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Bulletin of the American Physical Society

Prospects for a prototype quantum processor based on three-dimensional superconducting cavities and qubits

Chad T. Rigetti

Published Feb 29, 2012• 1 citation
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We present cavity QED experiments with an Er3+:Y2SiO5{\mathrm{Er}}^{3+}:{\mathrm{Y}}_{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ crystal magnetically coupled to a three-dimensional (3D) cylindrical sapphire loaded copper resonator. Such waveguide cavities are promising for the realization of a superconducting quantum processor. Here, we demonstrate the coherent integration of a rare-earth spin ensemble with the 3D architecture. The collective coupling strength of the Er3+{\mathrm{Er}}^{3+}Er3+ spins to the 3D cavity is 21 MHz. The cylindrical sapphire loaded resonator allowed us to explore the anisotropic collective coupling between the rare-earth doped crystal and the cavity. This work shows the potential of spin doped solids in 3D quantum circuits for application as microwave quantum memories as well as for prospective microwave to optical interfaces.
The prospects for a utility-scale, fault-tolerant superconducting quantum computer have grown increasingly promising through demonstrations of quantum advantage and quantum error correction. The next breakthrough will likely be driven by processors at scale, with qubit counts exceeding tens of thousands. Achieving this scale presents a formidable challenge for current architectures, including limitations in chip footprint, cryogenic cooling capacity, and control complexity. One approach to alleviating these constraints is to make each qubit better, thereby reducing the redundancy required for error correction. Protected qubits provide a promising path in this direction by encoding quantum information into a decoherence-free subspace. This can be achieved through Hamiltonian engineering in superconducting circuits, with representative examples including the 0–π qubit, the cos(2φ) qubit, and the GKP qubit. In this thesis, we present the superconducting grid states qubit, or gridium, as a strong candidate for protected qubits. The gridium consists of a Cooper-quartet tunneling element shunted by a quantum phase slip element. We show that the gridium realizes a decoherence-free subspace within a single mode and can achieve strong protection using readily available superconducting circuit elements. We further demonstrate that its key components—Cooper-quartet tunneling and quantum phase slip—can be realized and engineered through appropriate circuit design, as illustrated with a prototype device. By applying Hamiltonian engineering, we construct three gridium devices with varying levels of protection. We show that the measured spectra are consistent with numerical simulations of the theoretical model. From these fits, we extract the corresponding wavefunctions and observe enhanced protection as the number of grids increases. To directly probe this protection, we measure the depolarization times of the devices while dynamically tuning the protection via flux control. Our best device exhibits T1 = 2.4 ms, comparable to state-of-the-art superconducting qubits. The gridium architecture presented in this thesis demonstrates strong potential for realizing fault-tolerant quantum processors based on protected qubits.