Qubit readout using a transmon with a V-shaped energy diagram

The most widely used scheme to perform qubit readout in cQED relies on the dispersive coupling between a qubit and a harmonic oscillator. However, despite important progresses, implementing a fast and high fidelity readout remains nowadays a major challenge. Indeed, inferring a qubit state is limited by the trade-off between speed and accuracy due to Purcell effect and transitions induced by readout photons in the resonator. To overcome this, we introduce a new device: a transmon with a V-shaped energy diagram, embedded in 3D architecture. It is made of two transmons coupled via a large inductance [1]. The resulting circuit presents two qubits longitudinally coupled – called qubit and ancilla. Using symmetry rules [2], the ancilla can be strongly coupled to the cavity while the qubit remains uncoupled. However due to their strong longitudinal coupling, the qubit state can still be inferred through the ancilla state.
[1] É. Dumur, et al, Phys. Rev. B 92, 020515(R) (2015).
[2] É. Dumur, et al, IEEE Trans. On Appl. Supercond. 26, 1700304 (2016).