Experimental Demonstration of a Superconducting 0-π Qubit*

Encoding a qubit in logical quantum states with wavefunctions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing. Using a two-dimensional circuit-quantum-electrodynamics architecture, we experimentally realize a superconducting 0-π qubit, which hosts protected states suitable for quantum-information processing. Our multi-tone spectroscopy measurements reveal the energy level structure of the system, which can be precisely described by a simple two-mode Hamiltonian. We find that the circuit realizes an effective one-dimensional crystal with two sub-lattices, where the geometrical phase difference between Wannier states localized at adjacent phase unit cells leads to interference effects associated with tunneling of pairs of Cooper pairs. The parity symmetry of the qubit results in charge-insensitive levels connecting the protected states, allowing for logical operations. The measured relaxation (1.6 ms) and dephasing times (25 μs) demonstrate that our implementation of the 0-π circuit not only broadens the family of superconducting qubits but also represents a promising candidate for the building block of a fault-tolerant quantum computer.