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
16
Mä
2021
Merged-Element Transmons: Design and Qubit Performance
We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element
transmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with EJ/EC in the low transmon regime (EJ/EC ≲30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed mean T1 times typically in the range of 10-90 microseconds, with some annealed devices exhibiting T1 > 100 microseconds over several hours. The results suggest that energy relaxation in conventional, small-junction transmons is not limited by junction loss.
Anti-crosstalk high-fidelity state discrimination for superconducting qubits
Measurement for qubits plays a key role in quantum computation. Current methods for classifying states of single qubit in a superconducting multi-qubit system produce fidelities lower
than expected due to the existence of crosstalk, especially in case of frequency crowding. Here, We make the digital signal processing (DSP) system used in measurement into a shallow neural network and train it to be an optimal classifier to reduce the impact of crosstalk. The experiment result shows the crosstalk-induced readout error deceased by 100% after a 3-second optimization applied on the 6-qubit superconducting quantum chip.
15
Mä
2021
Fabrication of superconducting through-silicon vias
Increasing circuit complexity within quantum systems based on superconducting qubits necessitates high connectivity while retaining qubit coherence. Classical micro-electronic systems
have addressed interconnect density challenges by using 3D integration with interposers containing through-silicon vias (TSVs), but extending these integration techniques to superconducting quantum systems is challenging. Here, we discuss our approach for realizing high-aspect-ratio superconducting TSVs\textemdash 10 μm wide by 20 μm long by 200 μm deep\textemdash with densities of 100 electrically isolated TSVs per square millimeter. We characterize the DC and microwave performance of superconducting TSVs at cryogenic temperatures and demonstrate superconducting critical currents greater than 20 mA. These high-aspect-ratio, high critical current superconducting TSVs will enable high-density vertical signal routing within superconducting quantum processors.
Millisecond coherence in a superconducting qubit
Increasing the degree of control over physical qubits is a crucial component of quantum computing research. We report a superconducting qubit of fluxonium type with the Ramsey coherence
time reaching T∗2=1.48±0.13 ms, which exceeds the state of the art value by an order of magnitude. As a result, the average single-qubit gate fidelity grew above 0.9999, surpassing, to our knowledge, any other solid-state quantum system. Furthermore, by measuring energy relaxation of the parity-forbidden transition to second excited state, we exclude the effect of out-of-equilibrium quasiparticles on coherence in our circuit. Combined with recent demonstrations of two-qubit gates on fluxoniums, our result paves the way for the next generation of quantum processors.
14
Mä
2021
Effects of surface treatments on flux tunable transmon qubits
One of the main limitations in state-of-the art solid-state quantum processors are qubit decoherence and relaxation due to noise in their local environment. For the field to advance
towards full fault-tolerant quantum computing, a better understanding of the underlying microscopic noise sources is therefore needed. Adsorbates on surfaces, impurities at interfaces and material defects have been identified as sources of noise and dissipation in solid-state quantum devices. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure and ion irradiation treatments on coherence and slow parameter fluctuations of flux tunable superconducting transmon qubits. We analyse the effects of each of these surface treatments by comparing averages over many individual qubits and measurements before and after treatment. The treatments studied do not significantly impact the relaxation rate Γ1 and the echo dephasing rate Γe2, except for Ne ion bombardment which reduces Γ1. In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH3 treatments. Additionally, we demonstrate that SF6 ion bombardment can be used to adjust qubit frequencies in-situ and post fabrication without affecting qubit coherence at the sweet spot.
13
Mä
2021
Broadband Microwave Isolation with Adiabatic Mode Conversion in Coupled Superconducting Transmission Lines
We propose a traveling wave scheme for broadband microwave isolation using parametric mode conversion in conjunction with adiabatic phase matching technique in a pair of coupled nonlinear
transmission lines. This scheme is compatible with the circuit quantum electrodynamics architecture (cQED) and provides isolation without introducing additional quantum noise. We first present the scheme in a general setting then propose an implementation with Josephson junction transmission lines. Numerical simulation shows more than 20 dB isolation over an octave bandwidth (4-8\,GHz) in a 2000 unit cell device with less than 0.05 dB insertion loss dominated by dielectric loss.
Enhanced-coherence all-nitride superconducting qubit epitaxially grown on Si Substrate
We have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on Si substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions.
The all-nitride qubits have great advantages such as chemical stability against oxidation (resulting in fewer two-level fluctuators), feasibility for epitaxial tunnel barriers (further reducing energy relaxation and dephasing), and a larger superconducting gap of ∼5.2 meV for NbN compared to ∼0.3 meV for Al (suppressing the excitation of quasiparticles). Replacing conventional MgO by a Si substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time T1=16.3 μs and a spin-echo dephasing time T2=21.5 μs. These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to previously reported NbN-based qubits with MgO substrates (T1≈T2≈0.5 μs). These results are an important step towards constructing a new platform for superconducting quantum hardware.
Low-noise on-chip coherent microwave source
The increasing need for scaling up quantum computers operating in the microwave domain calls for advanced approaches for control electronics. To this end, integration of components
at cryogenic temperatures hosting also the quantum devices seems tempting. However, this comes with the limitations of ultra-low power dissipation accompanied by stringent signal-quality requirements to implement quantum-coherent operations. Here, we present a device and a technique to provide coherent continuous-wave microwave emission. We experimentally verify that its operation characteristics accurately follow our introduced theory based on a perturbative treatment of the capacitively shunted Josephson junction as a gain element. From phase noise measurements, we evaluate that the infidelity of typical quantum gate operations owing to this cryogenic source is less than 0.1% up to 10-ms evolution times, which is well below the infidelity caused by dephasing of the state-of-the-art superconducting qubits. Our device provides a coherent tone of 25 pW, corresponding to the total power needed in simultaneous control of thousands of qubits. Thus, together with future cryogenic amplitude and phase modulation techniques, our results may open pathways for scalable cryogenic control systems for quantum processors.
12
Mä
2021
Floating tunable coupler for scalable quantum computing architectures
We propose a floating tunable coupler that does not rely on direct qubit-qubit coupling capacitances to achieve the zero-coupling condition. We show that the polarity of the qubit-coupler
couplings can be engineered to offset the otherwise constant qubit-qubit coupling and attain the zero-coupling condition when the coupler frequency is above or below the qubit frequencies. We experimentally demonstrate these two operating regimes of the tunable coupler by implementing symmetric and asymmetric configurations of the coupler’s superconducting pads with respect to the qubits. Such a floating tunable coupler provides flexibility in designing large-scale quantum processors while reducing the always-on residual couplings.
11
Mä
2021
A Josephson junction supercurrent diode
Transport is called nonreciprocal when not only the sign, but also the absolute value of the current, depends on the polarity of the applied voltage. It requires simultaneously broken
inversion and time-reversal symmetries, e.g., by the interplay of spin-orbit coupling and magnetic field. So far, observation of nonreciprocity was always tied to resistivity, and dissipationless nonreciprocal circuit elements were elusive. Here, we engineer fully superconducting nonreciprocal devices based on highly-transparent Josephson junctions fabricated on InAs quantum wells. We demonstrate supercurrent rectification far below the transition temperature. By measuring Josephson inductance, we can link nonreciprocal supercurrent to the asymmetry of the current-phase relation, and directly derive the supercurrent magnetochiral anisotropy coefficient for the first time. A semi-quantitative model well explains the main features of our experimental data. Nonreciprocal Josephson junctions have the potential to become for superconducting circuits what pn-junctions are for traditional electronics, opening the way to novel nondissipative circuit elements.