R.Dassonneville, E. Dumur, B. Küng, A.K. Feofanov, T. Weissl, C. Naud, N. Roch, W. Guichard, O. Buisson

Institut Néel, CNRS–Université Joseph Fourier, BP 166, 38042 Grenoble Cedex 9, France

We present an experimental study on two transmons (i.e., small capacitively shunted Josephson junctions) coupled via a large inductance [1]. The resulting circuit exhibits a symmetric and an antisymmetric oscillation [2] which we use as a transmon and ancilla qubit, respectively. We observe a cross-Kerr-like coupling of the two oscillations which is explained by the Josephson nonlinearity [1]. This coupling leads the artificial atom to a have V-shape energy diagram.

We have predicted that such V-shape artificial atom allows to read out the transmon qubit state by using the ancilla qubit frequency [3]. In comparison with the most widely employed readout scheme for superconducting qubits, the dispersive readout in circuit quantum electrodynamics architecture, this approach promises a quantum non-demolition measurement with a significantly stronger measurement signal and without suffering from Purcell effect. In a measurement chain based on a state-of-the-art Josephson parametric amplifier, we predict a QND fidelity of up to 99.9% for a measurement time down to 60 ns [3]. This should allow the measurement of quantum trajectories and the testing of some new concepts of quantum thermodynamics.

Acknowledgment: this work is supported by the ANR-NSFC project.

[1] É. Dumur, et al, “A V-shape superconducting artificial atom based on two inductively coupled transmons”, arXiv1501.04892.

[2] F. Lecocq, et al, "Coherent Frequency Conversion in a Superconducting Artificial Atom with Two Internal Degrees of Freedom", Physical Review Letters 108, 107001 (2012).

[3] I. Diniz, et al, "Ultrafast quantum nondemolition measurements based on a diamond shaped artificial atom", Physical Review A 87, 033837 (2013).