge qubits and spin qubits using exchange coupling, charge noise directly modulates the energy splitting between states, resulting in inhomogeneous dephasing times that are typically only ∼10 times longer than gate operation times [1][2][3][4]. Recently a new semiconductor-based qubit, the gatemon, has been introduced [5,6]. This hybrid qubit is a superconducting transmon qubit that, crucially, features a semiconductor Josephson junction (JJ). Gatemons therefore combine the in situ tunability of a semiconductor with the simple connectivity and operation of transmons [7,8]. Initial experiments measured microsecond dephasing times that far exceeded ∼10 ns gate operation times [6], encouraging further investigation and optimization of this qubit.In this Letter, we explore coherence and gate operations of gatemons in a two-qubit circuit. We study the influence of the distinct gatemon spectrum on coherence and use Ramsey interferometry to precisely probe the stability of the semiconductor JJ. The excellent stability observed together with improved coherence allow for randomized benchmarking of singlequbit gates [9,10], including Z-rotations implemented with gate pulses [11,12]. We also demonstrate coherent capacitive coupling between two gatemons and coherent swap oscillations. Finally, with the implementation of a controlled-phase gate, we demonstrate that semiconductor-based gatemons are conceptually similar to transmons, but with the technological advantage of full voltage control, making them ideally suited for largescale quantum processors.Figure 1(a) shows the two-qubit device. As with conventional transmons, each gatemon operates as an LC oscillator with a nonlinear inductance due to the J...

The distribution of Coulomb blockade peak heights as a function of magnetic field is investigated experimentally in a Ge-Si nanowire quantum dot. Strong spin-orbit coupling in this hole-gas system leads to antilocalization of Coulomb blockade peaks, consistent with theory. In particular, the peak height distribution has its maximum away from zero at zero magnetic field, with an average that decreases with increasing field. Magnetoconductance in the open-wire regime places a bound on the spin-orbit length (l so < 20 nm), consistent with values extracted in the Coulomb blockade regime (l so < 25 nm).