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
17
Jun
2025
Tunable Hybrid-Mode Coupler Enabling Strong Interactions between Transmons at Centimeter-Scale Distance
The transmon, a fabrication-friendly superconducting qubit, remains a leading candidate for scalable quantum computing. Recent advances in tunable couplers have accelerated progress
toward high-performance quantum processors. However, extending coherent interactions beyond millimeter scales to enhance quantum connectivity presents a critical challenge. Here, we introduce a hybrid-mode coupler exploiting resonator-transmon hybridization to simultaneously engineer the two lowest-frequency mode, enabling high-contrast coupling between centimeter-scale transmons. For a 1-cm coupler, our framework predicts flux-tunable XX and ZZ coupling strengths reaching 23 MHz and 100 MHz, with modulation contrasts exceeding 102 and 104, respectively, demonstrating quantitative agreement with an effective two-channel model. This work provides an efficient pathway to mitigate the inherent connectivity constraints imposed by short-range interactions, enabling transmon-based architectures compatible with hardware-efficient quantum tasks.
Fabrication of airbridges with gradient exposure
In superconducting quantum circuits, airbridges are critical for eliminating parasitic slotline modes of coplanar waveguide circuits and reducing crosstalks between direct current magnetic
flux biases. Here, we present a technique for fabricating superconducting airbridges. With this technique, a single layer of photoresist is employed, and the gradient exposure process is used to define the profile of airbridges. In order to properly obtain the bridge profile, we design exposure dosage based on residual photoresist thickness and laser power calibrations. Compared with other airbridge fabrication techniques, the gradient exposure fabrication technique provides the ability to produce lossless superconducting airbridges with flexible size and, thus, is more suitable for large-scale superconducting quantum circuits. Furthermore, this method reduces the complexity of the fabrication process and provides a high fabrication yield.
Broadband merged-element Josephson parametric amplifier
Broadband quantum-limited amplifiers are essential for quantum information processing, yet challenges in design and fabrication continue to hinder their widespread applications. Here,
we introduce the broadband merged-element Josephson parametric amplifier in which the discrete parallel capacitor is directly integrated with the Josephson junctions. This merged-element design eliminates the shortcomings of discrete capacitors, simplifying the fabrication process, reducing the need for high-precision lithography tools, and ensuring compatibility with standard superconducting qubit fabrication procedures. Experimental results demonstrate a gain of 15 dB over a 500 MHz bandwidth, a mean saturation power of -116 dBm and near-quantum-limited noise performance. This robust readily implemented parametric amplifier holds significant promise for broader applications in superconducting quantum information and the advancement of quantum computation.
12
Jun
2025
Quantum secret sharing in a triangular superconducting quantum network
We present a three-node quantum communication testbed with a triangular topology, each side of the triangle formed by a 1.3-meter-long transmission line. We demonstrate state transfer
and entanglement generation between any two nodes, generate genuine multipartite entangled GHZ states, and implement quantum secret sharing (QSS) of classical information. Our experiments show that the QSS protocol can effectively detect eavesdropping, ensuring the secure sharing of secrets. This device forms a testbed for robust and secure quantum networking, enabling testing of more advanced quantum communication protocols.
11
Jun
2025
Real-time adaptive tracking of fluctuating relaxation rates in superconducting qubits
The fidelity of operations on a solid-state quantum processor is ultimately bounded by decoherence effects induced by a fluctuating environment. Characterizing environmental fluctuations
is challenging because the acquisition time of experimental protocols limits the precision with which the environment can be measured and may obscure the detailed structure of these fluctuations. Here we present a real-time Bayesian method for estimating the relaxation rate of a qubit, leveraging a classical controller with an integrated field-programmable gate array (FPGA). Using our FPGA-powered Bayesian method, we adaptively and continuously track the relaxation-time fluctuations of two fixed-frequency superconducting transmon qubits, which exhibit average relaxation times of approximately 0.17 ms and occasionally exceed 0.5 ms. Our technique allows for the estimation of these relaxation times in a few milliseconds, more than two orders of magnitude faster than previous nonadaptive methods, and allows us to observe fluctuations up to 5 times the qubit’s average relaxation rates on significantly shorter timescales than previously reported. Our statistical analysis reveals that these fluctuations occur on much faster timescales than previously understood, with two-level-system switching rates reaching up to 10 Hz. Our work offers an appealing solution for rapid relaxation-rate characterization in device screening and for improved understanding of fast relaxation dynamics.
10
Jun
2025
Optimizing Superconducting Three-Qubit Gates for Surface-Code Error Correction
Quantum error correction (QEC) is one of the crucial building blocks for developing quantum computers that have significant potential for reaching a quantum advantage in applications.
Prominent candidates for QEC are stabilizer codes for which periodic readout of stabilizer operators is typically implemented via successive two-qubit entangling gates, and is repeated many times during a computation. To improve QEC performance, it is thus beneficial to make the stabilizer readout faster and less prone to fault-tolerance-breaking errors. Here we design a 3-qubit CZZ gate for superconducting transmon qubits that maps the parity of two data qubits onto one measurement qubit in a single step. We find that the gate can be executed in a duration of 35ns with a fidelity of F=99.96%. To optimize the gate, we use an error model obtained from the microscopic gate simulation to systematically suppress Pauli errors that are particularly harmful to the QEC protocol. Using this error model, we investigate the implementation of this 3-qubit gate in a surface code syndrome readout schedule. We find that for the rotated surface code, the implementation of CZZ gates increases the error threshold by nearly 50\% to ≈1.2% and decreases the logical error rate, in the experimental relevant regime, by up to one order of magnitude, compared to the standard CZ readout protocol. We also show that for the unrotated surface code, strictly fault-tolerant readout schedules can be found. This opens a new perspective for below-threshold surface-code error correction, where it can be advantageous to use multi-qubit gates instead of two-qubit gates to obtain a better QEC performance.
09
Jun
2025
High Impedance Granular Aluminum Ring Resonators
Superconducting inductors with impedance surpassing the resistance quantum, i.e., superinductors, are important for quantum technologies because they enable the development of protected
qubits, enhance coupling to systems with small electric dipole moments, and facilitate the study of phase-slip physics. We demonstrate superinductors with densely packed meandered traces of granular aluminum (grAl) with inductances up to 4μH, achieving impedances exceeding 100kΩ in the 4−8GHz range. Ring resonators made with grAl meandered superinductors exhibit quality factors on the order of 105 in the single-photon regime and low non-linearity on the order of tens of Hz. Depending on the grAl resistivity, at 10Hz, we measure frequency noise spectral densities in the range of 102 to 103Hz/Hz‾‾‾√. In some devices, in the single-photon regime, we observe a positive Kerr coefficient of unknown origin. Using more complex fabrication, the devices could be released from the substrate, either freestanding or suspended on a membrane, thereby further improving their impedance by a factor of three.
Broadband and high-precision two-level system loss measurement using superconducting multi-wave resonators
Two-level systems (TLS) are known to be a dominant source of dissipation and decoherence in superconducting qubits. Superconducting resonators provide a convenient way to study TLS-induced
loss due to easier design and fabrication in comparison to devices that include non-linear elements. However, accurately measuring TLS-induced loss in a resonator in the quantum regime is challenging due to low signal-to-noise ratio (SNR) and the temporal fluctuations of the TLS, leading to uncertainties of 30% or more. To address these limitations, we develop a multi-wave resonator device that extends the resonator length from a standard quarter-wave λ/4 to Nλ/4 where N=37 at 6GHz. This design provides two key advantages: the TLS-induced fluctuations are reduced by a factor of N‾‾√ due to spatial averaging over an increased number of independent TLS, and the measurement SNR for a given intra-resonator energy density improves by a factor of N‾‾√. The multi-wave resonator also has fundamental and harmonic resonances that allow one to study the frequency dependence of TLS-induced loss. In this work we fabricate both multi-wave and quarter-wave coplanar waveguide resonators formed from thin-film aluminum on a silicon substrate, and characterize their TLS properties at both 10mK and 200mK. Our results show that the power-dependent TLS-induced loss measured from both types of resonators agree well, with the multi-wave resonators achieving a five-fold reduction in measurement uncertainty due to TLS fluctuations, down to 5%. The Nλ/4 resonator also provides a measure of the fully unsaturated TLS-induced loss due to the improved measurement SNR at low intra-resonator energy densities. Finally, measurements across seven harmonic resonances of the Nλ/4 resonator between 4GHz – 6.5GHz reveals no frequency dependence in the TLS-induced loss over this range.
05
Jun
2025
Full characterization of measurement-induced transitions of a superconducting qubit
Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced
by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive.
The Arm Qubit: A Superconducting Qubit Co-Designed for Coherence and Coupling
We present a superconducting qubit which consists of two strongly coupled modes: one for data storage and one for coupling, allowing faster, higher-fidelity entangling gates and readout.
The use of a dedicated coupling mode allows nonlinear couplings of several hundred MHz between the data mode and other elements, with minimal linear coupling to the data mode. Including decoherence, simulations show that this architecture enables microwave-only CZ gates with an infidelity of 8.6×10−5 in 17 ns and always-on ZZ interaction less than 0.4 kHz. Numerical simulations also show readout with state assignment error of 1×10−4 in 27 ns (assuming quantum efficiency η=0.5), Purcell-limited lifetime of 167 ms without a Purcell filter, and a mechanism to suppress shot-noise dephasing (1/Γϕ=15.8 ms). Single-qubit gate infidelities are below 1×10−5 including decoherence. These beyond experimental state-of-the-art gate and readout fidelities rely only on capacitive coupling between arm qubits, making the arm qubit a promising scalable building block for fault-tolerant quantum computers.