Non-Markovian exceptional points by interpolating quantum channels

Area of Science:

• Quantum Physics
• Non-Hermitian Systems
• Quantum Information

Background:

• Exceptional points (EPs) are critical phenomena in non-Hermitian systems where eigenvalues and eigenvectors coalesce.
• Traditional studies of EPs in open quantum systems often rely on simplified models like non-Hermitian Hamiltonians or Liouvillians, which do not capture the full complexity of quantum dynamics.

Purpose of the Study:

• To introduce a general strategy for creating quantum-channel exceptional points (EPs) in single-qubit systems.
• To explore the phase transitions and EP emergence in quantum channels without requiring additional symmetries.

Main Methods:

• Developed a theoretical framework for generating quantum-channel EPs.
• Investigated the natural phase transitions of quantum channels between purely real and complex-conjugate eigenvalue spectra.
• Experimentally simulated single-qubit quantum channels using nuclear magnetic resonance (NMR) quantum computing.

Main Results:

• Demonstrated that quantum channels exhibit two distinct phases (purely real or complex-conjugate eigenvalues) that can transition between each other.
• Experimentally observed second-order quantum-channel EPs with high fidelity (93%) using NMR.
• Extended the method to three channels, revealing exceptional lines and a third-order EP.

Conclusions:

• The proposed strategy provides a general method for generating quantum-channel EPs, offering a more complete description of open quantum dynamics.
• The findings open avenues for applications in enhanced quantum sensing, quantum control, and phenomena like Jordan-chain mediated asymmetric conversion.