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Enhanced coherence and Majorana qubits in superconducting circuits featuring π-periodic Josephson junctions

Luca Chirolli, Istituto Nanoscienze, Consiglio Nazionale delle Ricerche, Istituto CNR-NANO Pisa (Italy)

April 28th, 2022, 12:00 CEST

Salón de Actos, Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC and online.

A Josephson junction based on a π-periodic energy-phase relation has emerged as a novel element that can provide an augmented freedom of engineering in superconducting circuits. In systems interrupted by ordinary Josephson junctions, the dependence of the energy spectrum of the circuit on the offset charge on different islands is 2e periodic through the Aharonov-Casher effect and resembles a crystal band structure. The employment of cos(2φ) Josephson junctions enables tailoring of the Josephson potential and designing of spectra featuring multiplets of flat bands and Dirac points in the charge Brillouin zone. Flat bands provide noise-insensitive energy levels and engineering band pairs with flat spectral gaps can help improve the coherence of the system. More generally, a qubit based on a π-periodic Josephson junction features an increased degree of protection thanks to the suppression of individual Cooper pair tunneling at the junction and the resulting conserved boson parity of the qubit. We propose to couple such a parity-protected qubit to a Majorana qubit based on Majorana zero energy modes at the junction. By properly driving the system we can obtain a SWAP gate between the superconducting qubit and the Majorana qubit and employ the latter as a memory. The system enables fast gates and long-lived quantum memories, a key requirement for high fidelity quantum information processing in a noisy quantum computing environment.