The paper introduces a new type of superconducting qubit called the "transmon," which operates at a significantly higher ratio of Josephson energy to charging energy ($E_J/E_C$) compared to the Cooper pair box (CPB) qubit. The transmon is designed to reduce sensitivity to charge noise and increase qubit-photon coupling while maintaining sufficient anharmonicity for selective qubit control. The key advantages of the transmon include: 1. **Charge Noise Sensitivity**: The charge dispersion of the transmon decreases exponentially with $E_J/E_C$, leading to a dramatic reduction in sensitivity to charge noise. 2. **Anharmonicity**: While the charge dispersion decreases exponentially, the anharmonicity only decreases algebraically with a weak power law. This allows for a range of $E_J/E_C$ where the transmon has significantly improved charge noise insensitivity and sufficient anharmonicity for qubit operations. 3. **Circuit QED**: The transmon can be embedded in a superconducting transmission line resonator, enabling control and readout via resonant radiation, similar to CPB qubits. 4. **Coupling Strength**: Despite the reduced charge dispersion, the coupling between the transmon and the resonator increases with $E_J/E_C$, making it highly polarizable and responsive to electric fields at all frequencies. The paper provides a detailed analysis of the transmon's properties, including its Hamiltonian, charge dispersion, anharmonicity, and coupling strength. It also discusses the dispersive limit, where coherent control and readout can be achieved, and compares the transmon to other qubit designs, highlighting its potential for realizing a scalable quantum computer.The paper introduces a new type of superconducting qubit called the "transmon," which operates at a significantly higher ratio of Josephson energy to charging energy ($E_J/E_C$) compared to the Cooper pair box (CPB) qubit. The transmon is designed to reduce sensitivity to charge noise and increase qubit-photon coupling while maintaining sufficient anharmonicity for selective qubit control. The key advantages of the transmon include: 1. **Charge Noise Sensitivity**: The charge dispersion of the transmon decreases exponentially with $E_J/E_C$, leading to a dramatic reduction in sensitivity to charge noise. 2. **Anharmonicity**: While the charge dispersion decreases exponentially, the anharmonicity only decreases algebraically with a weak power law. This allows for a range of $E_J/E_C$ where the transmon has significantly improved charge noise insensitivity and sufficient anharmonicity for qubit operations. 3. **Circuit QED**: The transmon can be embedded in a superconducting transmission line resonator, enabling control and readout via resonant radiation, similar to CPB qubits. 4. **Coupling Strength**: Despite the reduced charge dispersion, the coupling between the transmon and the resonator increases with $E_J/E_C$, making it highly polarizable and responsive to electric fields at all frequencies. The paper provides a detailed analysis of the transmon's properties, including its Hamiltonian, charge dispersion, anharmonicity, and coupling strength. It also discusses the dispersive limit, where coherent control and readout can be achieved, and compares the transmon to other qubit designs, highlighting its potential for realizing a scalable quantum computer.