# High fidelity quantum measurement in transmon qubits

## We are studying the physics of Quantum Non-Demolition (QND) measurements of a superconducting transmon qubit. These experiments are performed through an original coupling, called “cross-Kerr”, between the qubit and the readout microwave radiation field. Preliminary measurements have demonstrated very high readout fidelity as well as very high QNDness, paving the way for a superconducting multi-qubit platform for quantum computing based on this novel readout.

### Goal

We are studying non-demolition quantum measurements in a quantum system consisting of a superconducting quantum bit (a “transmon qubit”) coupled to a radiative microwave field which carries the information to a classical detector.

### Background

In conventional qubits, increasing the coupling between the qubit and the detector usually comes at the expense of limiting the lifetime and coherence of the qubit. The originality in our experiment lies in a new nonlinear coupling between the qubit and the microwave radiation field, called “cross-Kerr” which will not induce lifetime reduction [1-3]. To engineer this coupling, we fabricate a circuit called “transmon molecule” composed of two nominally identical transmons coupled inductively and capacitively together [4]. The resulting circuit exhibits two degrees of freedom with the desired cross-kerr coupling.

### Performances

In practice, the circuit is patterned on a thin film of aluminum with Josephson tunnel junctions [2]. The circuit is anchored to the base plate of a dilution fridge (30 mK) to ensure that thermal fluctuations are not exciting the system in a home-made dilution cryostat. So far, we demonstrated single-shot readout fidelities of 97.4% for 50-ns readout pulses with a QND-ness of 99% [5,6].

### Some of our recent publications

[1] Non-linear coupling between the two oscillation modes of a dc-SQUID F. Lecocq, J. Claudon, O. Buisson, and P. Milman Phys. Rev. Lett. 107, 197002 (2011).

[2] Junction fabrication by shadow evaporation without a suspended bridge, F. Lecocq, I. M Pop, Z. Peng, I. Matei, T. Crozes, T. Fournier, C. Naud, W. Guichard and O. Buisson, Nanotechnology 22 315302 (2011).

[3] Ultrafast QND measurements based on diamond-shape artificial atom, I. Diniz, E. Dumur, O. Buisson and A. Auffeves. Phys. Rev. A 87, 033837 (2013)

[4] A V-shape superconducting artificial atom based on two inductively coupled transmons, É. Dumur, B. Küng, A. K. Feofanov, T. Weissl, N. Roch, C. Naud, W. Guichard, O. Buisson, arXiv:1501.04892, Phys. Rev. B 92, 020515(R) (2015)

[5] Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling, R. Dassonneville, T. Ramos, V. Milchakov, L. Planat, E. Dumur, F. Foroughi, J. Puertas,S. Leger, K. Bharadwaj, J. Delaforce, C. Naud, W. Hasch-Guichard, J. J. Garcıa-Ripoll, N. Roch, and O. Buisson, Phys. Rev. X 10, 011045 (2020)

[6] Transmon qubit readout using in-situ bifurcation amplification in the mesoscopic regime, R. Dassonneville, T. Ramos, V. Milchakov, C. Mori, L. Planat, E. Dumur, F. Foroughi, C. Naud, W. Hasch-Guichard, J. J. Garcıa-Ripoll, N. Roch, and O. Buisson, arXiv2210.04793 (2023).