## A charge-phase quantum bit circuit protected from decoherence: the quantronium

Achieving quantum coherence has been the main obstacle for the operation of quantum bit circuits. Although it was clearly understood that decoupling the circuit from the outside world was essential, no systematic strategy was available to achieve this goal. The quantronium circuit is the first example in which such a strategy was used.

Decoupling the qubit from the readout circuitry is the main problem in achieving good quantum because it provides an entry port for undesired noise, and because strong coupling is necessary at readout time. We have developed a general strategy based on operating the qubit at a working point where the transition frequency is stationary respectively to small changes in the control parameters, which results in an effective decoupling of the qubit from the readout circuitry. For the purpose of readout, the working point is then displaced. This key-strategy has been implemented in a modified Cooper pair box circuit called the quantronium because it behaves as an artificial atom.

The readout is performed by measuring the switching of the readout junction after a bias current pulse. The effective critical current is modified by the current of the qubit state in a small loop. The qubit is manipulating using microwave pulses applied to the box gate. A coherence time of 500ns was measured using a two pulse Ramsey interference scheme. This corresponds to a quality factor of about 25000, which is at the time of writing the best achieved for a qubit circuit.

The quantronium has been patented, including the key strategy to minimize decoherence. This method is now currently used by the project partners working with the Cooper pair box and with the flux qubit.

The most important publications describing this result, and the corresponding patent granted, are given in the document list. Other publications on the quantronium have not been mentioned there, but are listed in the final report of the SQUBIT project.

Decoupling the qubit from the readout circuitry is the main problem in achieving good quantum because it provides an entry port for undesired noise, and because strong coupling is necessary at readout time. We have developed a general strategy based on operating the qubit at a working point where the transition frequency is stationary respectively to small changes in the control parameters, which results in an effective decoupling of the qubit from the readout circuitry. For the purpose of readout, the working point is then displaced. This key-strategy has been implemented in a modified Cooper pair box circuit called the quantronium because it behaves as an artificial atom.

The readout is performed by measuring the switching of the readout junction after a bias current pulse. The effective critical current is modified by the current of the qubit state in a small loop. The qubit is manipulating using microwave pulses applied to the box gate. A coherence time of 500ns was measured using a two pulse Ramsey interference scheme. This corresponds to a quality factor of about 25000, which is at the time of writing the best achieved for a qubit circuit.

The quantronium has been patented, including the key strategy to minimize decoherence. This method is now currently used by the project partners working with the Cooper pair box and with the flux qubit.

The most important publications describing this result, and the corresponding patent granted, are given in the document list. Other publications on the quantronium have not been mentioned there, but are listed in the final report of the SQUBIT project.