&Bullet; physics 14, p70
Squeezed wave functions reshape the bulk boundary properties of an open quantum system and create a new class of parity-time symmetry.
Quantum systems that obey parity-time symmetry have “exception points” within their parameter space. In quantum optical systems, these are points where the energy gains and losses of a light beam are perfectly balanced, resulting in rich critical phenomena such as extreme sensitivity to external parameters. Parity-time symmetry can be incorporated into systems through designs that ensure a balance of incoming and outgoing energy flows (see point of view: PTSymmetry becomes quantum). Now, Lei Xiao of the Beijing Computational Science Research Center and colleagues are demonstrating a new way to create both parity-time symmetry and extraordinary points in a quantum optical circuit. Your study could allow tools to tune exotic behaviors in photonic, phononic, and cold atomic systems  .
The energy input-output balance required to generate parity-time symmetry is usually achieved by researchers with systems with spatially periodic structures. The emergence of exception points in such systems is predicted with the help of the Bloch band theory. In their experiment, Xiao and his colleagues observed the presence of parity-time symmetry and extraordinary points by measuring how photons interact with themselves as they propagate through a series of optical elements. Although this system has a periodic spatial structure in its volume, it has an open boundary condition, which means that its properties cannot be described by conventional Bloch band theory.
The researchers came to the conclusion that in their experiment a parity-time symmetry arose as a result of the “skin effect”, which means that wave functions are squeezed in the direction of a system boundary. Since it distorts the volume wave functions, the skin effect can only be described by an unconventional “non-Bloch band theory”. The development of artificial materials to use this mechanism could lead to quantum devices with high resolution sensor functions.
Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.
- L. Xiao et al., “Observation of non-Bloch parity-time symmetry and exception points”, Phys. Rev. Lett.126, 230402 (2021).