Protected Flux Pairing Qubit

We have studied the coherent flux tunneling in a qubit containing two submicron Josephson junctions shunted by a superinductor (a dissipationless inductor with an impedance much greater than the resistance quantum [1]). The two low energy quantum states of this device, ${\left|0\right\rangle}$ and ${\left|1\right\rangle}$, are represented by even and odd number of fluxes in the loop, respectively. This device is dual to the charge pairing Josephson rhombi qubit [2]. The spectrum of the device, studied by microwave spectroscopy, reflects the interference between coherent quantum phase slips in the two junctions (the Aharonov-Casher effect). The time domain measurements demonstrate the suppression of the qubit's energy relaxation in the protected regime, which illustrates the potential of this flux pairing device as a protected quantum circuit. \\[4pt] [1] M. Bell et al., ``Quantum Superinductor with Tunable Nonlinearity,'' PRL 109, 137003 (2012). \\[0pt] [2] S. Gladchenko et al., ``Superconducting Nanocircuits for Topologically Protected Qubits,'' Nature Physics 5, 48 (2009).