A 0-pi qubit with in-situ tunable protection and control

The 0-pi qubit is one of the very few known superconducting circuit types that offers exponential protection from both phase and bit flip errors simultaneously. This feature comes at the cost of complex readout and control, which was shown experimentally in [1]. Here, we propose a way to break the circuit symmetry, the key reason for the protection, to momentarily restore the ability to control and manipulate the qubit without the use of ancillary levels. An asymmetry in capacitances and inductances in the 0-pi circuit are detrimental since they lead to coupling of the protected state to the thermally occupied parasitic mode of the circuit [2]. However, here we try to exploit a controlled asymmetry in Josephson energies and show that this can be used as a tunable coupler between the protected states. In the future, this should allow to perform gate operations by dynamically controlling the asymmetry instead of driving the protected transition with microwave pulses. Therefore, we believe that the proposed method can make the use of protected qubits more practical in experimental realizations of quantum computing.

[1] Gyenis, A., Mundada, P. S., Di Paolo, A., Hazard, T. M., You, X., Schuster, D. I., ... & Houck, A. A. (2021). Experimental realization of a protected superconducting circuit derived from the 0–π qubit. PRX Quantum, 2(1), 010339.

[2] Dempster, J. M., Fu, B., Ferguson, D. G., Schuster, D. I., & Koch, J. (2014). Understanding degenerate ground states of a protected quantum circuit in the presence of disorder. Physical Review B, 90(9), 094518.