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
06
Okt
2025
Observation of Genuine Tripartite Non-Gaussian Entanglement from a Superconducting Three-Photon Spontaneous Parametric Down-Conversion Source
The generation of entangled photons through Spontaneous Parametric Down-Conversion (SPDC) is a critical resource for many key experiments and technologies in the domain of quantum optics.
Historically, SPDC was limited to the generation of photon pairs. However, the use of the strong nonlinearities in circuit quantum electrodynamics has recently enabled the observation of Three-Photon SPDC (3P-SPDC). Despite great interest in the entanglement structure of the resultant states, entanglement between photon triplets produced by a 3P-SPDC source has still has not been confirmed. Here, we report on the observation of genuine tripartite non-Gaussian entanglement in the steady-state output field of a 3P-SPDC source consisting of a superconducting parametric cavity coupled to a transmission line. We study this non-Gaussian tripartite entanglement using an entanglement witness built from three-mode correlation functions, and observe a maximum violation of the bound by 23 standard deviations of the statistical noise. Furthermore, we find strong agreement between the observed and the analytically predicted scaling of the entanglement witness. We then explore the impact of the temporal function used to define the photon mode on the observed value of the entanglement witness.
30
Sep
2025
Suppressing leakage and maintaining robustness in transmon qubits: Signatures of a trade-off relation
We study the problem of optimally generating quantum gates in a logical subspace embedded in a larger Hilbert space, where the dynamics is also affected by unknown static imperfections.
This general problem is widespread across various emergent quantum technology architectures. We derive the fidelity susceptibility in the computational subspace as a measure of robustness to perturbations, and define a cost function that quantifies leakage out of the subspace. We tackle both effects using a two-stage optimization where two cost functions are minimized in series. Specifically, we apply this framework to the generation of single-qubit gates in a superconducting transmon system, and find high-fidelity solutions robust to detuning and amplitude errors across various parameter regimes. We also show control pulses which maximize fidelity while minimizing leakage at all times during the evolution. However, finding control solutions that address both effects simultaneously is shown to be much more challenging, indicating the presence of a trade-off relation.
Robust NbN on Si-SiGe hybrid superconducting-semiconducting microwave quantum circuit
Advancing large-scale quantum computing requires superconducting circuits that combine long coherence times with compatibility with semiconductor technology. We investigate niobium
nitride (NbN) coplanar waveguide resonators integrated with Si/SiGe quantum wells, creating a hybrid platform designed for CMOS-compatible quantum hardware. Using temperature-dependent microwave spectroscopy in the single-photon regime, we examine resonance frequency and quality factor variations to probe the underlying loss mechanisms. Our analysis identifies the roles of two-level systems, quasiparticles, and scattering processes, and connects these losses to wafer properties and fabrication methods. The devices demonstrate reproducible performance and stable operation maintained for over two years, highlighting their robustness. These results provide design guidelines for developing low-loss, CMOS-compatible superconducting circuits and support progress toward resilient, scalable architectures for quantum information processing.
Gradiometric, Fully Tunable C-Shunted Flux Qubits
Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the
study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T1 = 25 μs and a frequency tunability range of ∼ 20 GHz at the flux-insensitive sweet spot. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.
29
Sep
2025
Quantum process tomography of a compressed time evolution circuit on superconducting quantum processors
As present day quantum hardware is limited by various noise mechanisms, quantum advantage can only be reached in the near-term by designing noise-resilient quantum algorithms. In this
work, we employ state-of-the-art quantum process tomography (QPT) techniques to characterize the noise channels of IBM quantum processors under realistic runtime constraints. As our main application, we compare the Trotter time-evolution of three- and four-qubit wave functions to a compressed quantum circuit version of the same evolution operator. By analysing the spectral properties of the two process channels, we find that the compressed circuit systematically yields larger eigenvalue moduli, demonstrating better noise resilience.
23
Sep
2025
Characterizing Noise in Controlling Superconducting Qubits
Meaningful quantum computing is currently bottlenecked by the error rates of current generation Noisy Intermediate Scale Quantum (NISQ) devices. To improve the fidelity of the quantum
logic gates, it is essential to recognize the contributions of various sources of errors, including background noise. In this work, we investigate the effects of noise when applied to superconducting qubit control pulses to observe the dependency of the gate fidelity with the signal-to-noise ratio (SNR). We propose a model on how the noise of the control electronics interacts with the qubit system and demonstrate a method for characterizing the noise environment of the qubit control.
Robust quantum communication through lossy microwave links
Entanglement generation lies at the heart of many quantum networking protocols as it enables distributed and modular quantum computing. For superconducting qubits, entanglement fidelity
is typically limited by photon loss in the links that connect these qubits together. We propose and realize a new scheme for heralded entanglement generation that almost entirely circumvents this limit. We produce Bell states with 92±1% state fidelity, including state preparation and measurement (SPAM) errors, between separated superconducting bosonic qubits in a high-loss regime where direct deterministic state transfer fails. Our scheme exploits simple but fundamental physics found in microwave links, specifically the ability to treat our communication channel as a single standing wave mode. Combining this with local measurements on bosonically encoded qubits allows us to herald entanglement with success probabilities approaching the scheme’s upper limit of 50% per attempt. We then use the heralded Bell state as a resource to deterministically teleport a qubit between modules with an average state transfer fidelity of 90±1%. This is achieved despite the link possessing a direct single photon transfer efficiency of 2%. Our work informs the design of future superconducting quantum networks, by demonstrating fast coupling rates and low loss links are no longer strict requirements for high-fidelity quantum communication in the microwave regime.
22
Sep
2025
Accelerated characterization of two-level systems in superconducting qubits via machine learning
We introduce a data-driven approach for extracting two-level system (TLS) parameters-frequency ωTLS, coupling strength g, dissipation time TTLS,1, and the pure dephasing time TϕTLS,2,
labelled as a 4-component vector q⃗ , directly from simulated spectroscopy data generated for a single TLS by a form of two-tone spectroscopy. Specifically, we demonstrate that a custom convolutional neural network model(CNN) can simultaneously predict ωTLS, g, TTLS,1 and TϕTLS,2 from the spectroscopy data presented in the form of images. Our results show that the model achieves superior performance to perturbation theory methods in successfully extracting the TLS parameters. Although the model, initially trained on noise-free data, exhibits a decline in accuracy when evaluated on noisy images, retraining it on a noisy dataset leads to a substantial performance improvement, achieving results comparable to those obtained under noise-free conditions. Furthermore, the model exhibits higher predictive accuracy for parameters ωTLS and g in comparison to TTLS,1 and TϕTLS,2.
Noise Protected Logical Qubit in a Open Chain of Superconducting Qubits with Ultrastrong Interactions
To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series
of superconducting qubits that alternate between XX and YY ultrastrong interactions. By considering the two-lowest energy eigenstates of this system as a {\it logical} qubit, we demonstrate that its coherence is significantly enhanced: both its pure dephasing and relaxation times are extended beyond those of individual {\it physical} qubits.
Specifically, we show that by increasing either the interaction strength or the number of physical qubits in the chain, the logical qubit’s pure dephasing rate is suppressed to zero, and its relaxation rate is reduced to half the relaxation rate of a single physical qubit. Single qubit and two-qubit gates can be performed with a high fidelity.
Analysis of polymerized superconducting circuits
We apply polymer quantization, a quantization technique sometimes used in high energy physics, to several superconducting circuits including: transmons, transmission line resonators,
and LC circuits. In the case of transmon qubits and transmission line resonators, experimental predictions are very close to what is found with canonical quantization, though in this approach constant charge offsets can be interpreted as quantization ambiguities. In the case of LC circuits, polymer quantization predicts nonlinearities which are not present in the canonical approach. Based on this analysis we design and analyze a qubit which uses a meander inductor instead of a Josephson junction. Implications for qubit performance and fabrication are discussed. Given a choice for an effective phase operator, relevant parameters such as anharmonicity, frequency, and dispersive shifts are calculated for this meander inductor based qubit.