We present a scalable scheme for executing the error-correction cycle of a monolithic surface-code fabric composed of fast-flux-tunable transmon qubits with nearest-neighbor coupling. An eight-qubit unit cell forms the basis for repeating both the quantum hardware and coherent control, enabling spatial multiplexing. This control uses three fixed frequencies for all single-qubit gates and a unique frequencydetuning pattern for each qubit in the cell. By pipelining the interaction and readout steps of ancilla-based X-and Z-type stabilizer measurements, we can engineer detuning patterns that avoid all second-order transmon-transmon interactions except those exploited in controlled-phase gates, regardless of fabric size. Our scheme is applicable to defect-based and planar logical qubits, including lattice surgery.

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.

Variational quantum eigensolvers offer a small-scale testbed to demonstrate the performance of error mitigation techniques with low experimental overhead. We present successful error mitigation by applying the recently proposed symmetry verification technique to the experimental estimation of the ground-state energy and ground state of the hydrogen molecule. A finely adjustable exchange interaction between two qubits in a circuit QED processor efficiently prepares variational ansatz states in the single-excitation subspace respecting the parity symmetry of the qubit-mapped Hamiltonian. Symmetry verification improves the energy and state estimates by mitigating the effects of qubit relaxation and residual qubit excitation, which violate the symmetry. A full-density-matrix simulation matching the experiment dissects the contribution of these mechanisms from other calibrated error sources. Enforcing positivity of the measured density matrix via scalable convex optimization correlates the energy and state estimate improvements when using symmetry verification, with interesting implications for determining system properties beyond the ground-state energy.Noisy intermediate-scale quantum (NISQ) devices [1], despite lacking layers of quantum error correction (QEC), may already be able to demonstrate quantum advantage over classical computers for select problems [2,3]. In particular, the hybrid quantum-classical variational quantum eigensolver (VQE) [4,5] may have sufficiently low experimental requirements to allow estimation of ground-state energies of quantum systems that are difficult to simulate purely classically [6][7][8][9]. To date, VQEs have been used to study small examples of the electronic structure problem, such as H 2 [10][11][12][13][14][15], HeH+ [4,16], LiH [13][14][15], and BeH 2 [14], as well as exciton systems [17], strongly correlated magnetic models [15], and the Schwinger model [18]. Although these experimental efforts have achieved impressive coherent control of up to 20 qubits, the error in the resulting estimations has remained relatively high due to performance limitations in the NISQ hardware. Consequently, much focus has recently been placed on developing error mitigation techiques that offer order-of-magnitude accuracy improvement without the costly overhead of full QEC. This may be achieved by using known properties of the target state, e.g., by checking known symmetries in a manner inspired by QEC stabilizer measurements [19,20], or by expanding around the experimentally-obtained state via a linear (or higher-order) response framework [21]. The former, termed symmetry verification (SV), is of particular interest because it is comparatively low-cost in terms of required hardware and additional measurements. Other mitigation techniques require understanding the underlying error models of the quantum device, allowing for an extrapolation of the calculation to the zero-error limit [22][23][24], or the summing of multiple calculations to probabilistically cancel errors [23,25,26].In this Rapid ...

A comparative analysis of various reconfigurable and multiband antenna concepts is presented. In order to satisfy the requirements for the advanced systems used in modern wireless and radar applications, different multiband and reconfigurable antennas have been proposed and investigated in the past years. In this paper, these design concepts have been classified into three basic approaches: tunable/switchable antenna integration with radio-frequency switching devices, wideband or multiband antenna integration with tunable filters, and array architectures with the same aperture utilized for different operational modes. Examples of each design approach are discussed along with their inherent benefits and challenges.

We present and demonstrate a general three-step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of post-measurement photons and optimal integration weight functions on two quadratures to maximize the signal-to-noise ratio of non-steady-state homodyne measurement. We derive analytically and demonstrate experimentally that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. We use the proven method to optimally bias a Josephson traveling-wave parametric amplifier and to quantify the different noise contributions in the readout amplification chain.Many protocols in quantum information processing, like quantum error correction 1,2 , require rapid interleaving of qubit gates and measurements. These measurements are ideally nondemolition, fast, and high fidelity.In circuit QED 3-5 , a leading platform for quantum computing, nondemolition readout is routinely achieved by off-resonantly coupling a qubit to a resonator. The qubit-state-dependent dispersive shift of the resonator frequency is inferred by measuring the resonator response to an interrogating pulse using homodyne detection. A key element setting the speed and fidelity of dispersive readout is the quantum efficiency 6 , which quantifies how the signal-to-noise ratio is degraded with respect to the limit imposed by quantum vacuum fluctuations.In recent years, the use of superconducting parametric amplifiers 7-11 as the front end of the readout amplification chain has boosted the quantum efficiency towards unity, leading to readout infidelity on the order of one percent 12,13 in individual qubits. Most recently, the development of traveling-wave parametric amplifiers 14,15 (TWPAs) has extended the amplification bandwidth from tens of MHz to several GHz and with sufficient dynamic range to readout tens of qubits. For characterization and optimization of the amplification chain, the ability to robustly determine the quantum efficiency is an important benchmark.A common method for quantifying the quantum efficiency η that does not rely on calibrated noise sources compares the information obtained in a weak qubit measurement (expressed by the signal-to-noise-ratio SNR) to the dephasing of the qubit (expressed by the decay of the off-diagonal elements of the qubit density matrix) 16,17 , η = SNR 2 4βm , with e −βm = |ρ01(T )| |ρ01(0)| , where T is the measurement duration. Previous experimental work 14,18-20 has been restricted to fast resonators driven under specific symmetry conditions such that information is contained in only one quadrature of the output field and in steady state. To allow in-situ calibration of η in multi-qubit devices under development 21-25 , a method is desirable that does not rely on either of these conditions.In this Letter, we present and demonstrate a general three-step method for extracting the quantum efficiency of linear dispersive readout in cQED. Our method disposes with previous requirements in both the dynamics an...

Simple tuneup of fast two-qubit gates is essential for the scaling of quantum processors. We introduce the sudden variant (SNZ) of the net zero scheme realizing controlled-Z (CZ) gates by flux control of transmon frequency. SNZ CZ gates realized in a multitransmon processor operate at the speed limit of transverse coupling between computational and noncomputational states by maximizing intermediate leakage. Beyond speed, the key advantage of SNZ is tuneup simplicity, owing to the regular structure of conditional phase and leakage as a function of two control parameters. SNZ is compatible with scalable schemes for quantum error correction and adaptable to generalized conditional-phase gates useful in intermediate-scale applications.

No abstract

No abstract