Abstract
Superconducting quantum circuits are potential candidates to realize a large-scale quantum computer. The envisioned large density of integrated components, however, requires a proper thermal management and control of dissipation. To this end, it is advantageous to utilize tunable dissipation channels and to exploit the optimized heat flow at exceptional points (EPs). Here, we experimentally realize an EP in a superconducting microwave circuit consisting of two resonators. The EP is a singularity point of the Hamiltonian, and corresponds to the most efficient heat transfer between the resonators without oscillation of energy. We observe a crossover from underdamped to overdamped coupling via the EP by utilizing photon-assisted tunneling as an \emph{in situ} tunable dissipative element in one of the resonators. The methods studied here can be applied to different circuits to obtain fast dissipation, for example, for initializing qubits to their ground states. In addition, these results pave the way towards thorough investigation of parity--time ($\mathcal{PT}$) symmetric systems and the spontaneous symmetry breaking in superconducting microwave circuits operating at the level of single energy quanta.
Highlights
Systems with effective non-Hermitian Hamiltonians have been actively studied in various setups in recent years [1,2,3,4,5,6,7]
These methods can be used to obtain fast dissipation, for example, for initializing qubits to their ground states. These results pave the way for thorough investigation of parity-time symmetry and the spontaneous symmetry breaking at the exceptional points (EPs) in superconducting quantum circuits operating at the level of single energy quanta
The fascinating effects of EPs include the disappearance of the beating Rabi oscillations [14], chiral states in microwave systems [15], and spontaneous symmetry breaking in systems with parity- and time-reversal (PT ) symmetry [16,17,18,19]
Summary
Systems with effective non-Hermitian Hamiltonians have been actively studied in various setups in recent years [1,2,3,4,5,6,7]. The recently developed quantum-circuit refrigerator (QCR) [46,47] provides great potential for both qubit initialization and thermal management since it enables tunability of energy dissipation rates over several orders of magnitude in a superconducting microwave resonator [48]. We utilize tunable dissipation to realize EPs, which correspond to critical coupling between two superconducting microwave resonators. To this end, we investigate a circuit consisting of two coupled resonators, one of which is equipped with NIS junctions and a flux-tunable resonance frequency (Fig. 1). Extended measurement results are shown in Appendix A, electron tunneling through NIS junctions is described in Appendices B and C, quantum-mechanical and classical models for our samples are discussed in detail in Appendices D–I, and experimental techniques are presented in Appendices J–L
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