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
Mä
2025
Strongly-anharmonic gateless gatemon qubits based on InAs/Al 2D heterostructure
The gatemon qubits, made of transparent super-semi Josephson junctions, typically have even weaker anharmonicity than the opaque AlOx-junction transmons. However, flux-frustrated gatemons
can acquire a much stronger anharmonicity, originating from the interference of the higher-order harmonics of the supercurrent. Here we investigate this effect of enhanced anharmonicity in split-junction gatemon devices based on InAs/Al 2D heterostructure. We find that anharmonicity in excess of 100% can be routinely achieved at the half-integer flux sweet-spot without any need for electrical gating or excessive sensitivity to the offset charge noise. We verified that such „gateless gatemon“ qubits can be driven with Rabi frequencies more than 100 MHz, enabling gate operations much faster than what is possible with traditional gatemons and transmons. Furthermore, by analyzing a relatively high-resolution spectroscopy of the device transitions as a function of flux, we were able to extract fine details of the current-phase relation, to which transport measurements would hardly be sensitive. The strong anharmonicity of our gateless gatemons, along with their bare-bones design, can prove to be a precious resource that transparent super-semi junctions bring to quantum information processing.
14
Mä
2025
Stacked Josephson junctions for quantum circuit applications
Low-loss inductors are essential components in various superconducting circuits, such as qubits or digital electronics. In this study, we investigate highly compact inductors formed
by vertical stacking of Josephson junctions. Our implementation employs multiple layers of aluminum separated by tunnel barriers. Individual stacks are connected by suspended superconducting bridges, which are free of additional dielectric materials and therefore should not contribute significantly to losses. We present implementation details, fabrication results, and device characterization measurements.
Charge Parity Rates in Transmon Qubits with Different Shunting Capacitors
The presence of non-equilibrium quasiparticles in superconducting resonators and qubits operating at millikelvin temperature has been known for decades. One metric for the number of
quasiparticles affecting qubits is the rate of single-electron change in charge on the qubit island (i.e. the charge parity rate). Here, we have utilized a Ramsey-like pulse sequence to monitor changes in the parity states of five transmon qubits. The five qubits have shunting capacitors with two different geometries and fabricated from both Al and Ta. The charge parity rate differs by a factor of two for the two transmon designs studied here but does not depend on the material of the shunting capacitor. The underlying mechanism of the source of parity switching is further investigated in one of the qubit devices by increasing the quasiparticle trapping rate using induced vortices in the electrodes of the device. The charge parity rate exhibited a weak dependence on the quasiparticle trapping rate, indicating that the main source of charge parity events is from the production of quasiparticles across the Josephson junction. To estimate this source of quasiparticle production, we simulate and estimate pair-breaking photon absorption rates for our two qubit geometries and find a similar factor of two in the absorption rate for a background blackbody radiation temperature of T∗∼ 350 mK.
High-Efficiency, Low-Loss Floquet-mode Traveling Wave Parametric Amplifier Characterization and Measurement
Advancing error-corrected quantum computing and fundamental science necessitates quantum-limited amplifiers with near-ideal quantum efficiency and multiplexing capability. However,existing solutions achieve one at the expense of the other. In this work, we experimentally demonstrate the first Floquet-mode traveling-wave parametric amplifier (Floquet TWPA). Fabricated in a superconducting-qubit process, our Floquet TWPA achieves minimal dissipation, quantum-limited noise performance, and broadband operation. Our device exhibits >20-dB amplification over a 3-GHz instantaneous bandwidth, <0.5-dB average in-band insertion loss, and the highest-reported intrinsic quantum efficiency for a TWPA of 92.1±7.6%, relative to an ideal phase-preserving amplifier. When measuring a superconducting qubit, our Floquet TWPA enables a system measurement efficiency of 65.1±5.8%, the highest-reported in a superconducting qubit readout experiment utilizing phase-preserving amplifiers to the best of our knowledge. These general-purpose Floquet TWPAs are suitable for fast, high-fidelity multiplexed readout in large-scale quantum systems and future monolithic integration with quantum processors.[/expand]
The waves-in-space Purcell effect for superconducting qubits
Quantum information processing, especially with quantum error correction, requires both long-lived qubits and fast, quantum non-demolition readout. In superconducting circuits this
leads to the requirement to both strongly couple qubits, such as transmons, to readout modes while also protecting them from associated Purcell decay through the readout port. So-called Purcell filters can provide this protection, at the cost of significant increases in circuit components and complexity. However, as we demonstrate in this work, visualizing the qubit fields in space reveals locations where the qubit fields are strong and cavity fields weak; simply placing ports at these locations provides intrinsic Purcell protection. For a λ/2 readout mode in the `chip-in-tube‘ geometry, we show both millisecond level Purcell protection and, conversely, greatly enhanced Purcell decay (qubit lifetime of 1~μs) simply by relocating the readout port. This method of integrating the Purcell protection into the qubit-cavity geometry can be generalized to other 3D implementations, such as post-cavities, as well as planar geometries. For qubit frequencies below the readout mode this effect is quite distinct from the multi-mode Purcell effect, which we demonstrate in a 3D-post geometry where we show both Purcell protection of the qubit while spoiling the quality factor of higher cavity harmonics to protect against dephasing due to stray photons in these modes.
Scaffold-Assisted Window Junctions for Superconducting Qubit Fabrication
The superconducting qubit is one of the promising directions in realizing fault-tolerant quantum computing (FTQC), which requires many high-quality qubits. To achieve this, it is desirable
to leverage modern semiconductor industry technology to ensure quality, uniformity, and reproducibility. However, conventional Josephson junction fabrication relies mainly on resist-assistant double-angle evaporation, posing integration challenges. Here, we demonstrate a lift-off-free qubit fabrication that integrates seamlessly with existing industrial technologies. This method employs a silicon oxide (SiO2) scaffold to define an etched window with a well-controlled size to form a Josephson junction. The SiO2, which has a large dielectric loss, is etched away in the final step using vapor HF leaving little residue. This Window junction (WJ) process mitigates the degradation of qubit quality during fabrication and allows clean removal of the scaffold. The WJ process is validated by inspection and Josephson junction measurement. The scaffold removal process is verified by measuring the quality factor of the resonators. Furthermore, compared to scaffolds fabricated by plasma-enhanced chemical vapor deposition (PECVD), qubits made by WJ through physical vapor deposition (PVD) achieve relaxation time up to 57μs. Our results pave the way for a lift-off-free qubit fabrication process, designed to be compatible with modern foundry tools and capable of minimizing damage to the substrate and material surfaces.
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.