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
18
Mä
2025
Protected phase gate for the 0-π qubit using its internal modes
Protected superconducting qubits such as the 0-π qubit promise to substantially reduce physical error rates through a multi-mode encoding. This protection comes at the cost of controllability,
as standard techniques for quantum gates are ineffective. We propose a protected phase gate for the 0-π qubit that utilises an internal mode of the circuit as an ancilla. The gate is achieved by varying the qubit-ancilla coupling via a tunable Josephson element. Our scheme is a modified version of a protected gate proposed by Brooks, Kitaev and Preskill that uses an external oscillator as an ancilla. We find that our scheme is compatible with the protected regime of the 0-π qubit, and does not suffer from spurious coupling to additional modes of the 0-π circuit. Through numerical simulations, we study how the gate error scales with the circuit parameters of the 0-π qubit and the tunable Josephson element that enacts the gate.
17
Mä
2025
Reversing Hydrogen-Related Loss in α-Ta Thin Films for Quantum Device Fabrication
α-Tantalum (α-Ta) is an emerging material for superconducting qubit fabrication due to the low microwave loss of its stable native oxide. However, hydrogen absorption during fabrication,
particularly when removing the native oxide, can degrade performance by increasing microwave loss. In this work, we demonstrate that hydrogen can enter α-Ta thin films when exposed to 10 vol% hydrofluoric acid for 3 minutes or longer, leading to an increase in power-independent ohmic loss in high-Q resonators at millikelvin temperatures. Reduced resonator performance is likely caused by the formation of non-superconducting tantalum hydride (TaHx) precipitates. We further show that annealing at 500°C in ultra-high vacuum (10−8 Torr) for one hour fully removes hydrogen and restores the resonators‘ intrinsic quality factors to ~4 million at the single-photon level. These findings identify a previously unreported loss mechanism in α-Ta and offer a pathway to reverse hydrogen-induced degradation in quantum devices based on Ta and, by extension also Nb, enabling more robust fabrication processes for superconducting qubits.
Optimizing the frequency positioning of tunable couplers in a circuit QED processor to mitigate spectator effects on quantum operations
We experimentally optimize the frequency of flux-tunable couplers in a superconducting quantum processor to minimize the impact of spectator transmons during quantum operations (single-qubit
gates, two-qubit gates and readout) on other transmons. We adapt a popular transmon-like tunable-coupling element, achieving high-fidelity, low-leakage controlled-Z gates with unipolar, fast-adiabatic pulsing only on the coupler. We demonstrate the ability of the tunable coupler to null residual ZZ coupling as well as exchange couplings in the one- and two-excitation manifolds. However, the nulling of these coherent interactions is not simultaneous, prompting the exploration of tradeoffs. We present experiments pinpointing spectator effects on specific quantum operations. We also study the combined effect on the three types of operations using repeated quantum parity measurements.
Low-loss Nb on Si superconducting resonators from a dual-use spintronics deposition chamber and with acid-free post-processing
Magnetic impurities are known to degrade superconductivity. For this reason, physical vapor deposition chambers that have previously been used for magnetic materials have generally
been avoided for making high-quality superconducting resonator devices. In this article, we show by example that such chambers can be used: with Nb films sputtered in a chamber that continues to be used for magnetic materials, we demonstrate compact (3 {\mu}m gap) coplanar waveguide resonators with low-power internal quality factors near one million. We achieve this using a resist strip bath with no post-fabrication acid treatment, which results in performance comparable to previous strip baths with acid treatments. We also find evidence that this improved resist strip bath provides a better surface chemical template for post-fabrication hydrogen fluoride processing. These results are consistent across three Si substrate preparation methods, including a \SI{700}{\celsius} anneal.
Realizing a Symmetry Protected Topological Phase in a Superconducting Circuit
We propose a superconducting quantum circuit whose low-energy degrees of freedom are described by the sine-Gordon (SG) quantum field theory. For suitably chosen parameters,
the circuit
hosts a symmetry protected topological (SPT) phase protected by a discrete ℤ2 symmetry. The ground state of the system is twofold degenerate and exhibits local spontaneous symmetry breaking of the ℤ2 symmetry close to the edges of the circuit, leading to spontaneous localized edge supercurrents. The ground states host Majorana zero modes (MZM) at the edges of the circuit. On top of each of the two ground states, the system exhibits localized bound states at both edges, which are topologically protected against small disorder in the bulk. The spectrum of these boundary excitations should be observable in a circuit-QED experiment with feasible parameter choices.
Mixed spin-boson coupling for qubit readout with suppressed residual shot-noise dephasing
Direct dipole coupling between a two-level system and a bosonic mode describes the interactions present in a wide range of physical platforms. In this work, we study a coupling that
is mixed between two pairs of quadratures of a bosonic mode and a spin. In this setting, we can suppress the dispersive shift while retaining a nonzero Kerr shift, which remarkably results in a cubic relationship between shot noise dephasing and thermal photons in the oscillator. We demonstrate this configuration with a simple toy model, quantify the expected improvements to photon shot-noise dephasing of the spin, and describe an approach to fast qubit readout via the Kerr shift. Further, we show how such a regime is achievable in superconducting circuits because magnetic and electric couplings can be of comparable strength, using two examples: the Cooper pair transistor and the fluxonium molecule.
16
Mä
2025
Performance Stabilization of High-Coherence Superconducting Qubits
Superconducting qubits have been used in the most advanced demonstrations of quantum information processing, and they can be manufactured at-scale using proven semiconductor techniques.
This makes them one of the leading technologies in the race to demonstrate useful quantum computers. Since their initial demonstration, advances in design, fabrication, and materials have extended the timescales over which fragile quantum information can be stored and manipulated on superconducting qubits. Ubiquitous atomic-scale material defects have been identified as a primary cause of qubit energy-loss and decoherence. Here we study transmon qubits that exhibit energy relaxation times exceeding 2.5 ms. Even at these long timescales, our qubit energy loss is dominated by two level systems (TLS). We observe large variations in these energy-loss times that would make it extremely difficult to accurately evaluate and compare qubit fabrication processes and to perform studies that require precise measurements of energy loss. To address this issue, we present a technique for characterizing qubit quality factor. In this method, we apply a slowly varying electric field to TLS near the qubit to stabilize the measured energy relaxation time, enabling us to replace hundreds of hours of measurements with ones that span several minutes.
Performance Characterization of a Multi-Module Quantum Processor with Static Inter-Chip Couplers
Three-dimensional integration technologies such as flip-chip bonding are a key prerequisite to realize large-scale superconducting quantum processors. Modular architectures, in which
circuit elements are spread over multiple chips, can further improve scalability and performance by enabling the integration of elements with different substrates or fabrication processes, by increasing the fabrication yield of completed devices, and by physically separating the qubits onto distinct modules to avoid correlated errors mediated by a common substrate. We present a design for a multi-chip module comprising one carrier chip and four qubit modules. Measuring two of the qubits, we analyze the readout performance, finding a mean three-level state-assignment error of 9×10−3 in 200 ns. We calibrate single-qubit gates and measure a mean simultaneous randomized benchmarking error of 6×10−4, consistent with the coherence times of the qubits. Using a wiring-efficient static inter-module coupler featuring galvanic inter-chip transitions, we demonstrate a controlled-Z two-qubit gate in 100 ns with an error of 7×10−3 extracted from interleaved randomized benchmarking. Three-dimensional integration, as presented here, will continue to contribute to improving the performance of gates and readout as well as increasing the qubit count in future superconducting quantum processors.
15
Mä
2025
Niobium Air Bridges as a Low-Loss Component for Superconducting Quantum Hardware
Scaling up superconducting quantum processors requires a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device
performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting CPW resonators incorporating multiple niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding Qint=8.2×106. Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium compared to conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration into transmon qubits by achieving median qubit lifetimes of T1=51.6μs.
Automatic Characterization of Fluxonium Superconducting Qubits Parameters with Deep Transfer Learning
Accurate determination of qubit parameters is critical for the successful implementation of quantum information and computation applications. In solid state systems, the parameters
of individual qubits vary across the entire system, requiring time consuming measurements and manual fitting processes for characterization. Recent developed superconducting qubits, such as fluxonium or 0-pi qubits, offer improved fidelity operations but exhibit a more complex physical and spectral structure, complicating parameter extraction. In this work, we propose a machine learning (ML)based methodology for the automatic and accurate characterization of fluxonium qubit parameters. Our approach utilized the energy spectrum calculated by a model Hamiltonian with various magnetic fields, as training data for the ML model. The output consists of the essential fluxonium qubit energy parameters, EJ, EC, and EL in Hamiltonian. The ML model achieves remarkable accuracy (with an average accuracy 95.6%) as an initial guess, enabling the development of an automatic fitting procedure for direct application to realistic experimental data. Moreover, we demonstrate that similar accuracy can be retrieved even when the input experimental spectrum is noisy or incomplete, highlighting the model robustness. These results suggest that our automated characterization method, based on a transfer learning approach, provides a reliable framework for future extensions to other superconducting qubits or different solid-state systems. Ultimately, we believe this methodology paves the way for the construction of large-scale quantum processors.