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
12
Mä
2024
Designing high-fidelity two-qubit gates between fluxonium qubits
We take a bottom-up, first-principles approach to design a two-qubit gate between fluxonium qubits for minimal error, speed, and control simplicity. Our proposed architecture consists
of two fluxoniums coupled via a linear resonator. Using a linear coupler introduces the possibility of material optimization for suppressing its loss, enables efficient driving of state-selective transitions through its large charge zero point fluctuation, reduces sensitivity to junction aging, and partially mitigates coherent coupling to two-level systems. Crucially, a resonator-as-coupler approach also suggests a clear path to increased connectivity between fluxonium qubits, by reducing capacitive loading when the coupler has a high impedance. After performing analytic and numeric analyses of the circuit Hamiltonian and gate dynamics, we tune circuit parameters to destructively interfere sources of coherent error, revealing an efficient, fourth-order scaling of coherent error with gate duration. For component properties from the literature, we predict an open-system average CZ gate infidelity of 1.86×10−4 in 70ns.
10
Mä
2024
Higher-order exceptional surface in a pseudo-Hermitian superconducting circuit
In the last few years, much attention has been paid to exceptional surfaces (ESs) owing to various important physical phenomena and potential applications. However, high-order ESs in
pseudo-Hermitian systems have not been reported until now. Here, we study the high-order ES in a pseudo-Hermitian superconducting (SC) circuit system. In our proposal, the SC circuit system is composed of three circularly coupled SC cavities, where the gain and loss are balanced. According to the eigenvalue properties of the pseudo-Hermitian Hamiltonian, we derive the general pseudo-Hermitian conditions for the ternary SC system. In the special pseudo-Hermitian case with parity-time symmetry, all third-order exceptional points (EP3s) of the SC system form a third-order exceptional line in the parameter space. Under the general pseudo-Hermitian conditions, more EP3s are found, and all EP3s are located on a surface, i.e., a third-order exceptional surface is constructed. Moreover, we also investigate the eigenvalues of the pseudo-Hermitian SC circuit around EP3s. Our work opens up a door for exploring high-order ESs and related applications in pseudo-Hermitian systems.
07
Mä
2024
Bloch oscillations in a transmon embedded in a resonant electromagnetic environment
Recently developed Josephson junction array transmission lines implement strong-coupling circuit electrodynamics compatible with a range of superconducting quantum devices. They provide
both the high impedance which allows for strong quantum fluctuations, and photon modes with which to probe a quantum device, such as a small Josephson junction. In this high-impedance environment, current through the junction is accompanied by charge Bloch oscillations analogous to those in crystalline systems. However, the interplay between Bloch oscillations and environmental photon resonances remains largely unexplored. Here we describe the Bloch oscillations in a transmon-type qubit attached to high-impedance transmission lines with discrete photon spectra. Transmons are characterized by well-separated charge bands, favoring Bloch oscillations over Landau-Zener tunneling. We find resonances in the voltage–current relation and the spectrum of photons emitted by the Bloch oscillations. The transmon also scatters photons inelastically; we find the cross-section for a novel anti-Stokes-like process whereby photons gain a Bloch oscillation quantum. Our results outline how Bloch oscillations leave fingerprints for experiments across the DC, MHz, and GHz ranges.
06
Mä
2024
Mechanically Designing Protected Superconducting Qubits
Significant progress is required in the engineering of large, interacting quantum systems in order to realize the promises of gate-model quantum computing. Designing such systems is
challenging, as the dynamics of continuous variable quantum systems are generally unintuitive, and brute-force numerical solutions are difficult to impossible in more than a few dimensions. In this work, I draw analogies between modern superconducting qubits and mechanical mass-spring systems in attempt to gain a simple intuition for what makes each design special. In particular, I analyze superconducting qubits that are inherently protected from noise, and connect this protection to features of the corresponding mechanical system. The hope is that intuition gained from analyzing these systems mechanically will allow for intuitive design of useful superconducting circuits in the future.
A multiplexed control architecture for superconducting qubits with row-column addressing
In state-of-the-art superconducting quantum processors, each qubit is controlled by at least one control line that delivers control pulses generated at room temperature to qubits at
millikelvin temperatures. This strategy has been successfully applied to control hundreds of qubits but is unlikely to be scalable to control thousands of qubits, let alone millions or even billions of qubits needed in fault-tolerance quantum computing. The reason for this is due to the wiring challenge, the number of accommodated control lines is limited by factors, such as the cooling power and physical space of the cryogenic system, the control footprint area at the qubit chip level, and so on. Here, we introduce a multiplexed control architecture for superconducting qubits with two types of shared control lines, row and column lines, providing an efficient approach for parallel controlling N qubits with O(N‾‾√) control lines. With the combination of the two-type shared lines, unique pairs of control pulses are delivered to qubits on each row-column intersection, enabling parallel qubit addressing. Of particular concern here is that, unlike traditional gate schemes, both single- and two-qubit gates are implemented with pairs of control pulses. Considering the inherent parallelism and the control limitations, the integration of the architecture into quantum computing systems should be tailored as much as possible to the specific properties of the quantum circuits to be executed. As such, the architecture could be scalable for executing structured quantum circuits, such as quantum error correction circuits.
High-Impedance Microwave Resonators with Two-Photon Nonlinear Effects
In this article, we present an experimental study of a Josephson junction -based high-impedance resonator. By taking the resonator to the limit of consisting effectively only of one
junction, results in strong non-linear effects already for the second photon while maintaining a high impedance of the resonance mode. Our experiment yields thus resonators with the strong interactions both between individual resonator photons and from the resonator photons to other electric quantum systems. We also present an energy diagram technique which enables to measure, identify and analyse different multi-photon optics processes along their energy conservation lines.
05
Mä
2024
Quasiparticle effects in magnetic-field-resilient 3D transmons
Recent research shows that quasiparticle-induced decoherence of superconducting qubits depends on the superconducting-gap asymmetry originating from the different thicknesses of the
top and bottom films in Al/AlOx/Al junctions. Magnetic field is a key tuning knob to investigate this dependence as it can change the superconducting gaps in situ. We present measurements of the parity-switching time of a field-resilient 3D transmon with in-plane field up to 0.41T. At low fields, small parity splitting requires qutrit pulse sequences for parity measurements. We measure a non-monotonic evolution of the parity lifetime with in-plane magnetic field, increasing up to 0.2T, followed by a decrease at higher fields. We demonstrate that the superconducting-gap asymmetry plays a crucial role in the observed behavior. At zero field, the qubit frequency is nearly resonant with the superconducting-gap difference, favoring the energy exchange with the quasiparticles and so enhancing the parity-switching rate. With a higher magnetic field, the qubit frequency decreases and gets detuned from the gap difference, causing the initial increase of the parity lifetime, while photon-assisted qubit transitions increase, producing the subsequent decrease at higher fields. Besides giving a deeper insight into the parity-switching mechanism in conventional transmon qubits, we establish that Al-AlOx-Al JJs could be used in architectures for the parity-readout and manipulation of topological qubits based on Majorana zero modes.
Quantum refrigeration powered by noise in a superconducting circuit
While dephasing noise frequently presents obstacles for quantum devices, it can become an asset in the context of a Brownian-type quantum refrigerator. Here we demonstrate a novel quantum
thermal machine that leverages noise-assisted quantum transport to fuel a cooling engine in steady state. The device exploits symmetry-selective couplings between a superconducting artificial molecule and two microwave waveguides. These waveguides act as thermal reservoirs of different temperatures, which we regulate by employing synthesized thermal fields. We inject dephasing noise through a third channel that is longitudinally coupled to an artificial atom of the molecule. By varying the relative temperatures of the reservoirs, and measuring heat currents with a resolution below 1 aW, we demonstrate that the device can be operated as a quantum heat engine, thermal accelerator, and refrigerator. Our findings open new avenues for investigating quantum thermodynamics using superconducting quantum machines coupled to thermal microwave waveguides.
04
Mä
2024
Wafer-scale uniformity improvement of Dolan-bridge Josephson junctions by optimization of shadow evaporation technique
One of the practical limitations of solid-state quantum computer manufacturing is the low reproducibility of the superconducting qubits resonance frequency. It makes hard demands on
the Josephson junction fabrication process, producing a nonlinear inductance of the qubit. In this work, we demonstrate for 100 mm wafer decreasing of the room temperature resistance variation coefficient to 6.0% for 150×170 nm2 Al/AlOx/Al Josephson junction area and to 4.0% for 150×670 nm2 Al/AlOx/Al Josephson junction area. These results were achieved by the development of the shadow evaporation process model considering the Josephson junction area variation on the wafer. Our model allows us to provide the junction area variation coefficient of about 1.0% for Josephson junction characteristic dimensions from 100 nm to 700 nm. In addition, we show the junction oxidation technic optimization. Our improvements can be scalable on the wafer with a large diameter, which allows to manufacturing of the quantum processor with high reproducibility of electrical parameters.
Recovering quantum coherence of a cavity qubit through environment monitoring and active feedback
Decoherence in qubits, caused by their interaction with a noisy environment, poses a significant challenge to developing reliable quantum processors. Monitoring the qubit’s environment
enables not only to flag decoherence events but also to reverse these errors, thereby restoring the qubit coherence. This approach is particularly beneficial for superconducting cavity qubits, whose unavoidable interaction with auxiliary transmons impacts their coherence. In this work, we uncover the intricate dynamics of cavity qubit decoherence by tracking the noisy trajectory of a transmon acting as the cavity’s environment. Using real-time feedback, we successfully recover the lost coherence of the cavity qubit, achieving a fivefold increase in its dephasing time. Alternatively, by detecting transmon errors and converting them into erasures, we improve the cavity phase coherence by more than an order of magnitude. These advances are essential for using cavity qubits with low photon loss rates as long-lived quantum memories with high-fidelity gates and can enable more efficient bosonic quantum error correction codes.