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. 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.