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
09
Apr
2025
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.
02
Apr
2025
Variational preparation of entangled states in a system of transmon qubits
The conventional method for generating entangled states in qubit systems relies on applying precise two-qubit entangling gates alongside single-qubit rotations. However, achieving high-fidelity
entanglement demands high accuracy in two-qubit operations, requiring complex calibration protocols. In this work, we use a minimally calibrated two-qubit iSwap-like gate, tuned via straightforward parameter optimization (flux pulse amplitude and duration), to prepare Bell states and GHZ states experimentally in systems of two and three transmon qubits. By integrating this gate into a variational quantum algorithm (VQA), we bypass the need for intricate calibration while maintaining high fidelity. Our proposed methodology employs variational quantum algorithms (VQAs) to create the target quantum state through imperfect multiqubit operations. Furthermore, we experimentally demonstrate a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality for Bell states, confirming their high fidelity of preparation.
31
Mä
2025
Implementation and readout of maximally entangled two-qubit gates quantum circuits in a superconducting quantum processor
Besides noticeable challenges in implementing low-error single- and two-qubit quantum gates in superconducting quantum processors, the readout technique and analysis are a key factor
in determining the efficiency and performance of quantum processors. Being able to efficiently implement quantum algorithms involving entangling gates and asses their output is mandatory for quantum utility. In a transmon-based 5-qubit superconducting quantum processor, we compared the performance of quantum circuits involving an increasing level of complexity, from single-qubit circuits to maximally entangled Bell circuits. This comparison highlighted the importance of the readout analysis and helped us optimize the protocol for more advanced quantum algorithms. Here we report the results obtained from the analysis of the outputs of quantum circuits using two readout paradigms, referred to as „multiplied readout probabilities“ and „conditional readout probabilities“. The first method is suitable for single-qubit circuits, while the second is essential for accurately interpreting the outputs of circuits involving two-qubit gates.
25
Mä
2025
Highly efficient microwave memory using a superconducting artificial chiral atom
A microwave memory using a superconducting artificial chiral atom embedded in a one-dimensional open transmission line is theoretically investigated. By applying a coupling field to
a single artificial atom, we modify its dispersion, resulting in a slow probe pulse similar to electromagnetically induced transparency. The single atom’s intrinsic chirality, along with optimal control of the coupling field, enables a storage efficiency exceeding 99% and near-unity fidelity across a broad range of pulse durations. Our scheme provides a feasible pathway toward highly efficient quantum information processing in superconducting circuits.
24
Mä
2025
Scalable architecture for dark photon searches: Superconducting-qubit proof of principle
The dark photon is a well-motivated candidate of dark matter due to its potential to open the window of new physics beyond the Standard Model. A fundamental mass-range-sensitivity dilemma
is always haunting the dark photon searching experiments: The resonant haloscopes have excellent sensitivity but are narrowband, and vice versa for the non-resonant ones. A scalable architecture integrating numerous resonant haloscopes will be a desirable solution to this dilemma. However, even the concept of scalable searching remains rarely explored, due to the size limitation of conventional haloscopes imposed by the dark photon wavelength. Here we propose and demonstrate a novel architecture using superconducting qubits as sub-wavelength haloscope units. By virtue of the scalability of superconducting qubits, it is possible to integrate multiple qubits with different frequencies on a chip-scale device. Furthermore, the frequencies of the qubits can be tuned to extend the searching mass range. Thus, our architectures allow for searching for dark photons in a broad mass range with high sensitivity. As a proof-of-principle experiment, we designed and fabricated a three-qubit chip and successfully demonstrated a scalable dark-photon searching. Our work established constraints on dark photons in the mass range of 15.632 μeV∼15.638 μeV, 15.838 μeV∼15.845 μeV, and 16.463 μeV∼16.468 μeV, simultaneously, and the constraints are much more stringent than the cosmology constraints. Our work can be scaled up in the future to boost the scrutiny of new physics and extended to search for more dark matter candidates, including dark photons, axions and axion-like particles.
20
Mä
2025
Non-Markovian Relaxation Spectroscopy of Fluxonium Qubits
Recent studies have shown that parasitic two-level systems (TLS) in superconducting qubits, which are a leading source of decoherence, can have relaxation times longer than the qubits
themselves. However, the standard techniques used to characterize qubit relaxation is only valid for measuring T1 under Markovian assumptions and could mask such non-Markovian behavior of the environment in practice. Here, we introduce two-timescale relaxometry, a technique to probe the qubit and environment relaxation simultaneously and efficiently. We apply it to high-coherence fluxonium qubits over a frequency range of 0.1-0.4 GHz, which reveals a discrete spectrum of TLS with millisecond lifetimes. Our analysis of the spectrum is consistent with a random distribution of TLS in the aluminum oxide tunnel barrier of the Josephson junction chain of the fluxonium with an average density and electric dipole similar to previous TLS studies at much higher frequencies. Our study suggests that investigating and mitigating TLS in the junction chain is crucial to the development of various types of noise-protected qubits in circuit QED.
19
Mä
2025
Quantifying Trapped Magnetic Vortex Losses in Niobium Resonators at mK Temperatures
Trapped magnetic vortices in niobium can introduce microwave losses in superconducting devices, affecting both niobium-based qubits and resonators. While our group has extensively studied
this problem at temperatures above 1~K, in this study we quantify for the first time the losses driven by magnetic vortices for niobium-based quantum devices operating down to millikelvin temperature, and in the low photon counts regime. By cooling a single interface system a 3-D niobium superconducting cavity in a dilution refrigerator through the superconducting transition temperature in controlled levels of magnetic fields, we isolate the flux-induced losses and quantify the added surface resistance per unit of trapped magnetic flux. Our findings indicate that magnetic flux introduces approximately 2~nΩ/mG at 10~mK and at 6~GHz in high RRR niobium. We find that the decay rate of a 6~GHz niobium cavity at 10~mK which contains a native niobium pentoxide will be dominated by the TLS oxide losses until vortices begin to impact T1 for trapped magnetic field (Btrap) levels of >100~mG. In the absence of the niobium pentoxide, Btrap=~10~mG limits Q0∼~10\textsuperscript{10}, or T1∼~350~ms, highlighting the importance of magnetic shielding and magnetic hygiene in enabling T1>~1~s. We observe that the flux-induced resistance decreases with temperature-yet remains largely field-independent, qualitatively explained by thermal activation of vortices in the flux-flow regime. We present a phenomenological model which captures the salient experimental observations. Scaling our findings to typical transmon qubit dimensions suggests that these 2-D structures could be robust against vortex dissipation up to several hundreds~mG. We are directly addressing vortex losses in transmon qubits made with low RRR Nb films in a separate experimental study.