Rapid Microwave-Only Characterization and Readout of Quantum Dots Using Multiplexed Gigahertz-Frequency Resonators

Superconducting resonators enable fast characterization and readout of mesoscopic quantum devices. Finding ways to perform measurements of interest on such devices using resonators only is therefore of great practical relevance. We report an experimental investigation of an InAs nanowire multiquantum dot device by probing gigahertz resonators connected to the device. First, we demonstrate accurate extraction of the dc conductance from measurements of the high-frequency admittance. Because our technique does not rely on dc calibration, it could potentially obviate the need for dc measurements in semiconductor qubit devices. Second, we demonstrate multiplexed gate sensing and the detection of charge tunneling on microsecond timescales. The gigahertz detection of dispersive resonator shifts allows rapid acquisition of charge stability diagrams, as well as resolving charge tunneling in the device with a signal-to-noise ratio of up to 15 in 1μs. Our measurements show that gigahertz-frequency resonators may serve as a universal tool for fast tuneup and high-fidelity readout of semiconductor qubits.QRD/Kouwenhoven LabQN/Kouwenhoven LabBUS/Quantum Delf

Time-domain characterization and correction of on-chip distortion of control pulses in a quantum processor

We introduce Cryoscope, a method for sampling on-chip baseband pulses used to dynamically control qubit frequency in a quantum processor. We specifically use Cryoscope to measure the step response of the dedicated flux control lines of two-junction transmon qubits in circuit QED processors with the temporal resolution of the room-temperature arbitrary waveform generator producing the control pulses. As a first application, we iteratively improve this step response using optimized real-time digital filters to counter the linear-dynamical distortion in the control line, as needed for high-fidelity repeatable one- and two-qubit gates based on dynamical control of qubit frequency.Accepted Author ManuscriptQCD/DiCarlo LabQuTechQRD/Kouwenhoven LabGeneralQN/DiCarlo La

Radio-Frequency C - V Measurements with Subattofarad Sensitivity

We demonstrate the use of radio-frequency (rf) resonators to measure the capacitance of nanoscale semiconducting devices in field-effect transistor configurations. The rf resonator is attached to the gate or the lead of the device. Consequently, tuning the carrier density in the conducting channel of the device affects the resonance frequency, quantitatively reflecting its capacitance. We test the measurement method on InSb and InAs nanowires at dilution-refrigerator temperatures. The measured capacitances are consistent with those inferred from the periodicity of the Coulomb blockade of quantum dots realized in the same devices. In an implementation of the resonator using an off-chip superconducting spiral inductor we find the measurement sensitivity values reaching down to 75zF/Hz at 1 kHz measurement bandwidth, and noise down to 0.45 aF at 1 Hz bandwidth. We estimate the sensitivity of the method for a number of other implementations. In particular, we predict a typical sensitivity of about 40zF/Hz at room temperature with a resonator composed of off-the-shelf components. Of several proposed applications, we demonstrate two: the capacitance measurement of several identical 80-nm-wide gates with a single resonator, and the field-effect mobility measurement of an individual nanowire with the gate capacitance measured in situ. QRD/Kouwenhoven LabQuTechBUS/Quantum DelftQN/Kouwenhoven La

Variable and Orbital-Dependent Spin-Orbit Field Orientations in an InSb Double Quantum Dot Characterized via Dispersive Gate Sensing

Utilizing dispersive gate sensing (DGS), we investigate the spin-orbit field (BSO) orientation in a many-electron double quantum dot (DQD) defined in an InSb nanowire. While characterizing the interdot tunnel couplings, we find the measured dispersive signal depends on the electron-charge occupancy, as well as on the amplitude and orientation of the external magnetic field. The dispersive signal is mostly insensitive to the external field orientation when a DQD is occupied by a total odd number of electrons. For a DQD occupied by a total even number of electrons, the dispersive signal is reduced when the finite external magnetic field aligns with the effective BSO orientation. This fact enables the identification of BSO orientations for different DQD electron occupancies. The BSO orientation varies drastically between charge transitions, and is generally neither perpendicular to the nanowire nor in the chip plane. Moreover, BSO is similar for pairs of transitions involving the same valence orbital, and varies between such pairs. Our work demonstrates the practicality of DGS in characterizing spin-orbit interactions in quantum dot systems, without requiring any current flow through the device. Qubit Research DivisionQRD/Wimmer GroupBUS/Quantum DelftQRD/Kouwenhoven LabQN/Kouwenhoven LabBUS/TNO STAF

Fast, High-Fidelity Conditional-Phase Gate Exploiting Leakage Interference in Weakly Anharmonic Superconducting Qubits

Conditional-phase (cz) gates in transmons can be realized by flux pulsing computational states towards resonance with noncomputational ones. We present a 40 ns cz gate based on a bipolar flux pulse suppressing leakage (0.1%) by interference and approaching the speed limit set by exchange coupling. This pulse harnesses a built-in echo to enhance fidelity (99.1%) and is robust to long-timescale distortion in the flux-control line, ensuring repeatability. Numerical simulations matching experiment show that fidelity is limited by high-frequency dephasing and leakage by short-timescale distortion.QCD/DiCarlo LabQuTechQCD/Terhal GroupQRD/Kouwenhoven LabGeneralBusiness DevelopmentQuantum ComputingQN/DiCarlo La

Realization of a minimal Kitaev chain in coupled quantum dots

Majorana bound states constitute one of the simplest examples of emergent non-Abelian excitations in condensed matter physics. A toy model proposed by Kitaev shows that such states can arise at the ends of a spinless p-wave superconducting chain1. Practical proposals for its realization2,3 require coupling neighbouring quantum dots (QDs) in a chain through both electron tunnelling and crossed Andreev reflection4. Although both processes have been observed in semiconducting nanowires and carbon nanotubes5–8, crossed-Andreev interaction was neither easily tunable nor strong enough to induce coherent hybridization of dot states. Here we demonstrate the simultaneous presence of all necessary ingredients for an artificial Kitaev chain: two spin-polarized QDs in an InSb nanowire strongly coupled by both elastic co-tunnelling (ECT) and crossed Andreev reflection (CAR). We fine-tune this system to a sweet spot where a pair of poor man’s Majorana states is predicted to appear. At this sweet spot, the transport characteristics satisfy the theoretical predictions for such a system, including pairwise correlation, zero charge and stability against local perturbations. Although the simple system presented here can be scaled to simulate a full Kitaev chain with an emergent topological order, it can also be used imminently to explore relevant physics related to non-Abelian anyons.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QRD/Kouwenhoven LabQRD/Wimmer GroupBUS/Quantum DelftCommunication QuTechQRD/Goswami LabApplied SciencesBUS/TNO STAFFQN/Wimmer GroupQN/Kouwenhoven La