Blochnium: superconducting quasicharge qubit

Figure

Description

Blochnium is proposed as the dual of the transmon qubit, which exploits a coherent insulating response of a single Josephson junction that emerges from the extension of phase fluctuations beyond 2π. A weak junction is shunted by an extremely high inductance. The insulating character of the junction is manifested by the vanishing flux sensitivity of the qubit transition between the ground state and the first excited state, which recovers rapidly for transitions to higher-energy states. The spectrum agrees with a duality mapping of blochnium onto a transmon, which replaces the external flux by the offset charge and introduces a new collective quasicharge variable instead of the superconducting phase.

Motivation

Three fundamental types of superconducting qubit are known, each reflecting a distinct behaviour of quantum fluctuations in a Cooper pair condensate: single-charge tunnelling (charge qubit), single-flux tunnelling (flux qubit) and phase oscillations (phase qubit or transmon). Yet, the dual nature of charge and flux suggests that circuit atoms must come in pairs, ‘blochnium’ is proposed as the dual of the transmon.

References

• https://www.nature.com/articles/s41586-020-2687-9

Linked Papers

• pechenezhskiy-2020-blochnium

Related Entries

• transmon
• cooper-pair-box-charge-qubit
• fluxonium

Seed Metadata

• date_published: 2020-09-16

Physics

Operates in the extreme fluxonium limit $E_L\ll E_C,E_J$, where the superconducting phase $\phi$ becomes quasi-continuous. The qubit transition is between states in the lowest Bloch band of the Josephson cosine potential, with the conjugate “quasicharge” $p$ (Bloch momentum) as the good quantum number. Hamiltonian same as fluxonium:

$H=4E_C(\hat{n}-n_g{)}^2-E_J\cos \hat{\phi }+\frac{1}{2}{E}_L(\hat{\phi }-2\pi {\Phi }_{\text{ext}}/{\Phi }_0{)}^2$

Dual of the transmon: where the transmon flattens charge dispersion via $E_J/E_C\gg 1$, blochnium flattens flux dispersion via $E_L\to 0$. Exactly insensitive to offset charge; flux sensitivity can also be suppressed.

Key Metrics

Metric	Value	Notes	Fidelity reference
Qubit coherence $T_1$	10–100 μs	Early devices (2020)	Pechenezhskiy et al. 2020
Qubit coherence $T_2$	1–10 μs	Echo measurement	Pechenezhskiy et al. 2020
Charge sensitivity	Exactly zero	No island in circuit	—
Anharmonicity	Large	Unlike transmon	—
Operating temperature	10–20 mK	Dilution refrigerator	—
Qubit footprint	~500 μm	Superinductance chain	—

Related Qubits

• fluxonium — parent architecture
• transmon — dual qubit (charge vs flux continuous)
• cooper-pair-box-charge-qubit — island-based ancestor