High coherence bosonic modes in long Josephson junction chains

Owing to high coherence and easy fabrication, 3-D Superconducting microwave cavities coupled to ancilla qubits are an attractive platform for the storage and manipulation of continuous variable quantum information. Recent experiments have demonstrated high fidelity QND readout, arbitrary control and remote entanglement of photonic states in these cavities[1]. Despite the progress, large size of the 3-D cavities poses a significant hurdle in scaling up the architecture. Here we present a way to get around the problem by using standing wave modes in 6mm long transmission line made out of two parallel chains of Josephson junctions[2]. Remarkably, we measure energy relaxation times exceeding 1ms in a setup with more than 20,000 junctions in one device.

[1] Y. Gao, B. Lester, K. Chou, L. Frunzio, M. Devoret, L. Jiang, S. Girvin, R. Schoelkopf. Nature, 566, 509-512 (2019)
[2] R. Kuzmin, R. Mencia, N. Grabon, N. Mehta, Y.-H. Lin, V. E. Manucharyan, Nature Physics, 15, 930-934 (2019)