| 1. |
- Osman, Amr, 1993, et al.
(författare)
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Simplified Josephson-junction fabrication process for reproducibly high-performance superconducting qubits
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:6
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a simplified fabrication technique for Josephson junctions and demonstrate superconducting Xmon qubits with T1 relaxation times averaging above 50 μs (Q > 1.5 × 1 0 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 × 0.5-cm2 chips, with junction areas ranging between 0.01 and 0.16 μm2. 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.
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| 2. |
- Chen, Liangyu, 1994, et al.
(författare)
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Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier
- 2023
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Ingår i: npj Quantum Information. - : Springer Science and Business Media LLC. - 2056-6387. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that reduces the contribution of decay error during readout, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140 ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout–without using a quantum-limited amplifier.
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| 3. |
- Grigoras, K., et al.
(författare)
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Qubit-Compatible Substrates With Superconducting Through-Silicon Vias
- 2022
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Ingår i: IEEE Transactions on Quantum Engineering. - 2689-1808. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- We fabricate and characterize superconducting through-silicon vias and electrodes suitable for superconducting quantum processors. We measure internal quality factors of a million for test resonators excited at single-photon levels, on chips with superconducting vias used to stitch ground planes on the front and back sides of the chips. This resonator performance is on par with the state of the art for silicon-based planar solutions, despite the presence of vias. Via stitching of ground planes is an important enabling technology for increasing the physical size of quantum processor chips, and is a first step toward more complex quantum devices with three-dimensional integration.
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| 4. |
- Kosen, Sandoko, 1991, et al.
(författare)
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Building blocks of a flip-chip integrated superconducting quantum processor
- 2022
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Ingår i: Quantum Science and Technology. - : IOP Publishing. - 2058-9565. ; 7:3
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Tidskriftsartikel (refereegranskat)abstract
- We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips-one quantum chip and one control chip-that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90 mu s, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.
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| 5. |
- Li, Hangxi, 1994, et al.
(författare)
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Experimentally Verified, Fast Analytic, and Numerical Design of Superconducting Resonators in Flip-Chip Architectures
- 2023
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Ingår i: IEEE Transactions on Quantum Engineering. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- In superconducting quantum processors, the predictability of device parameters is of increasing importance as many laboratories scale up their systems to larger sizes in a 3-D-integrated architecture. In particular, the properties of superconducting resonators must be controlled well to ensure high-fidelity multiplexed readout of qubits. Here, we present a method, based on conformal mapping techniques, to predict a resonator's parameters directly from its 2-D cross-section, without computationally heavy and time-consuming 3-D simulation. We demonstrate the method's validity by comparing the calculated resonator frequency and coupling quality factor with those obtained through 3-D finite-element-method simulation and by measurement of 15 resonators in a flip-chip-integrated architecture. We achieve a discrepancy of less than 2% between designed and measured frequencies for 6-GHz resonators. We also propose a design method that reduces the sensitivity of the resonant frequency to variations in the interchip spacing.
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| 6. |
- Osman, Amr, 1993, et al.
(författare)
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Mitigation of frequency collisions in superconducting quantum processors
- 2023
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Ingår i: Physical Review Research. - 2643-1564. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- The reproducibility of qubit parameters is a challenge for scaling up superconducting quantum processors. Signal cross talk imposes constraints on the frequency separation between neighboring qubits. The frequency uncertainty of transmon qubits arising from the fabrication process is attributed to deviations in the Josephson junction area, tunnel barrier thickness, and the qubit shunt capacitor. We decrease the sensitivity to these variations by fabricating larger Josephson junctions and reduce the wafer-level standard deviation in resistance down to 2%. We characterize 32 identical transmon qubits and demonstrate the reproducibility of the qubit frequencies with a 40 MHz standard deviation (i.e., 1%) with qubit quality factors exceeding 2 million. We perform two-level-system (TLS) spectroscopy and observe no significant increase in the number of TLSs causing qubit relaxation. We further show by simulation that for our parametric-gate architecture, and accounting only for errors caused by the uncertainty of the qubit frequency, we can scale up to 100 qubits with an average of only three collisions between quantum-gate transition frequencies, assuming 2% cross talk and 99.9% target gate fidelity.
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