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
14
Apr
2025
Scalable fluxonium qubit architecture with tunable interactions between non-computational levels
The fluxonium qubit has emerged as a promising candidate for superconducting quantum computing due to its long coherence times and high-fidelity gates. Nonetheless, further scaling
up and improving performance remain critical challenges for establishing fluxoniums as a viable alternative to transmons. A key obstacle lies in developing scalable coupling architectures. In this work, we introduce a scalable fluxonium architecture that enables decoupling of qubit states while maintaining tunable couplings between non-computational states. Beyond the well-studied ZZ crosstalk, we identify that an always-on interaction involving non-computational levels can significantly degrade the fidelities of initialization, control, and readout in large systems, thereby impeding scalability. We demonstrate that this issue can be mitigated by implementing tunable couplings for fluxonium’s plasmon transitions, meanwhile enabling fast, high-fidelity gates with passive ZZ suppression. Furthermore, since fluxonium transitions span multiple frequency octaves, we emphasize the importance of carefully designing coupling mechanisms and parameters to suppress residual interactions.
Cross-talk in superconducting qubit lattices with tunable couplers – comparing transmon and fluxonium architectures
Cross-talk between qubits is one of the main challenges for scaling superconducting quantum processors. Here, we use the density-matrix renormalization-group to numerically analyze
lattices of superconducting qubits from a perspective of many-body localization. Specifically, we compare different architectures that include tunable couplers designed to decouple qubits in the idle state, and calculate the residual ZZ interactions as well as the inverse participation ratio in the computational basis states. For transmon qubits outside of the straddling regime, the results confirm that tunable C-shunt flux couplers are significantly more efficient in mitigating the ZZ interactions than tunable transmons. A recently proposed fluxonium architecture with tunable transmon couplers is demonstrated to also maintain its strong suppression of the ZZ interactions in larger systems, while having a higher inverse participation ratio in the computational basis states than lattices of transmon qubits. Our results thus suggest that fluxonium architectures may feature lower cross talk than transmon lattices when designed to achieve similar gate speeds and fidelities.
11
Apr
2025
Analog Quantum Simulation of Dirac Hamiltonians in Circuit QED Using Rabi Driven Qubits
Quantum simulators hold promise for solving many intractable problems. However, a major challenge in quantum simulation, and quantum computation in general, is to solve problems with
limited physical hardware. Currently, this challenge is tackled by designing dedicated devices for specific models, thereby allowing to reduce control requirements and simplify the construction. Here, we suggest a new method for quantum simulation in circuit QED, that provides versatility in model design and complete control over its parameters with minimal hardware requirements. We show how these features manifest through examples of quantum simulation of Dirac dynamics, which is relevant to the study of both high-energy physics and 2D materials. We conclude by discussing the advantages and limitations of the proposed method.
10
Apr
2025
Localized quasiparticles in a fluxonium with quasi-two-dimensional amorphous kinetic inductors
Disordered superconducting materials with high kinetic inductance are an important resource to generate nonlinearity in quantum circuits and create high-impedance environments. In thin
films fabricated from these materials, the combination of disorder and the low effective dimensionality leads to increased order parameter fluctuations and enhanced kinetic inductance values. Among the challenges of harnessing these compounds in coherent devices are their proximity to the superconductor-insulator phase transition, the presence of broken Cooper pairs, and the two-level systems located in the disordered structure. In this work, we fabricate tungsten silicide wires from quasi-two-dimensional films with one spatial dimension smaller than the superconducting coherence length and embed them into microwave resonators and fluxonium qubits, where the kinetic inductance provides the inductive part of the circuits. We study the dependence of loss on the frequency, disorder, and geometry of the device, and find that the loss increases with the level of disorder and is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap.
Stable and Efficient Charging of Superconducting C-shunt Flux Quantum Batteries
Quantum batteries, as miniature energy storage devices, have sparked significant research interest in recent years. However, achieving rapid and stable energy transfer in quantum batteries
while obeying quantum speed limits remains a critical challenge. In this work, we experimentally optimize the charging process by leveraging the unique energy level structure of a superconducting capacitively-shunted flux qubit, using counterdiabatic pulses in the stimulated Raman adiabatic passage. Compared to previous studies, we impose two different norm constraints on the driving Hamiltonian, achieving optimal charging without exceeding the overall driving strength. Furthermore, we experimentally demonstrate a charging process that achieves the quantum speed limit. In addition, we introduce a dimensionless parameter to unify charging speed and stability, offering a universal metric for performance optimization. In contrast to metrics such as charging power and thermodynamic efficiency, the criterion quantitatively captures the stability of ergentropy while also considering the charging speed. Our results highlight the potential of the capacitively-shunted qubit platform as an ideal candidate for realizing three-level quantum batteries and deliver novel strategies for optimizing energy transfer protocols.
09
Apr
2025
Dissipation and noise in strongly driven Josephson junctions
In circuit quantum electrodynamics systems, the quasiparticle-related losses in Josephson junctions are suppressed due to the gap in the superconducting density of states which is much
higher than the typical energy of a microwave photon. In this work, we show that a strong drive even at frequency lower than the double superconducting gap enables dissipation in the junctions due to photon-assisted breaking of the Cooper pairs. Both the decay rate and noise strength associated with the losses are sensitive to the dc phase bias of the junction and can be tuned in a broad range by the amplitude and the frequency of the external driving field, making the suggested mechanism potentially attractive for designing tunable dissipative elements. Furthermore, pronounced memory effects in the driven Josephson junctions render them perspective for both theoretical and experimental study of non-Markovian physics in superconducting quantum circuits. We illustrate our theoretical findings by studying the spectral properties and the steady state population of a low impedance resonator coupled to the driven Josephson junction.
Noise-Aware Entanglement Generation Protocols for Superconducting Qubits with Impedance-Matched FBAR Transducers
Connecting superconducting quantum processors to telecommunications-wavelength quantum networks is critically necessary to enable distributed quantum computing, secure communications,
and other applications. Optically-mediated entanglement heralding protocols offer a near-term solution that can succeed with imperfect components, including sub-unity efficiency microwave-optical quantum transducers. The viability and performance of these protocols relies heavily on the properties of the transducers used: the conversion efficiency, resonator lifetimes, and added noise in the transducer directly influence the achievable entanglement generation rate and fidelity of an entanglement generation protocol. Here, we use an extended Butterworth-van Dyke (BVD) model to optimize the conversion efficiency and added noise of a Thin Film Bulk Acoustic Resonator (FBAR) piezo-optomechanical transducer. We use the outputs from this model to calculate the fidelity of one-photon and two-photon entanglement heralding protocols in a variety of operating regimes. For transducers with matching circuits designed to either minimize the added noise or maximize conversion efficiency, we theoretically estimate that entanglement generation rates of greater than 160kHz can be achieved at moderate pump powers with fidelities of >90%. This is the first time a BVD equivalent circuit model is used to both optimize the performance of an FBAR transducer and to directly inform the design and implementation of an entanglement generation protocol. These results can be applied in the near term to realize quantum networks of superconducting qubits with realistic experimental parameters.
07
Apr
2025
Fast gates for bit-flip protected superconducting qubits
Superconducting qubits offer an unprecedentedly high degree of flexibility in terms of circuit encoding and parameter choices. However, in designing the qubit parameters one typically
faces the conflicting goals of long coherence times and simple control capabilities. Both are determined by the wavefunction overlap of the qubit basis states and the corresponding matrix elements. Here, we address this problem by introducing a qubit architecture with real-time tunable bit-flip protection. In the first, the `heavy‘ regime, the energy relaxation time can be on the order of hours for fluxons located in two near-degenerate ground states, as recently demonstrated in Ref. [Hassani et al., Nat.~Commun.~14 (2023)]. The second, `light‘ regime, on the other hand facilitates high-fidelity control on nanosecond timescales without the need for microwave signals. We propose two different tuning mechanisms of the qubit potential and show that base-band flux-pulses of around 10 ns are sufficient to realize a universal set of high-fidelity single- and two-qubit gates. We expect that the concept of real-time wavefunction control can also be applied to other hardware-protected qubit designs.
04
Apr
2025
Native-oxide-passivated trilayer junctions for superconducting qubits
Superconducting qubits in today’s quantum processing units are typically fabricated with angle-evaporated aluminum–aluminum-oxide–aluminum Josephson junctions. However,
there is an urgent need to overcome the limited reproducibility of this approach when scaling up the number of qubits and junctions. Fabrication methods based on subtractive patterning of superconductor–insulator–superconductor trilayers, used for more classical large-scale Josephson junction circuits, could provide the solution but they in turn often suffer from lossy dielectrics incompatible with high qubit coherence. In this work, we utilize native aluminum oxide as a sidewall passivation layer for junctions based on aluminum–aluminum-oxide–niobium trilayers, and use such junctions in qubits. We design the fabrication process such that the few-nanometer-thin native oxide is not exposed to oxide removal steps that could increase its defect density or hinder its ability to prevent shorting between the leads of the junction. With these junctions, we design and fabricate transmon-like qubits and measure time-averaged coherence times up to 30 μs at a qubit frequency of 5 GHz, corresponding to a qubit quality factor of one million. Our process uses subtractive patterning and optical lithography on wafer scale, enabling high throughput in patterning. This approach provides a scalable path toward fabrication of superconducting qubits on industry-standard platforms.
03
Apr
2025
An Overview of Josephson Junctions Based QPUs
Quantum processing units (QPUs) based on superconducting Josephson junctions promise significant advances in quantum computing. However, they face critical challenges. Decoherence,
scalability limitations, and error correction overhead hinder practical, fault-tolerant implementations. This paper investigates these issues by exploring both fundamental quantum phenomena and practical engineering challenges. We analyze key quantum mechanical principles such as superposition, entanglement, and decoherence that govern the behavior of superconducting qubits. We also discuss quantum tunneling, Cooper pair formation, and the operational mechanics of Josephson junctions in detail. Additionally, we present a comparative analysis with alternative architectures, including ion trap and photonic systems. This comparison highlights the unique advantages and trade-offs of Josephson junction-based QPUs. Our findings emphasize the critical role of material innovations and optimized control techniques. These advances are essential for mitigating noise and decoherence and for realizing robust, scalable quantum computing.