Abstract
The high kinetic inductance offered by granular aluminum (grAl) has recently been employed for linear inductors in superconducting high-impedance qubits and kinetic inductance detectors. Because of its large critical current density compared to typical Josephson junctions, its resilience to external magnetic fields, and its low dissipation, grAl may also provide a robust source of nonlinearity for strongly driven quantum circuits, topological superconductivity, and hybrid systems. Having said that, can the grAl nonlinearity be sufficient to build a qubit? Here we show that a small grAl volume () shunted by a thin film aluminum capacitor results in a microwave oscillator with anharmonicity two orders of magnitude larger than its spectral linewidth , effectively forming a transmon qubit. With increasing drive power, we observe several multiphoton transitions starting from the ground state, from which we extract . Resonance fluorescence measurements of the transition yield an intrinsic qubit linewidth , corresponding to a lifetime of , as confirmed by pulsed time-domain measurements. This linewidth remains below for in-plane magnetic fields up to .
- Received 10 January 2020
- Accepted 12 June 2020
DOI:https://doi.org/10.1103/PhysRevX.10.031032
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Superconducting quantum circuits constitute a versatile hardware platform for engineering interactions between light and matter, most prominently finding application in quantum information processing schemes. Recently, researchers have turned their attention to combining the unique properties of superconducting circuits with other quantum degrees of freedom such as molecular magnets and spin qubits. To that end, it is critically important in these so-called hybrid superconducting circuits to identify nonlinear inductive elements—an essential requirement for coherent state manipulation and amplification—that are resilient to magnetic fields. We demonstrate for the first time that the intrinsic nonlinearity of a disordered superconductor with a critical magnetic field in the tesla range, namely, granular aluminum, can be used to build a superconducting qubit.
We find that the granular aluminum qubit has some remarkable radio-frequency properties: state-of-the-art quantum coherence resilient to magnetic fields up to 0.1 T and robustness to strong microwave pump drives, showcased by multiphoton transitions. These properties make the granular aluminum qubit appealing for several communities working on hybrid quantum circuits for quantum information processing, as well as microwave detectors and fundamental mechanisms of superconductivity.
Following our proof-of-principle demonstration, future developments will focus on the enhancement of the magnetic field resilience, the nonlinearity, and the quantum coherence, as well as on coupling schemes to other quantum systems.
