Simplified Josephson-junction fabrication process for reproducibly high-performance superconducting qubits

Abstract

We introduce a simplified fabrication technique for Josephson junctions and demonstrate superconducting Xmon qubits with T 1 relaxation times averaging above 50 ls (Q > 1:5 Â 10 6 ). Current shadow-evaporation techniques for aluminum-based Josephson junctions require a separate lithography step to deposit a patch that makes a galvanic, superconducting connection between the junction electrodes and the circuit wiring layer. The patch connection eliminates parasitic junctions, which otherwise contribute significantly to dielectric loss. In our patch-integrated cross-type junction technique, we use one lithography step and one vacuum cycle to evaporate both the junction electrodes and the patch. This eliminates a key bottleneck in manufacturing superconducting qubits by reducing the fabrication time and cost. In a study of more than 3600 junctions, we show an average resistance variation of 3.7% on a wafer that contains forty 0:5 Â 0:5-cm 2 chips, with junction areas ranging between 0.01 and 0.16 lm 2 . The average on-chip spread in resistance is 2.7%, with 20 chips varying between 1.4% and 2%. For the junction sizes used for transmon qubits, we deduce a wafer-level transition-frequency variation of 1.7%-2.5%. We show that 60%-70% of this variation is attributed to junction-area fluctuations, while the rest is caused by tunnel-junction inhomogeneity. Such high frequency predictability is a requirement for scaling-up the number of qubits in a quantum computer.

In-situ bandaged Josephson junctions for superconducting quantum processors

“…We note that a similar bandaging technique has recently been developed independently by Osman et al and demonstrated for Manhattan-style Josephson junctions [21].…”

TOF-SIMS Analysis of Decoherence Sources in Nb Superconducting Resonators

“…One potential solution to reduce the presence of these species would be to define all relevant patterns using a hard, physical masks. [43,44] In addition to the aforementioned contaminants, we also probe the distribution of other anionic and cationic contaminants in the Nb film. In the case of hydroxides, fluorides and chlorides, we observe a very similar behavior in the blanket films compared to the lithographically defined capacitance pads.…”

“…We hypothesize that these halogen species are introduced through the reactive ion etching and wet etching processes, which further highlights the value in potentially employing physical masks to define relevant features in the qubit structure. [43][44][45] Finally, a distribution of cationic contaminants are provided in Figure 4. In this case, as well, the concentration of species such as K + , Na + , and Ca + are amplified in the thin film region of the pattern sample.…”

Qubit-compatible substrates with superconducting through-silicon vias

“…Flipchip bonded modules of two chips connected by superconducting bumps increase the layer count to two and air bridges further alleviate routing challenges. These have indeed been used successfully to construct processors of several dozen qubits [8]- [10], although with coherence times and gate fidelities significantly lower than in planar [11]- [15] or flipchip bonded [16] single-or few-qubit devices.…”