Enhancement of Quantum Heat Engine by Encircling a Liouvillian Exceptional Point

Quantum heat engines are expected to outperform the classical counterparts due to quantum coherences involved. Here we experimentally execute a single-ion quantum heat engine and demonstrate, for the first time, the dynamics and the enhanced performance of the heat engine originating from the Liouvillian exceptional points (LEPs). In addition to the topological effects related to LEPs, we focus on thermodynamic effects, which can be understood by the Landau-Zener-Stückelberg process under decoherence. We witness a positive net work from the quantum heat engine if the heat engine cycle dynamically encircles a LEP. Further investigation reveals that a larger net work is done when the system is operated closer to the LEP. We attribute the enhanced performance of the quantum heat engine to the Landau-Zener-Stückelberg process, enabled by the eigenenergy landscape in the vicinity of the LEP, and the exceptional point-induced topological transition. Therefore, our results open new possibilities toward LEP-enabled control of quantum heat engines and of thermodynamic processes in open quantum systems.

Hierarchical Construction of Higher-Order Exceptional Points

The realization of higher-order exceptional points (HOEPs) can lead to orders of magnitude enhancement in light-matter interactions beyond the current fundamental limits. Unfortunately, implementing HOEPs in the existing schemes is a rather difficult task, due to the complexity and sensitivity to fabrication imperfections. Here we introduce a hierarchical approach for engineering photonic structures having HOEPs that are easier to build and more resilient to experimental uncertainties. We demonstrate our technique by an example that involves parity-time symmetric optical microring resonators with chiral coupling among the internal optical modes of each resonator. Interestingly, we find that the uniform coupling profile is not required to achieve HOEPs in this system-a feature that implies the emergence of HOEPs from disorder and provides resilience against some fabrication errors. Our results are confirmed by using full-wave simulations based on Maxwell's equation in realistic optical material systems.

Controlling directional absorption with chiral exceptional surfaces

Significant efforts have been dedicated to engineering optical systems with predefined, excitation-dependent light absorption. An important concept along this line is that of exceptional points which allow for engineering directional light absorbing schemes. Current systems, however, do not lend themselves to easy design criterion or robust experimental realization. Here we demonstrate that an optical microring resonator coupled to a waveguide terminated with a mirror supports a chiral exceptional surface that can be used as a platform for tailoring directional light absorption in a straightforward fashion. We further demonstrate that this configuration can be used to implement a unidirectional coherent perfect absorber with controllable differential loss by tuning only a single parameter.

Sensing with Exceptional Surfaces in Order to Combine Sensitivity with Robustness

Exceptional points (EPs) are singularities that arise in non-Hermitian physics. Current research efforts focus only on systems supporting isolated EPs characterized by increased sensitivity to external perturbations, which makes them potential candidates for building next generation optical sensors. On the downside, this feature is also the Achilles heel of these devices: they are very sensitive to fabrication errors and experimental uncertainties. To overcome this problem, we introduce a new design concept for implementing photonic EPs that combine the robustness required for practical use together with their hallmark sensitivity. Particularly, our proposed structure exhibits a hypersurface of Jordan EPs embedded in a larger space, and having the following peculiar features: (1) A large class of undesired perturbations shift the operating point along the exceptional surface (ES), thus, leaving the system at another EP which explains the robustness; (2) Perturbations due to back reflection or backscattering force the operating point out of the ES, leading to enhanced sensitivity. Importantly, our proposed geometry is relatively easy to implement using standard photonics components and the design concept can be extended to other physical platforms such as microwave or acoustics.

High-order exceptional points in optomechanics

We study mechanical cooling in systems of coupled passive (lossy) and active (with gain) optical resonators. We find that for a driving laser which is red-detuned with respect to the cavity frequency, the supermode structure of the system is radically changed, featuring the emergence of genuine high-order exceptional points. This in turn leads to giant enhancement of both the mechanical damping and the spring stiffness, facilitating low-power mechanical cooling in the vicinity of gain-loss balance. This opens up new avenues of steering micromechanical devices with exceptional points beyond the lowest-order two.

Visible light emission from a silica microbottle resonator by second- and third-harmonic generation

We report the first observation of nonlinear harmonic generation and sum frequency generation (SFG) coupled with stimulated Raman scattering (SRS) via the second-order (χ⁽²⁾) and the third-order (χ⁽³⁾) nonlinearities in a silica microbottle resonator. The visible light emission due to third-harmonic generation (THG) was observed in both the output of a tapered fiber and the optical microscope images, which can be used to identify the axial mode profiles. SFG enabled by three- and four-wave mixing processes between the pump light and the light generated via SRS was also observed. Second-harmonic generation (SHG) and the SFG are enabled by χ⁽²⁾ induced in silica by surface effects and multipole excitations.