realization of a universal quantum computer [2]. In this project, we undertake the task of implementing an extensible wiring method for the operation of a quantum processor based on solid-state devices, e.g., superconducting qubits [3][4][5]. Possible experimental solutions based on wafer bonding techniques [6-9] or coaxial through-silicon vias [10] as well as theoretical proposals [1,11] have recently addressed the wiring issue, highlighting it as a priority for quantum computing.Building a universal quantum computer [12-17] will make it possible to execute quantum algorithms [18], which would have profound implications on scientific research and society. For a quantum computer to be competitive with the most advanced classical computer, it is widely believed that the qubit operations will require error rates on the order of 10 −15 or less. Achieving such error rates is only possible by means of quantum error correction (QEC) algorithms [13,15,19], which allow for arXiv:1606.00063v1 [quant-ph]

Quantum bits (qubits) with long coherence times are an important element for the implementation of medium-and large-scale quantum computers. In the case of superconducting planar qubits, understanding and improving qubits' quality can be achieved by studying superconducting planar resonators. In this Paper, we fabricate and characterize coplanar waveguide resonators made from aluminum thin films deposited on silicon substrates. We perform three different substrate treatments prior to aluminum deposition: One chemical treatment based on a hydrofluoric acid clean; one physical treatment consisting of a thermal annealing at 880 • C in high vacuum; one combined treatment comprising both the chemical and the physical treatments. The aim of these treatments is to remove the two-level state defects hosted by the native oxides residing at the various sample's interfaces. We first characterize the fabricated samples through cross-sectional tunneling electron microscopy acquiring electron energy loss spectroscopy maps of the samples' cross sections. These measurements show that both the chemical and the physical treatments almost entirely remove native silicon oxide from the substrate surface and that their combination results in the cleanest interface. We then study the quality of the resonators by means of microwave measurements in the "quantum regime", i.e., at a temperature T ∼ 10 mK and at a mean microwave photon number n ph ∼ 1. In this regime, we find that both surface treatments independently improve the resonator's intrinsic quality factor and that the highest quality factor is obtained for the combined treatment, Q i ∼ 0.8 million. Finally, we find that the TLS quality factor averaged over a time period of 3 h is ∼ 3 million at n ph ∼ 10, indicating that substrate surface engineering can potentially reduce the TLS loss below other losses such as quasiparticle and vortex loss.

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Scalable architectures characterized by quantum bits (qubits) with low error rates are essential to the development of a practical quantum computer. In the superconducting quantum computing implementation, understanding and minimizing materials losses is crucial to the improvement of qubit performance. A new material that has recently received particular attention is indium, a low-temperature superconductor that can be used to bond pairs of chips containing standard aluminum-based qubit circuitry. In this work, we characterize microwave loss in indium and aluminum/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level states (TLSs) are the dominating loss mechanism at low photon number and temperature. Thermally evaporated indium is determined to have a TLS loss tangent due to indium oxide of ∼ 5 × 10 −5 . The molecular beam epitaxial films show evidence of formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance.

Extensible quantum computing architectures require a large array of quantum devices operating with low error rates. A quantum processor based on superconducting quantum bits can be scaled up by stacking microchips that each perform different computational functions. In this article, we experimentally demonstrate a thermocompression bonding technology that utilizes indium films as a welding agent to attach pairs of lithographically-patterned chips. We perform chip-to-chip indium bonding in vacuum at 190 • C with indium film thicknesses of 150 nm. We characterize the dc and microwave performance of bonded devices at room and cryogenic temperatures. At 10 mK, we find a dc bond resistance of 515 nΩ mm −2 . Additionally, we show minimal microwave reflections and good transmission up to 6.8 GHz in a tunnel-capped, bonded device as compared to a similar uncapped device. As a proof of concept, we fabricate and measure a set of tunnelcapped superconducting resonators, demonstrating that our bonding technology can be used in quantum computing applications.