Charge-insensitive qubit design derived from the Cooper pair box

Introduces the transmon qubit: a Cooper pair box operated at large $E_J/E_C$ ratio (~50), where charge dispersion is exponentially suppressed while anharmonicity decreases only as a power law. This tradeoff makes the transmon practically immune to charge noise — the dominant decoherence source for the CPB — at the cost of reduced anharmonicity (~200–300 MHz). The transmon has become the most widely deployed superconducting qubit.

Key Results

• Charge dispersion exponentially suppressed: ${ϵ}_m\propto e^{-\sqrt{8E_J/E_C}}$
• Anharmonicity decreases algebraically: $\alpha \approx -E_C$
• Sweet spot: $E_J/E_C\approx 50$ gives charge sensitivity <1% with ~300 MHz anharmonicity
• T₁ ~ 1–5 μs (2007, limited by dielectric loss)
• Foundation for Google, IBM, Rigetti, and AWS quantum processors

Links

• Journal: Phys. Rev. A 76, 042319 (2007)
• arXiv: cond-mat/0703002

Zoo Links

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