A stepped-impedance low-pass filter with integrated hollow waveguide absorbers is presented. The filter combines low insertion loss in the passband with strong attenuation at high frequencies, making it well suited for superconducting quantum computing applications, where qubits are sensitive to both near-band and far out-of-band radiation. The structure is implemented in a rectangular coaxial geometry, with inductive sections coupled to circular hollow waveguides oriented orthogonally to the transmission axis. Above their cutoff frequency, these waveguides efficiently couple to radiation inside the stepped-impedance filter, absorbing energy that would otherwise cause Cooper pair breaking in conventional superconductors. Optimal dimensions were obtained using a differential evolution (DE) algorithm applied to interpolated electromagnetic simulation data. A prototype was fabricated and characterized using a calibrated vector network analyzer (VNA) up to 67 GHz. Measurements confirm a 3-dB cutoff frequency at 13.7 GHz, insertion loss below 0.4 dB for frequencies under 8 GHz, and more than 48.5-dB rejection above 17.3 GHz. The design offers a compact, low-loss solution for near-band filtering and suppression of quasiparticle-generating radiation in cryogenic quantum systems.

Abstract On-demand qubit-state initialization is a prerequisite for quantum computation. We demonstrate such a protocol in a device consisting of fixed-frequency transmon qubits pair-wise coupled via tunable couplers — an architecture that is also compatible with the surface code. We use tunable couplers to transfer any undesired qubit excitation to the readout resonator of the qubit, from which this excitation decays into the feedline. In total, the combination of multi-level qubit reset, leakage reduction, and coupler reset takes only 88 ns to complete. Our reset scheme is fast, unconditional, and achieves fidelities above 99%, thus enabling fixed-frequency qubit architectures as future implementations of fault-tolerant quantum computers.

It is advantageous for any quantum processor to support different classes of two-qubit quantum logic gates when compiling quantum circuits, a property that is typically not present in existing platforms. In particular, access to a gate set that includes support for the CZ-, iSWAP-, and SWAP-type families of gates renders conversions between these gate families unnecessary during compilation, as any two-qubit Clifford gate can be executed using at most one two-qubit gate from this set, plus additional single-qubit gates. We experimentally demonstrate that a SWAP gate can be decomposed into one iSWAP gate followed by one CZ gate, affirming a more efficient compilation strategy over the conventional approach that relies on three iSWAP or three CZ gates to replace a SWAP gate. Our implementation makes use of a superconducting quantum processor design based on fixed-frequency transmon qubits coupled together by a parametrically modulated tunable transmon coupler, extending this platform’s native gate set so that any two-qubit Clifford unitary matrix can be realized using no more than two two-qubit gates and single-qubit gates.

Flux-tunable qubits and couplers are common components in superconducting quantum processors. However, dynamically controlling these elements via current pulses poses challenges due to distortions and transients in the propagating signals. In particular, long-time transients can persist, adversely affecting subsequent qubit control operations. We model the flux control line as a first-order RC circuit and introduce a class of pulses designed to mitigate long-time transients. We theoretically demonstrate the robustness of these pulses against parameter mischaracterization and provide experimental evidence of their effectiveness in mitigating transients when applied to a flux-tunable qubit coupler. The proposed pulse design offers a practical solution for mitigating long-time transients, enabling efficient and reliable experiment tune-ups without requiring detailed flux line characterization.

We investigate the required peripheral circuits to enable ideal performance for a high-gain travelling-wave parametric amplifier (TWPA) based on three-wave mixing (3WM). By embedding the TWPA in a network of superconducting diplexers, hybrid couplers and impedance matching networks, the amplifier can deliver a high stable gain with near-quantum-limited noise performance, with suppressed gain ripples, while eliminating the reflections of the signal, the idler and the pump as well as the transmission of all unwanted tones. We also demonstrate a configuration where the amplifier can isolate. We call this technique Wideband Idler Filtering (WIF). The theory is supported by simulations that predict over 20 dB gain in the 4-8 GHz band with 10 dB isolation for a single amplifier and 30 dB isolation for two cascaded amplifiers. We demonstrate how the WIF-TWPAs can be used to construct controllable isolators with over 40 dB isolation over the full 4-8 GHz band.

Detecting quasiparticle tunneling events in superconducting circuits provides information about the population and dynamics of non-equilibrium quasiparticles. Such events can be detected by monitoring changes in the frequency of an offset-charge-sensitive superconducting qubit. This monitoring has so far been performed by Ramsey interferometry assisted by a readout resonator. Here, we demonstrate a quasiparticle detector based on a superconducting qubit directly coupled to a waveguide. We directly measure quasiparticle number parity on the qubit island by probing the coherent scattering of a microwave tone, offering simplicity of operation, fast detection speed, and a large signal-to-noise ratio. We observe tunneling rates between 0.8 and $7~\rm{s}^{-1}$, depending on the average occupation of the detector qubit, and achieve a temporal resolution below $10~\mu\rm{s}$ without a quantum-limited amplifier. Our simple and efficient detector lowers the barrier to perform studies of quasiparticle population and dynamics, facilitating progress in fundamental science, quantum information processing, and sensing.

We characterise a small footprint travelling-wave parametric amplifier (TWPA). The TWPA is built with magnetically flux-tunable superconducting nonlinear asymmetric inductive elements (SNAILs) and parallel-plate capacitors. It implements three-wave mixing (3WM) with resonant phase matching (RPM), a small cutoff frequency for high gain per unitcell and impedance matching networks for large bandwidth impedance matching. The device has 200 unitcells and a physical footprint of only 1.1 mm^2, yet demonstrates an average parametric gain of 19 dB over a 3 GHz bandwidth, an average effective signal-to-noise ratio improvement of 10 dB and a clear speedup of qubit readout time.

Quantum processors require a signal-delivery architecture with high addressability (low crosstalk) to ensure high performance already at the scale of dozens of qubits. Signal crosstalk causes inadvertent driving of quantum gates, which will adversely affect quantum-gate fidelities in scaled-up devices. Here, we demonstrate packaged flip-chip superconducting quantum processors with signal-crosstalk performance competitive with those reported in other platforms. For capacitively coupled qubit-drive lines, we find on-resonant crosstalk better than -27 dB (average -37 dB). For inductively coupled magnetic-flux-drive lines, we find less than 0.13 % direct-current flux crosstalk (average 0.05 %). These observed crosstalk levels are adequately small and indicate a decreasing trend with increasing distance, which is promising for further scaling up to larger numbers of qubits. We discuss the implication of our results for the design of a low-crosstalk, on-chip signal delivery architecture, including the influence of a shielding tunnel structure, potential sources of crosstalk, and estimation of crosstalk-induced qubit-gate error in scaled-up quantum processors.

The realization of fault-tolerant quantum computing requires the execution of quantum error-correction (QEC) schemes, to mitigate the fragile nature of qubits. In this context, to ensure the success of QEC, a protocol capable of implementing both qubit reset and leakage reduction is highly desirable. We demonstrate such a protocol in an architecture consisting of fixed-frequency transmon qubits pair-wise coupled via tunable couplers -- an architecture that is compatible with the surface code. We use tunable couplers to transfer any undesired qubit excitation to the readout resonator of the qubit, from which this excitation decays into the feedline. In total, the combination of qubit reset, leakage reduction, and coupler reset takes only 83ns to complete. Our reset scheme is fast, unconditional, and achieves fidelities well above 99%, thus enabling fixed-frequency qubit architectures as future implementations of fault-tolerant quantum computers. Our protocol also provides a means to both reduce QEC cycle runtime and improve algorithmic fidelity on quantum computers.

In superconducting quantum processors, the predictability of device parameters is of increasing importance as many labs scale up their systems to larger sizes in a 3D-integrated architecture. In particular, the properties of superconducting resonators must be controlled well to ensure high-fidelity multiplexed readout of qubits. Here we present a method, based on conformal mapping techniques, to predict a resonator's parameters directly from its 2D cross-section, without computationally heavy and time-consuming 3D simulation. We demonstrate the method's validity by comparing the calculated resonator frequency and coupling quality factor with those obtained through 3D finite-element-method simulation and by measurement of 15 resonators in a flip-chip-integrated architecture. We achieve a discrepancy of less than 2% between designed and measured frequencies, for 6-GHz resonators. We also propose a design method that reduces the sensitivity of the resonant frequency to variations in the inter-chip spacing.

A new type of low-pass filter based on a leaky coaxial waveguide is presented. The filter has minimal insertion loss in the passband, while at the same time, high attenuation in the stopband is achieved. Thanks to its arrangement, the filter does not present parasitic leakage paths, so that, unlike conventional resonant filters, the stopband extends to very high frequencies. It is shown that a particular stopband attenuation can be obtained by adding or removing leaking sections. The coupling between the center coaxial structure and the leaking holes is investigated. A prototype is manufactured, and scattering parameters are measured up to 145 GHz. The prototype shows an insertion loss of less than 0.15 dB up to 10 GHz and an attenuation in excess of 60 dB above 70 GHz. The proposed filter is suitable for superconducting quantum computing applications, in which quantum bits are sensitive to radiation with energy high enough to break Cooper pairs.

We investigate the required peripheral circuits to enable ideal performance for a high-gain travelling-wave parametric amplifier (TWPA) based on three-wave mixing (3WM). By embedding the TWPA in a network of superconducting diplexers, hybrid couplers and impedance matching networks, the amplifier can deliver a high stable gain with near-quantum-limited noise performance, with suppressed gain ripples, while eliminating the reflections of the signal, the idler and the pump as well as the transmission of all unwanted tones. We also demonstrate a configuration where the amplifier can isolate. We call this technique Wideband Idler Filtering (WIF). The theory is supported by simulations that predict over 20 dB gain in the 4-8 GHz band with 10 dB isolation for a single amplifier and 30 dB isolation for two cascaded amplifiers. We demonstrate how the WIF-TWPAs can be used to construct controllable isolators with over 40 dB isolation over the full 4-8 GHz band.

The effects of manufacturing tolerances on the quiet zone quality of a plane wave generator have been investigated. Waveguide power dividers required in the passive power distribution network of a plane wave generator have been designed. Monte Carlo simulations of the S-parameters of the designed power splitters have been performed by varying the power dividers' dimensions within manufacturing tolerances assuming a normal distribution. Furthermore, the splitting ratios resulting from the randomization are assumed to be uniformly distributed as the input for evaluating the quiet zone by means of Monte Carlo simulations. Assuming a 20 by 20 squared cm quiet zone, it has been found that for the considered setup that: (i) the phase deviation in the quiet zone never exceeds +/- 4.79 degrees, and (ii) for the amplitude deviation, never exceeds +/- 0.91 dB within a single simulated quiet zone.

A novel type of hybrid measurement facility comprising a chamber antenna array (CAA) inside an overmoded waveguide (WG) is proposed and analyzed numerically. The reflecting walls of the metal rectangular WG are used in conjunction with a CAA to synthesize obliquely incident plane-wave (PW) fields at the device under test (DUT). This enables increased flexibility in emulating almost any PW multipath testing conditions in the WG chamber without the high cost and complexity of classical anechoic measurement systems employing relatively large phase-steered PW generators (PWGs). A modeling framework is proposed that has been used to devise first-order design rules (e.g., the number of independent propagating modes, dimensions of the WG, CAA, and DUT). Afterward, an optimally beamformed CAA example is presented to numerically validate the quality of the on- and off-axis PW fields in the test zone (TZ). This study shows design tradeoffs between the amplitude ripple in the DUT region, the total power focused in this region, the DUT size, and the angle of incidence.

A joint design approach for cointegrated antenna and power amplifier (PA), employing a high-efficiency Doherty PA (DPA) architecture and including a bandpass RF filter, is proposed. This design is realized through the optimally distributed and balanced multiport feeding of the cavity-backed patch antenna element that provides the desired (unique) loading conditions for the main and auxiliary PA branches and tailored power combining. A novelty and advantage of this feeding solution is that each pair of feeding points forms a virtual common feeding center of the radiating element; as a result, the radiation pattern remains power invariant when the port excitations change. A joint optimization of the integrated antenna-DPA transmitter is carried out to enhance the overall performance and maximize the bandwidth. This optimization is demonstrated through an example design for the sub-6 GHz telecommunication applications that target high power efficiency (>50%) at the 6 dB backed-off power levels and require RF-filtering in a compact integrated design. The latter challenge leads to a nonconventional implementation, which generally does not require the filter to be inserted between the antenna and the final output stage of the PA, and can be embedded in the topology with complex-valued source/load impedance values. The results of numerical studies are supported by measurements obtained with the antenna-DPA-filter prototype system.

It has been shown by means of simulations that the rich isotropic multipath and the random line-of-sight channels and other Rician channels in-between can be successfully generated in the same over-the-air hybrid measurement chamber without the use of additional static or moving scatterers.

Multipath fields with ascribed Rician distribution parameters have been simulated in a novel hybrid chamber for over the air characterization of antenna systems and wireless devices. Simulations are based on waveguide mode summations representing a combination of the half-space rich isotropic multipath component and the on-axis line-of-sight component. It has been shown that the Rician distributed channels can be emulated without the use of additional static or moving scatterers.

Optimal loading conditions of a reverberation chamber were investigated. The relevant figure of merit is discussed and defined. Multiple loading configurations were tested and results are reported. A simple model of how loading affects measurement uncertainty was developed and used to find a loading configuration that vastly improve results with regards to the defined figure of merit compared to other configurations.

We investigate the use of a hybrid over-the-air (OTA) measurement chamber to emulate far-field testing conditions. The focus has been on the generation of a plane wave arriving at an arbitrary angle-of-arrival (AoA) to the device-under-test (DUT) without mechanical steering. The chamber consists of an oversized waveguide (WG) and a planar array antenna with adaptive beamforming. A modified linear constrained minimum variance (LCMV) beamforming algorithm is used to form a quiet zone (QZ) in the DUT region by using the reflections from the WG’s metal walls. The LCMV beamformer formulation has been extended to achieve a volumetric QZ with the desired amplitude and phase uniformity. Numerical studies for the QZ size corresponding to an FR1 base station demonstrate promising results with the low levels of amplitude and phase variation. The AoA coverage of this chamber meets 3GPP specification requirements for OTA conformance testing of base stations with active antenna systems.