Probing Sensitivity near a Quantum Exceptional Point using Waveguide Quantum Electrodynamics

TL;DR

This study emulates a passive PT dimer with superconducting qubits in waveguide QED, characterizing its dynamics near an exceptional point and assessing its potential for quantum sensing.

Contribution

It demonstrates the use of waveguide quantum electrodynamics to explore non-Hermitian quantum dynamics and evaluates the sensing capabilities near a quantum exceptional point.

Findings

No sensitivity enhancement observed near the quantum exceptional point.

System dynamics are broadly consistent with an ideal passive PT dimer, with some corrections.

Waveguide QED is validated as a platform for non-Hermitian quantum dynamics exploration.

Abstract

Non-Hermitian Hamiltonians with complex eigenenergies are useful tools for describing the dynamics of open quantum systems. In particular, parity and time (PT) symmetric Hamiltonians have generated interest due to the emergence of exceptional-point degeneracies, where both eigenenergies and eigenvectors coalesce as the energy spectrum transitions from real- to complex-valued. Because of the abrupt spectral response near exceptional points, such systems have been proposed as candidates for precision quantum sensing. In this work, we emulate a passive PT dimer using a two-mode, non-Hermitian system of superconducting qubits comprising one high-coherence qubit coupled to an intentionally lossy qubit via a tunable coupler. The loss is introduced by strongly coupling the qubit to a continuum of photonic modes in an open waveguide environment. Using both pulsed and continuous-wave measurements, we characterize the system dynamics near the exceptional point. We observe a behavior broadly consistent with an ideal passive PT dimer with some corrections due to the tunable coupler element. We extract the complex eigenenergies associated with the two modes and calculate the sensitivity as a function of the coupling strength. Confirming theoretical predictions, we observe no sensitivity enhancement near the quantum exceptional point. This work elucidates the limitations of exceptional-point systems as candidates for quantum-enhanced sensing. We establish waveguide quantum electrodynamics as a versatile platform for exploring non-Hermitian quantum dynamics in superconducting circuits.