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
15
Mai
2025
Light-Matter Interaction in dispersive Superconducting Circuit QED
It is well known that superconducting waveguides strongly attenuate the propagation of electromagnetic waves with frequencies beyond the superconducting gap. In circuit QED, the interaction
between non-linear charge qubits and superconducting resonators invariably involves the qubit coupling to a large set of resonator modes. So far, strong dispersion effects near and beyond the superconducting-gap have been ignored in quantization models. Rather, it is assumed that the superconducting resonator behaves ideally across the large frequency intervals. We present a quantization approach which includes the superconducting frequency-dependent surface impedance and demonstrate that superconducting dispersion plays a role in determining the effective light-matter interaction cut-off.
13
Mai
2025
Simultaneous sweet-spot locking of gradiometric fluxonium qubits
Efforts to scale up superconducting processors that employ flux-qubits face numerous challenges, among which is the crosstalk created by neighboring flux lines, which are necessary
to bias the qubits at the zero-field and Φ0/2 sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux-bias within Φeff=−3×10−4Φ0 from the target, corresponding to only 15 % degradation in T2,E when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing cross-talk and potentially eliminating the need for local flux bias.
High-contrast interaction between remote superconducting qubits mediated by multimode cable coupling
Superconducting quantum processors offer a promising path towards practical quantum computing. However, building a fault-tolerant quantum computer with millions of superconducting qubits
is hindered by wiring density, packaging constraints and fabrication yield. Interconnecting medium-scale processors via low-loss superconducting links provides a promising alternative. Yet, achieving high-fidelity two-qubit gates across such channels remains difficult. Here, we show that a multimode coaxial cable can mediate high-contrast interaction between spatially separated super-conducting qubits. Leveraging interference between cable modes, we can implement high-fidelity controlled-Z and ZZ-free iSWAP gates by simply modulating qubit frequencies. Numerical simulations under realistic coherence and coupling parameters predict fidelities above 99% for both gate schemes. Our approach provides a versatile building block for modular superconducting architectures and facilitates distributed quantum error correction and large-scale fault-tolerant quantum computing.
CMOS-Compatible, Wafer-Scale Processed Superconducting Qubits Exceeding Energy Relaxation Times of 200us
We present the results of an industry-grade fabrication of superconducting qubits on 200 mm wafers utilizing CMOS-established processing methods. By automated waferprober resistance
measurements at room temperature, we demonstrate a Josephson junction fabrication yield of 99.7% (shorts and opens) across more than 10000 junctions and a qubit frequency prediction accuracy of 1.6%. In cryogenic characterization, we provide statistical results regarding energy relaxation times of the qubits with a median T1 of up to 100 us and individual devices consistently approaching 200 us in long-term measurements. This represents the best performance reported so far for superconducting qubits fabricated by industry-grade, wafer-level subtractive processes.
12
Mai
2025
Recovery dynamics of a gap-engineered transmon after a quasiparticle burst
Ionizing radiation impacts create bursts of quasiparticle density in superconducting qubits. These bursts severely degrade qubit coherence for a prolonged period of time and can be
detrimental for quantum error correction. Here, we experimentally resolve quasiparticle bursts in 3D gap-engineered transmon qubits by continuously monitoring qubit transitions. Gap engineering allowed us to reduce the burst detection rate by a factor of a few. This modest reduction falls several orders of magnitude short of the reduction expected if the quasiparticles quickly thermalize to the cryostat temperature. We associate the limited effect of gap engineering with the slow thermalization of the phonons in our chips after the burst.
10
Mai
2025
Protected Symmetrical Superconducting Qubit Based on Quantum Flux Parametron
Conventional Quantum Flux Parametrons (QFPs) have historically been used for storing classical bits in Josephson junction-based computers. In this work, we propose a novel QFP-based
topology dubbed „Degenerium“ qubit, to process and compute quantum information. Degenerium combines principles from the 0-π qubit and flux qubit to create ideally degenerate quantum ground states, while significantly simplifying the 0-π qubit structure. The symmetrical design of Degenerium enables easier qubit control and fabrication. We demonstrate that due to the inherent symmetry of Degenerium, our designed qubit is insensitive to fabrication-induced variations in critical current (Ic) of the Josephson junctions. Our calculations of depolarization and dephasing rates due to charge, flux, and critical current noise sources result in depolarization and dephasing times of 1.25 s and 90 μs, respectively. Further parameter tuning and optimization is possible to meet specific application demands.
09
Mai
2025
Quantitative calibration of a TWPA applied to an optomechanical platform
In the last decade, the microwave quantum electronics toolbox has been enriched with quantum limited detection devices such as Traveling Wave Parametric Amplifiers (TWPAs). The extreme
sensitivity they provide is not only mandatory for some physics applications within quantum information processing, but is also the key element that will determine the detection limit of quantum sensing setups. In the framework of microwave optomechanical systems, an unprecedented range of small motions and forces is accessible, for which a specific quantitative calibration becomes necessary. We report on near quantum-limited measurements performed with an aluminum drumhead mechanical device within the temperature range 4 mK – 400 mK. The whole setup is carefully calibrated, especially taking into account the power-dependence of microwave absorption in the superconducting optomechanical cavity. This effect is commonly attributed to Two-Level-Systems (TLSs) present in the metal oxide. We demonstrate that a similar feature exists in the TWPA, and can be phenomenologically fit with adapted expressions. The power and temperature dependence is studied over the full parameter range, leading to an absolute definition of phonon population (i.e. Brownian motion amplitude), with an uncertainty +-20 %.
Non-degenerate pumping of superconducting resonator parametric amplifier with evidence of phase-sensitive amplification
Superconducting resonator parametric amplifiers are potentially important components for a wide variety of fundamental physics experiments and utilitarian applications. We propose and
realise an operation scheme that achieves amplification through the use of non-degenerate pumps, which addresses two key challenges in the design of parametric amplifiers: non-continuous gain across the band over which amplification is possible, and pump-tone removal. We have experimentally demonstrated the non-degenerate pumping scheme using a half-wave resonator amplifier based on NbN thin-film, and measured peak gain of 26 dB and 3-dB bandwidth of 0.5 MHz. The two non-degenerate pump tones were positioned ~10 bandwidths above and below the frequency at which peak gain occurs. We have found the non-degenerate pumping scheme to be more stable compared to the usual degenerate pumping scheme in terms of gain drift against time, by a factor of 4. This scheme also retains the usual flexibility of NbN resonator parametric amplifiers in terms of reliable amplification in a ~4 K environment, and is suitable for cross-harmonic amplification. The use of pump at different frequencies allows phase-sensitive amplification when the signal tone is degenerate with the idler tone. A gain of 23 dB and squeezing ratio of 6 dB have been measured.
08
Mai
2025
A Circuit-QED Lattice System with Flexible Connectivity and Gapped Flat Bands for Photon-Mediated Spin Models
Quantum spin models are ubiquitous in solid-state physics, but classical simulation of them remains extremely challenging. Experimental testbed systems with a variety of spin-spin interactions
and measurement channels are therefore needed. One promising potential route to such testbeds is provided by microwave-photon-mediated interactions between superconducting qubits, where native strong light-matter coupling enables significant interactions even for virtual-photon-mediated processes. In this approach, the spin-model connectivity is set by the photonic mode structure, rather than the spatial structure of the qubit. Lattices of coplanar-waveguide (CPW) resonators have been demonstrated to allow extremely flexible connectivities and can therefore host a huge variety of photon-mediated spin models. However, large-scale CPW lattices have never before been successfully combined with superconducting qubits. Here we present the first such device featuring a quasi-1D CPW lattice with a non-trivial band structure and multiple transmon qubits. We demonstrate that superconducting-qubit readout and diagnostic techniques can be generalized to this highly multimode environment and observe the effective qubit-qubit interaction mediated by the bands of the resonator lattice. This device completes the toolkit needed to realize CPW lattices with qubits in one or two Euclidean dimensions, or negatively-curved hyperbolic space, and paves the way to driven-dissipative spin models with a large variety of connectivities.
07
Mai
2025
3D-Integrated Superconducting qubits: CMOS-Compatible, Wafer-Scale Processing for Flip-Chip Architectures
In this article, we present a technology development of a superconducting qubit device 3D-integrated by flip-chip-bonding and processed following CMOS fabrication standards and contamination
rules on 200 mm wafers. We present the utilized proof-of-concept chip designs for qubit- and carrier chip, as well as the respective front-end and back-end fabrication techniques. In characterization of the newly developed microbump technology based on metallized KOH-etched Si-islands, we observe a superconducting transition of the used metal stacks and radio frequency (RF) signal transfer through the bump connection with negligible attenuation. In time-domain spectroscopy of the qubits we find high yield qubit excitation with energy relaxation times of up to 15 us.