I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
13
Mä
2025
Fabrication of Metal Air Bridges for Superconducting Circuits using Two-photon Lithography
Extraneous high frequency chip modes parasitic to superconducting quantum circuits can result in decoherence when these modes are excited. To suppress these modes, superconducting air
bridges (AB) are commonly used to electrically connect ground planes together when interrupted by transmission lines. Here, we demonstrate the use of two-photon photolithography to build a supporting 3D resist structure in conjunction with a lift-off process to create AB. The resulting aluminum AB, have a superconducting transition temperature Tc=1.08 K and exhibit good mechanical strength up to lengths of 100 μm. A measurable amount of microwave loss is observed when 35 AB were placed over a high-Q Ta quarter-wave coplanar waveguide resonator.
Efficient spectrum analysis for multi-junction nonlinear superconducting circuit
The extraction of transition frequencies from a spectrum has conventionally relied on empirical methods, and particularly in complex systems it is a time-consuming and cumbersome process.
To address this challenge, we establish an semi-automated efficient and precise spectrum analysis method. It, at first, employs image processing methods to extract transition frequencies, subsequently estimates Hamiltonians of superconducting quantum circuit containing multiple Josephson junctions. Additionally, we determine the suitable range of approximations in simulation methods, evaluating the physical reliability of analyses.
Sub-resonant wideband superconducting Purcell filters
In superconducting quantum devices, Purcell filters protect qubit information from decaying into external lines by reducing external coupling at qubit frequencies while maintaining
it at readout frequencies. Here, we introduce and demonstrate a novel Purcell filter design that places the readout resonator frequencies in a „linewidth plateau“ below the filter’s first resonant mode. This approach, based on direct admittance engineering, can simultaneously achieve strong qubit protection and nearly constant external coupling across a wide readout bandwidth, addressing the traditional tradeoff between these properties. We first present a lumped-element analysis of our filters. We then experimentally demonstrate a compact on-chip linewidth-plateau filter, coupled to four resonators across its approximately 1 GHz readout band. We compare the measured linewidths to numerical predictions, and show how the filter protects a frequency-tunable transmon qubit from external decay. We envision that our flexible design paradigm will aid in efforts to create multiplexed readout architectures for superconducting quantum circuits, with well-controlled external couplings.
Utilizing discrete variable representations for decoherence-accurate numerical simulation of superconducting circuits
Given the prevalence of superconducting platforms for uses in quantum computing and quantum sensing, the simulation of quantum superconducting circuits has become increasingly important
for identifying system characteristics and modeling their relevant dynamics. Various numerical tools and software packages have been developed with this purpose in mind, typically utilizing the harmonic oscillator basis or the charge basis to represent a Hamiltonian. In this work, we instead consider the use of discrete variable representations (DVRs) to model superconducting circuits. In particular, we use `sinc DVRs‘ of both charge number and phase to approximate the eigenenergies of several prototypical examples, exploring their use and effectiveness in the numerical analysis of superconducting circuits. We find that not only are these DVRs capable of achieving decoherence-accurate simulation, i.e., accuracy at the resolution of experiments subject to decay, decoherence, and dephasing, they also demonstrate improvements in efficiency with smaller basis sizes and better convergence over standard approaches, showing that DVRs are an advantageous alternative for representing superconducting circuits.
Bias-preserving and error-detectable entangling operations in a superconducting dual-rail system
For useful quantum computation, error-corrected machines are required that can dramatically reduce the inevitable errors experienced by physical qubits. While significant progress has
been made in approaching and exceeding the surface-code threshold in superconducting platforms, large gains in the logical error rate with increasing system size remain out of reach. This is due both to the large number of required physical qubits and the need to operate far below threshold. Importantly, by exploiting the biases and structure of the physical errors, this threshold can be raised. Erasure qubits achieve this by detecting certain errors at the hardware level. Dual-rail qubits encoded in superconducting cavities are a promising erasure qubit wherein the dominant error, photon loss, can be detected and converted to an erasure. In these approaches, the complete set of operations, including two qubit gates, must be high performance and preserve as much of the desirable hierarchy or bias in the errors as possible. Here, we design and realize a novel two-qubit gate for dual-rail erasure qubits based on superconducting microwave cavities. The gate is high-speed (∼500 ns duration), and yields a residual gate infidelity after error detection below 0.1%. Moreover, we experimentally demonstrate that this gate largely preserves the favorable error structure of idling dual-rail qubits, making it ideal for error correction. We measure low erasure rates of ∼0.5% per gate, as well as low and asymmetric dephasing errors that occur at least three times more frequently on control qubits compared to target qubits. Bit-flip errors are practically nonexistent, bounded at the few parts per million level. This error asymmetry has not been well explored but is extremely useful in quantum error correction and flag-qubit contexts, where it can create a faster path to effective error-corrected systems.
A Superconducting Qubit-Resonator Quantum Processor with Effective All-to-All Connectivity
In this work we introduce a superconducting quantum processor architecture that uses a transmission-line resonator to implement effective all-to-all connectivity between six transmon
qubits. This architecture can be used as a test-bed for algorithms that benefit from high connectivity. We show that the central resonator can be used as a computational element, which offers the flexibility to encode a qubit for quantum computation or to utilize its bosonic modes which further enables quantum simulation of bosonic systems. To operate the quantum processing unit (QPU), we develop and benchmark the qubit-resonator conditional Z gate and the qubit-resonator MOVE operation. The latter allows for transferring a quantum state between one of the peripheral qubits and the computational resonator. We benchmark the QPU performance and achieve a genuinely multi-qubit entangled Greenberger-Horne-Zeilinger (GHZ) state over all six qubits with a readout-error mitigated fidelity of 0.86.
12
Mä
2025
Quantum Computer Controlled by Superconducting Digital Electronics at Millikelvin Temperature
Current superconducting quantum computing platforms face significant scaling challenges, as individual signal lines are required for control of each qubit. This wiring overhead is a
result of the low level of integration between control electronics at room temperature and qubits operating at millikelvin temperatures, which raise serious doubts among technologists about whether utility-scale quantum computers can be built. A promising alternative is to utilize cryogenic, superconducting digital control electronics that coexist with qubits. Here, we report the first multi-qubit system integrating this technology. The system utilizes digital demultiplexing, breaking the linear scaling of control lines to number of qubits. We also demonstrate single-qubit fidelities above 99%, and up to 99.9%. This work is a critical step forward in realizing highly scalable chip-based quantum computers.
Broad Spectrum Coherent Frequency Conversion with Kinetic Inductance Superconducting Metastructures
arametric frequency converters (PFCs) play a critical role in bridging the frequency gap between quantum information carriers. PFCs in the microwave band are particularly important
for superconducting quantum processors, but their operating bandwidth is often strongly limited. Here, we present a multimode kinetic metastructure for parametric frequency conversion between broadly spanning frequency modes. This device comprises a chain of asymmetric kinetic inductance grids designed to deliver efficient three-wave mixing nonlinearity. We demonstrate high efficient coherent conversion among broadly distributed modes, and the mode frequency is continuously tunable by controlling the external magnetic field strength, making it ideally suited for quantum computing and communication applications requiring flexible and efficient frequency conversion.
Eigen-SNAP gate for photonic qubits in a cavity-transmon system
In the pursuit of robust quantum computing, we put forth a platform based on photonic qubits in a circuit-QED environment. Specifically, we propose a versatile two-qubit gate based
on two cavities coupled via a transmon, constituting a selective number-dependent phase gate operating on the in-phase eigenmodes of the two cavities, the Eigen-SNAP gate. This gate natively operates in the dispersive coupling regime of the cavities and the transmon, and operates by driving the transmon externally, to imprint desired phases on the number states. As an example for the utility of the Eigen-SNAP gate, we implement a SWAP‾‾‾‾‾‾‾√ gate on a system of two logical bosonic qubits encoded in the cavities. Further, we use numerical optimization to determine the optimal implementation of the SWAP‾‾‾‾‾‾‾√. We find that the fidelities of these optimal protocols are only limited by the coherence times of the system’s components. These findings pave the way to continuous variable quantum computing in cavity-transmon systems.
SU(4) gate design via unitary process tomography: its application to cross-resonance based superconducting quantum devices
We present a novel approach for implementing pulse-efficient SU(4) gates on cross resonance (CR)-based superconducting quantum devices. Our method introduces a parameterized unitary
derived from the CR-Hamiltonian propagator, which accounts for static-ZZ interactions. Leveraging the Weyl chamber’s geometric structure, we successfully realize a continuous 2-qubit basis gate, RZZ(θ), as an echo-free pulse schedule on the IBM Quantum device ibm_kawasaki. We evaluate the average fidelity and gate time of various SU(4) gates generated using the RZZ(θ) to confirm the advantages of our implementation.