Materials improvements in transmon qubits have yielded remarkable advances in qubit lifetimes (T1)and coherence times (T2). However, state-of-the-art superconducting qubits experiencetemporal fluctuations in their performance, which limit gate fidelities and quantum processor performance. We observe correlated increases in qubit relaxation and dephasing rates that last for minutes to hours, which we call T1 and T2 excursions. Using interleaved lifetime and coherence time measurements, noise spectroscopy, and two-level-system (TLS) spectroscopy, we find that these excursions are caused by single or few strongly-coupled TLS defects that spectrally diffuse into resonance with the qubit frequency. Our measurement protocol provides a powerful tool for understanding the noise sources that cause temporal fluctuations in superconducting qubit performance.
Assuming only the solubility of Kirchhoff’s laws, the conservation of energy, and the form of the constitutive relations, we present a flux-charge symmetric Hamiltonian formalismthat always admits a symplectic reduction for nonsingular, well-posed circuits involving gyrators, inductors, and capacitors (LCG circuits) with arbitrary non-linearity. The formalism introduces a notion of circuit duality on non-linear non-reciprocal circuits and a family of point transformations that inserts, removes, and moves reciprocal elements across gyrators while preserving the Hamiltonian spectrum. With these transformations, we extend Tellegen’s classic result that gyrators make reciprocal elements behave as their dual to non-linear reciprocal elements such as the Josephson junction and quantum phase slip. Furthermore, we present conditions under which non-reciprocal circuits can be transformed to dynamically equivalent reciprocal circuits.
Many-body systems with strong interactions often exhibit macroscopic behavior markedly absent in single-particle or noninteracting limits. Such emergent phenomena are well exemplifiedin lattice Hubbard models, where the interplay between interactions, geometric frustration, and magnetic flux gives rise to rich physics. Superconducting qubits naturally enable analog quantum simulation of Bose-Hubbard models, while offering tunable parameters, site-resolved control, and rapid experimental repetition rates. Here, we study a superconducting-qubit device that realizes the Bose-Hubbard model on a triangular-ladder lattice. By tuning the magnitude and sign of couplings, we engineer a synthetic magnetic flux to characterize the resulting half-filling ground state for various parameter regimes. We measure observables analogous to current-current correlators and bond kinetic energies, finding signatures consistent with chiral superfluids, Meissner superfluids, and bond-ordered insulators. Our results establish superconducting circuits as a platform for robustly probing quantum phases of matter in frustrated Bose-Hubbard systems, even in strongly correlated and gapless regimes.
Tantalum-based transmon qubits are a promising platform for building large-scale quantum processors. So far, these qubits have been made from tantalum films grown exclusively in thealpha phase ({\alpha}-Ta). The beta phase of tantalum (\{beta}-Ta) readily nucleates at room temperature, making it attractive for scalable qubit fabrication. However, \{beta}-Ta is widely believed to be detrimental to qubit performance because it has a lower superconducting critical temperature than {\alpha}-Ta. We challenge this prevailing belief by fabricating low-loss transmon qubits from \{beta}-Ta films on sapphire. Across 11 qubits, the mean time-averaged quality factor is (5.6 +/- 2.3) x 10^6, with the best qubit recording a time-averaged quality factor of (10.1 +/- 1.3) x 10^6. Resonator studies demonstrate that the dominant microwave loss channel is surface two-level systems, with the surface loss contribution for \{beta}-Ta being about twice that of {\alpha}-Ta. \{beta}-Ta films exhibit significant kinetic inductance, consistent with an estimated magnetic penetration depth of (1.78 +/- 0.02) {\mu}m. This work establishes \{beta}-Ta on sapphire as a material platform for realizing low-loss transmon qubits and other superconducting devices such as compact resonators, kinetic inductance detectors, and quasiparticle traps.
The recent realization of millisecond-scale coherence with tantalum-on-silicon transmon qubits showed that depositing the Al/AlOx/Al Josephson junction in a high purity, ultrahigh vacuumenvironment was critical for achieving lifetime-limited coherence, motivating careful examination of the aluminum surface two-level system (TLS) bath. Here, we measure the microwave absorption arising from surface TLSs in superconducting aluminum resonators, following methodology developed for tantalum resonators. We vary film and surface properties and correlate microwave measurements with materials characterization. We find that the lifetimes of superconducting aluminum resonators are primarily limited by surface losses associated with TLSs in the 2.7 nm-thick native AlOx. Treatment with 49% HF removes surface AlOx completely; however, rapid oxide regrowth limits improvements in surface loss and long term device stability. Using these measurements we estimate that TLSs in aluminum interfaces contribute around 27% of the relaxation rate of state-of-the-art tantalum-on-silicon qubits that incorporate aluminum-based Josephson junctions.
Utilizing tantalum (Ta) in superconducting circuits has led to significant improvements, such as high qubit lifetimes and quality factors in both qubits and resonators, underscoringthe importance of material optimization in quantum device performance. In this work, we explore superconducting gap engineering in Ta-based devices as a strategy to expand the range of viable host materials. By alloying 20 atomic percent hafnium (Hf) into Ta thin films, we achieve a superconducting transition temperature (Tc) of 6.09~K, as measured by DC transport, reflecting an increased superconducting gap. We systematically vary deposition conditions to control film orientation and transport properties of the Ta-Hf alloy films. The enhancement in Tc is further confirmed by microwave measurements at millikelvin temperatures. Despite the 40\% increase in Tc relative to pure Ta, the loss contributions from two-level systems (TLS) and quasiparticles (QPs) remain unchanged in the low-temperature regime. These findings highlight the potential of material engineering to improve superconducting circuit performance and motivate further exploration of engineered alloys for quantum technologies.
Materials improvements are a powerful approach to reducing loss and decoherence in superconducting qubits because such improvements can be readily translated to large scale processors.Recent work improved transmon coherence by utilizing tantalum (Ta) as a base layer and sapphire as a substrate. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q) exceeding 1.5 x 10^7, reaching a maximum Q of 2.5 x 10^7, corresponding to a lifetime (T_1) of up to 1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T_1. We achieve these material improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single qubit gates with 99.994% fidelity. The Ta-on-Si platform comprises a simple material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale quantum processors.
In superconducting quantum devices, Purcell filters protect qubit information from decaying into external lines by reducing external coupling at qubit frequencies while maintainingit 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.
In this work, we introduce new methods for the quantization, decomposition, and extraction (from electromagnetic simulations) of lumped-element circuit models for superconducting quantumdevices. Our flux-charge symmetric procedures center on the network matrix, which encodes the connectivity of a circuit’s inductive loops and capacitive nodes. First, we use the network matrix to demonstrate a simple algorithm for circuit quantization, giving novel predictions for the Hamiltonians of circuits with both Josephson junctions and quantum phase slip wires. We then show that by performing pivoting operations on the network matrix, we can decompose a superconducting circuit model into its simplest equivalent „fundamental“ form, in which the harmonic degrees of freedom are separated out from the Josephson junctions and phase slip wires. Finally, we illustrate how to extract an exact, transformerless circuit model from electromagnetic simulations of a device’s hybrid admittance/impedance response matrix, by matching the lumped circuit’s network matrix to the network topology of the physical layout. Overall, we provide a toolkit of intuitive methods that can be used to construct, analyze, and manipulate superconducting circuit models.
The lifetime of superconducting qubits is limited by dielectric loss, and a major source of dielectric loss is the native oxide present at the surface of the superconducting metal.Specifically, tantalum-based superconducting qubits have been demonstrated with record lifetimes, but a major source of loss is the presence of two-level systems (TLSs) in the surface tantalum oxide. Here, we demonstrate a strategy for avoiding oxide formation by encapsulating the tantalum with noble metals that do not form native oxide. By depositing a few nanometers of Au or AuPd alloy before breaking vacuum, we completely suppress tantalum oxide formation. Microwave loss measurements of superconducting resonators reveal that the noble metal is proximitized, with a superconducting gap over 80% of the bare tantalum at thicknesses where the oxide is fully suppressed. We find that losses in resonators fabricated by subtractive etching are dominated by oxides on the sidewalls, suggesting total surface encapsulation by additive fabrication as a promising strategy for eliminating surface oxide TLS loss in superconducting qubits.