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
23
Mai
2024
Parametrically controlled chiral interface for superconducting quantum devices
Nonreciprocal microwave routing plays a crucial role for measuring quantum circuits, and allows for realizing cascaded quantum systems for generating and stabilizing entanglement between
non-interacting qubits. The most commonly used tools for implementing directionality are ferrite-based circulators. These devices are versatile, but suffer from excess loss, a large footprint, and fixed directionality. For utilizing nonreciprocity in scalable quantum circuits it is desirable to develop efficient integration of low-loss and in-situ controllable directional elements. Here, we report the design and experimental realization of a controllable directional interface that may be integrated directly with superconducting qubits. In the presented device, nonreciprocity is realized through a combination of interference and phase-controlled parametric pumping. We have achieved a maximum directionality of around 30\,dB, and the performance of the device is predicted quantitatively from independent calibration measurements. Using the excellent agreement of model and experiment, we predict that the circuit will be useable as a chiral qubit interface with inefficiencies at the one-percent level or below. Our work provides a route toward isolator-free qubit readout schemes and high-fidelity entanglement generation in all-to-all connected networks of superconducting quantum devices.
21
Mai
2024
Circuit QED theory of direct and dual Shapiro steps with finite-size transmission line resonators
We investigate the occurrence of direct and dual Shapiro steps for a Josephson junction coupled to a finite-size transmission line resonator. We treat both problems through a circuit
QED approach with a large, but finite number of photon modes. For the dual case, we do not assume the (approximate) charge-phase duality, but include the full multi-band dynamics for the Josephson junction. Mean-field equations within such Hamiltonian approach reproduce the result obtained through a dissipative classical equation when the number of transmission line modes is large enough. To account for quantum and thermal fluctuations, we go beyond the mean-field treatment within a truncated Wigner approach. The fluctuations are shown to modify both the direct and the dual steps. We show how the dual steps are very sensitive to these fluctuations and identify the key physical parameters for the junction and the transmission line controlling their robustness, which is essential for applications to close the quantum metrological triangle.
20
Mai
2024
Cryogenic growth of tantalum thin films for low-loss superconducting circuits
Motivated by recent advancements highlighting Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have exploredthe effect of cryogenic temperatures on the growth of Ta and its integration in superconducting circuits. Cryogenic growth of Ta using a low temperature molecular beam epitaxy (MBE) system is found to stabilize single phase α-Ta on several different substrates, which include Al2O3(0001), Si(001), Si(111), SiNx, and GaAs(001). The substrates are actively cooled down to cryogenic temperatures and remain < 20 K during the Ta deposition. X-ray θ-2θ diffraction after warming to room temperature indicates the formation of polycrystalline α-Ta. The 50 nm α-Ta films grown on Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (ρ300K) of 13.4 μΩcm, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature (TC) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an RMS roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher ρ300K, lower RRR and Tc, and increased β-Ta content. Coplanar waveguide resonators with a gap width of 3 μm fabricated from cryogenically grown Ta on Si(111) and Al2O3(0001) show low power Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline α-Ta films may be promising for superconducting qubit applications even though they are not fully epitaxial.[/expand]
16
Mai
2024
Interferometric Purcell suppression of spontaneous emission in a superconducting qubit
In superconducting qubits, suppression of spontaneous emission is essential to achieve fast dispersive measurement and reset without sacrificing qubit lifetime. We show that resonator-mediated
decay of the qubit mode to the feedline can be suppressed using destructive interference, where the readout resonator is coupled to the feedline at two points. This „interferometric Purcell filter“ does not require dedicated filter components or impedance mismatch in the feedline, making it suitable for applications such as all-pass readout. We design and fabricate a device with the proposed scheme and demonstrate suppression of resonator-mediated decay that exceeds 2 orders of magnitude over a bandwidth of 400 MHz.
13
Mai
2024
Efficiently Building and Characterizing Electromagnetic Models of Multi-Qubit Superconducting Circuits
In an attempt to better leverage superconducting quantum computers, scaling efforts have become the central concern. These efforts have been further exacerbated by the increased complexity
of these circuits. The added complexity can introduce parasitic couplings and resonances, which may hinder the overall performance and scalability of these devices. We explore a method of modeling and characterization based on multiport impedance functions that correspond to multi-qubit circuits. By combining vector fitting techniques with a novel method for interconnecting rational impedance functions, we are able to efficiently construct Hamiltonians for multi-qubit circuits using electromagnetic simulations. Our methods can also be applied to circuits that contain both lumped and distributed element components. The constructed Hamiltonians account for all the interactions within a circuit that are described by the impedance function. We then present characterization methods that allow us to estimate effective qubit coupling rates, state-dependent dispersive shifts of resonant modes, and qubit relaxation times.
A logical qubit-design with geometrically tunable error-resistibility
Breaking the error-threshold would mark a milestone in establishing quantum advantage for a wide range of relevant problems. One possible route is to encode information redundantly
in a logical qubit by combining several noisy qubits, providing an increased robustness against external perturbations. We propose a setup for a logical qubit built from superconducting qubits (SCQs) coupled to a microwave cavity-mode. Our design is based on a recently discovered geometric stabilizing mechanism in the Bose-Hubbard wheel (BHW), which manifests as energetically well-separated clusters of many-body eigenstates. We investigate the impact of experimentally relevant perturbations between SCQs and the cavity on the spectral properties of the BHW. We show that even in the presence of typical fabrication uncertainties, the occurrence and separation of clustered many-body eigenstates is extremely robust. Introducing an additional, frequency-detuned SCQ coupled to the cavity yields duplicates of these clusters, that can be split up by an on-site potential. We show that this allows to (i) redundantly encode two logical qubit states that can be switched and read out efficiently and (ii) can be separated from the remaining many-body spectrum via geometric stabilization. We demonstrate at the example of an X-gate that the proposed logical qubit reaches single qubit-gate fidelities >0.999 in experimentally feasible temperature regimes ∼10−20mK.
09
Mai
2024
Achieving millisecond coherence fluxonium through overlap Josephson junctions
Fluxonium qubits are recognized for their high coherence times and high operation fidelities, attributed to their unique design incorporating over 100 Josephson junctions per superconducting
loop. However, this complexity poses significant fabrication challenges, particularly in achieving high yield and junction uniformity with traditional methods. Here, we introduce an overlap process for Josephson junction fabrication that achieves nearly 100% yield and maintains uniformity across a 2-inch wafer with less than 5% variation for the phase slip junction and less than 2% for the junction array. Our compact junction array design facilitates fluxonium qubits with energy relaxation times exceeding 1 millisecond at the flux frustration point, demonstrating consistency with state-of-the-art dielectric loss tangents and flux noise across multiple devices. This work suggests the scalability of high coherence fluxonium processors using CMOS-compatible processes, marking a significant step towards practical quantum computing.
Self-correcting GKP qubit and gates in a driven-dissipative circuit
We propose a circuit architecture for a dissipatively error-corrected GKP qubit. The device consists of a high-impedance LC circuit coupled to a Josephson junction and a resistor
via a controllable switch. When the switch is activated via a particular family of stepwise protocols, the resistor absorbs all noise-induced entropy, resulting in dissipative error correction of both phase and amplitude errors. This leads to an exponential increase of qubit lifetime, reaching beyond 10ms in simulations with near-feasible parameters. We show that the lifetime remains exponentially long in the presence of extrinsic noise and device/control imperfections (e.g., due to parasitics and finite control bandwidth) under specific thresholds. In this regime, lifetime is likely only limited by phase slips and quasiparticle tunneling. We show that the qubit can be read out and initialized via measurement of the supercurrent in the Josephson junction. We finally show that the qubit supports native self-correcting single-qubit Clifford gates, where dissipative error-correction of control noise leads to exponential suppression of gate infidelity.
06
Mai
2024
Flux-Tunable Regimes and Supersymmetry in Twisted Cuprate Heterostructures
Van der Waals assembly allows for the creation of Josephson junctions in an atomically sharp interface between two exfoliated Bi2Sr2CaCu2O8+δ (Bi-2212) flakes that are twisted relative
to each other. In a narrow range of angles close to 45∘, the junction exhibits a regime where time-reversal symmetry can be spontaneously broken and it can be used to encode an inherently protected qubit called flowermon. In this work we investigate the physics emerging when two such junctions are integrated in a SQuID circuit threaded by a magnetic flux. We show that the flowermon qubit regime is maintained up to a finite critical value of the magnetic field and, under appropriate conditions, it is protected against both charge and flux noise. For larger external fluxes, the interplay between the inherent twisted d-wave nature of the order parameter and the external magnetic flux enables the implementation of different artificial atoms, including a flux-biased protected qubit and a supersymmetric quantum circuit.
05
Mai
2024
Minimizing Kinetic Inductance in Tantalum-Based Superconducting Coplanar Waveguide Resonators for Alleviating Frequency Fluctuation Issues
Advancements in the fabrication of superconducting quantum devices have highlighted tantalum as a promising material, owing to its low surface oxidation microwave loss at low temperatures.
However, tantalum films exhibit significantly larger kinetic inductances compared to materials such as aluminum or niobium. Given the inevitable variations in film thickness, this increased kinetic inductance leads to considerable, uncontrolled frequency variances and shifts in components like superconducting coplanar waveguide (SCPW) resonators. Achieving high precision in resonator frequencies is crucial, particularly when multiple resonators share a common Purcell filter with limited bandwidth in superconducting quantum information processors. Here, we tackle this challenge from both fabrication and design perspectives, achieving a reduction in resonator frequency fluctuation by a factor of more than 100. Concurrently, the internal quality factor of the SCPW resonator remains at high level. Our findings open up new avenues for the enhanced utilization of tantalum in large-scale superconducting chips.