Transmon

The transmon is a type of superconducting charge qubit widely used in quantum computing. It is designed to reduce the sensitivity to charge noise, a significant source of decoherence in earlier charge qubit designs.

The core of a transmon qubit is a Cooper pair box, a superconducting island connected to a superconducting reservoir via a Josephson junction. A key characteristic of the transmon is a large ratio of the Josephson energy (EJ) to the charging energy (EC), typically EJ/EC >> 1. This large ratio makes the qubit states exponentially insensitive to charge fluctuations on the island.

By decreasing the charging energy relative to the Josephson energy, the transmon's energy levels become less dependent on the precise number of Cooper pairs on the superconducting island. This effectively protects the qubit from dephasing caused by stray charges fluctuating in the environment.

The energy levels of a transmon are still anharmonic, meaning they are not equally spaced. This anharmonicity allows for selective excitation of the qubit's two lowest energy levels, representing the |0⟩ and |1⟩ states, using microwave pulses. The anharmonicity also prevents unwanted transitions to higher energy levels during qubit manipulation.

While reducing charge sensitivity, increasing EJ/EC also reduces the qubit's sensitivity to control signals. Therefore, transmon design involves carefully balancing robustness against charge noise with controllability. Different variations of the transmon exist, such as the Xmon and fluxmon, which further optimize specific aspects of the qubit's performance.

The transmon's relatively long coherence times and ease of fabrication have made it a leading candidate for building larger-scale quantum computers.