A superconducting qubit with noise-insensitive plasmon levels and decay-protected fluxon states

Abstract

The inductively shunted transmon (IST) is a superconducting qubit with exponentially suppressed fluxon transitions and a plasmon spectrum approximating that of the transmon. It shares many characteristics with the transmon but offers charge offset insensitivity for all levels and precise flux tunability with quadratic flux noise suppression. In this work we propose and realize IST qubits deep in the transmon limit where the large geometric inductance acts as a mere perturbation. With a flux dispersion of only 5.1 MHz we reach the 'sweet-spot everywhere' regime of a SQUID device with a stable coherence time over a full flux quantum. Close to the flux degeneracy point the device reveals tunneling physics between the two quasi-degenerate ground states with typical observed lifetimes on the order of minutes. In the future, this qubit regime could be used to avoid leakage to unconfined transmon states in high-power read-out or driven-dissipative bosonic qubit realizations. Moreover, the combination of well controllable plasmon transitions together with stable fluxon states in a single device might offer a way forward towards improved qubit encoding schemes.

Observation of Josephson Harmonics in Tunnel Junctions

“…Note that the series inductance does not break the 2π periodicity of the Hamiltonian, despite the occurrence of ϕ 2 . This is in contrast to a shunt inductance, for which the flux ϕ and the charge n would need to be treated as non-compact operators with continuous spectrum R, see [85][86][87][88]. For a series inductance, though, due to the two symmetry properties of ϕ ∆ (ϕ), the two ϕ-dependent terms of Eq.…”

Entangling transmons with low-frequency protected superconducting qubits

“…Motivated by the challenge of exploring complementary qubit modalities, several alternative qubit encodings have been proposed [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and experimentally studied [33][34][35][36][37][38][39][40][41][42][43][44][45]. A particular class of such novel qubit encodings are Cooper-pair parity-protected qubits (PPQ) [30][31][32][33][34], which rely on a special Josephson element that only permits tunneling of pairs of Cooper-pairs.…”