Observation of the parity-time symmetry breaking transition in a dissipative superconducting qubit

Non-Hermitian systems that exhibit parity-time (PT) symmetry have been recently studied in electrical, optical, and mechanical classical systems revealing applications in non-reciprocal devices and sensing. While questions have remained as to whether PT-symmetric systems are realizable in the quantum regime due to the role of quantum noise in amplifiers, the topological features associated with PT-symmetric Hamiltonians are shared by dissipative systems with selective losses, which can be realized at the quantum level. We employ bath engineering techniques to realize a non-Hermitian Hamiltonian that has effective PT symmetry for a superconducting circuit. We use quantum state tomography to observe the PT symmetry breaking transition as manifested in the evolution of both diagonal and off-diagonal elements of the qubit density matrix