Chiral bound states in the continuum: a higher-order singularity for on-chip control of quantum emission

Abstract

We demonstrate a fully integrable and reconfigurable platform for controlling quantum emission by harnessing chiral bound states in the continuum (BICs) as a higher-order non-Hermitian singularity. Our architecture employs dual-microring resonators evanescently coupled to two waveguides, supporting symmetry-protected BICs. By integrating an integrated reflector coupled with one resonator as a unidirectional feedback, a pair of orthogonal BICs gets transformed into a single, chiral BIC residing on an exceptional surface. The phase terms in external coupling and inter-modal coupling serve as two independent tuning knobs, enabling unprecedented dynamic control over the spontaneous emission dynamics of individual quantum emitters, including the Purcell enhancement and the emission lineshape. The efficiency in reconfiguring the output intensity gets promoted by more than a factor of two compared to alternative schemes, offering a promising path toward high-speed quantum optical switches and active lifetime control in integrated quantum photonic circuits.

Figures (4)

• Figure 1: (a) Schematic of S-BICs in a dual-ring resonator system. (b) Formation of a chiral BIC by introducing a unidirectional coupling term A. (c) Numerically calculated imaginary parts of the eigenvalue surfaces in the $\varepsilon$--$\Delta\beta$ parameter space for the S-BIC (left) and chiral-BIC (right) systems. Here, we set $\gamma_{1} = \gamma_{2} + \gamma_{\kappa} = 0.2~\mathrm{GHz}$, $\gamma_{\mathrm{wg}} = 0.4~\mathrm{GHz}$, and $\Delta\omega = 0$. (d) Realization of a circuit-level cQED system harnessing a chiral quasi-BIC, including a QE embedded in cavity II and three phase shifters (PS1, PS2, and PS3). The QE couples to both CW and CCW modes with an identical electric dipole moment.
• Figure 2: (a--b) Calculated $\eta_{\mathrm{int}}$ versus frequency and phase difference $\Delta\varphi$ for $\Delta\beta = \pi$ (a) and $\Delta\beta = \pi/2$ (b). Here, we set $\Delta\omega = 0$, $\gamma_{\kappa} = 2.5~\mathrm{GHz}$, $\gamma_{\mathrm{wg}} = 5~\mathrm{GHz}$, and $\gamma_{1} = \gamma_{2} = 0~\mathrm{GHz}$. (c) Calculated $\eta_{\mathrm{FP}}$ as a function of $\Delta\beta$ and the sum of the intrinsic cavity losses, $\gamma_{1} + \gamma_{2}$. The dashed line denotes the case for a typical TFLN-based microring resonator with an intrinsic $Q$ factor of 20000. (d) Calculated $\eta_{\mathrm{int}}$ as a function of $\Delta\beta_{1}$ and $\Delta\beta_{2}$ at zero frequency detuning and $\Delta\varphi = \pi$. The dashed line denotes the ideal symmetric external coupling case where $\Delta\beta_{1} = \Delta\beta_{2}$.
• Figure 3: (a) Simulated output spectra for the chiral quasi-BIC ($\Delta\varphi = \pi$, $\Delta\beta = \pi$), chiral leaky state ($\Delta\varphi = \pi$, $\Delta\beta = \pi/2$), and regular quasi-BIC ($\Delta\beta = \pi$). Simulations use a refractive index contrast of 2/1.55 between LN waveguides and silica cladding. The two identical microrings have a radius of 10 $\mu$m, bus waveguides and rings are 400 nm wide with a 250 nm coupling gap. The feedback waveguide has a 300 nm gap and a perfect electric conductor at its end as a reflector. The QE is modeled as a point dipole mimicking a hybrid InGaAs quantum dot at $\sim$901 nm. (b) Mode electric field intensity $|E|^2$ on a logarithmic scale for the three cases in (a). Insets: zoomed-in views of the region surrounding the embedded QE. (c) Extracted output normalized intensity versus $\Delta n$ applied to the bus waveguides. Solid line: theoretical fit.
• Figure 4: (a) Calculated $\eta_{\mathrm{int}}$ versus applied phase for three different configurations, including the conventional microring system operated at a DP (top), the single microring system operated at a chiral ES (middle), and a dual-ring system operated around a chiral quasi-BIC (bottom). Insets: corresponding system schematics. (b) Calculated lifetime traces of a single QE upon reconfiguration of $\Delta\beta$. (c) Calculated emission spectra upon reconfiguration of $\Delta\beta$. The cases under a DP condition are also presented in (b--c) for comparison. (d) Map of $\eta_{\mathrm{int}}$ as a function of $\Delta\beta$ and the loss contrast $\gamma_{\mathrm{wg}}/(\gamma_{1} + \gamma_{2})$. The dashed lines indicate the phase amplitude required for a 20 dB dynamic range of $\eta_{\mathrm{int}}$.