Transmon qubits: a charge-insensitive superconducting design

Short answer: The transmon is a superconducting qubit design proposed in 2007 by a team that included Jens Koch. It is derived from the Cooper pair box but operated at a significantly higher ratio of Josephson energy to charging energy. The authors predicted that this makes it much less sensitive to charge noise while keeping enough anharmonicity to control the qubit selectively. This article describes that proposal, as written in the paper.

The problem being solved

The paper "Charge-insensitive qubit design derived from the Cooper pair box", published in Physical Review A in October 2007, begins by saying short dephasing times pose one of the main challenges in realizing a quantum computer. It says different approaches have been devised for superconducting qubits, a prime example being operation at optimal working points called sweet spots. That approach led to significant improvement of the T2 time in Cooper pair box qubits, in earlier work by Vion and colleagues.

What the transmon changes

The authors introduce a new type of superconducting qubit that they call the transmon. Unlike the charge qubit, they say, the transmon is designed to operate in a regime of significantly increased ratio of Josephson energy to charging energy, written EJ over EC. In the introduction they describe it as a transmission-line shunted plasma oscillation qubit, closely related to the Cooper pair box but operated at a significantly different ratio.

Two quantities that matter

The paper explains that two quantities are crucial to the operation of a Cooper pair box: the anharmonicity and the charge dispersion of the energy levels. A sufficiently large anharmonicity is needed to prevent qubit operations from exciting other transitions in the system. The charge dispersion describes the variation of the energy levels with respect to environmental offset charge and gate voltage, and determines the sensitivity to charge noise: the smaller the charge dispersion, the less the qubit frequency changes in response to gate charge fluctuations.

The trade-off the authors describe

The abstract says the transmon benefits from the fact that its charge dispersion decreases exponentially with EJ over EC, while its loss in anharmonicity is described by a weak power law. As a result, the authors say they predict a drastic reduction in sensitivity to charge noise relative to the Cooper pair box and an increase in the qubit-photon coupling, while maintaining sufficient anharmonicity for selective qubit control. They add that their detailed analysis of the full system shows the gain is not compromised by increased noise in other known channels. The introduction says the design choice should lead to dramatically improved dephasing times.

What this does not tell you

This article summarises the abstract and introduction of a 2007 theory and design paper. The statements are the authors' predictions at the time. It reports no measured coherence times and does not say how any current chip performs. Later experimental papers would be needed for that.

FAQ

Who proposed the transmon?

A team including Jens Koch, Terri Yu, Jay Gambetta and others, in a 2007 Physical Review A paper.

What is it derived from?

The Cooper pair box qubit.

What is the key design change?

Operating at a significantly increased ratio of Josephson energy to charging energy.

Why does that help?

The authors say charge dispersion falls exponentially with that ratio, which they predict greatly reduces sensitivity to charge noise.

What is the cost?

Anharmonicity falls, but the authors describe the loss as a weak power law and say enough remains for selective control.