As superconducting qubit circuits become more complex, addressing a large array of qubits becomes a challenging engineering problem. Dense arrays of qubits benefit from, and may require,access via the third dimension to alleviate interconnect crowding. Through-silicon vias (TSVs) represent a promising approach to three-dimensional (3D) integration in superconducting qubit arrays — provided they are compact enough to support densely-packed qubit systems without compromising qubit performance or low-loss signal and control routing. In this work, we demonstrate the integration of superconducting, high-aspect ratio TSVs — 10 μm wide by 20 μm long by 200 μm deep — with superconducting qubits. We utilize TSVs for baseband control and high-fidelity microwave readout of qubits using a two-chip, bump-bonded architecture. We also validate the fabrication of qubits directly upon the surface of a TSV-integrated chip. These key 3D integration milestones pave the way for the control and readout of high-density superconducting qubit arrays using superconducting TSVs.
As designs for superconducting qubits become more complex, 3D integration of two or more vertically bonded chips will become necessary to enable increased density and connectivity.Precise control of the spacing between these chips is required for accurate prediction of circuit performance. In this paper, we demonstrate an improvement in the planarity of bonded superconducting qubit chips while retaining device performance by utilizing hard-stop silicon spacer posts. These silicon spacers are defined by etching several microns into a silicon substrate and are compatible with 3D-integrated qubit fabrication. This includes fabrication of Josephson junctions, superconducting air-bridge crossovers, underbump metallization and indium bumps. To qualify the integrated process, we demonstrate high-quality factor resonators on the etched surface and measure qubit coherence (T1, T2,echo > 40 {\mu}s) in the presence of silicon posts as near as 350 {\mu}m to the qubit.
Superconducting qubits are leading candidates in the race to build a quantum computer capable of realizing computations beyond the reach of modern supercomputers. The superconductingqubit modality has been used to demonstrate prototype algorithms in the `noisy intermediate scale quantum‘ (NISQ) technology era, in which non-error-corrected qubits are used to implement quantum simulations and quantum algorithms. With the recent demonstrations of multiple high fidelity two-qubit gates as well as operations on logical qubits in extensible superconducting qubit systems, this modality also holds promise for the longer-term goal of building larger-scale error-corrected quantum computers. In this brief review, we discuss several of the recent experimental advances in qubit hardware, gate implementations, readout capabilities, early NISQ algorithm implementations, and quantum error correction using superconducting qubits. While continued work on many aspects of this technology is certainly necessary, the pace of both conceptual and technical progress in the last years has been impressive, and here we hope to convey the excitement stemming from this progress.
The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separatedby more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.