Quantum Interference Amplifies Weak Chirality into Giant Quantum Nonreciprocity

Abstract: Quantum nonreciprocity at few-photon level typically requires strong symmetry breaking, posing significant experimental challenges. Here we demonstrate that phase-controlled quantum interference can amplify weak chirality into giant quantum nonreciprocity. We consider two phase-programmable atoms coupled to a spinning whispering-gallery-mode resonator, where interference dramatically amplifies the effect of weak Fizeau splitting. This mechanism generates pronounced directional asymmetry in photon statistics, featuring bright antibunched emission in one direction and strongly bunched emission in the opposite direction. Remarkably, both correlation and brightness isolations obey phase-controlled power-law scaling with Fizeau splitting, reaching up to 65~dB and 17.3~dB, respectively. Our results establish interference-enhanced weak chirality as a powerful route toward directional nonclassical light sources.

• The paper demonstrates how phase-controlled quantum interference amplifies weak Fizeau-induced chirality into giant nonreciprocity, achieving photon isolation ratios up to 65.7 dB.
• The methodology employs a cavity-QED system with two phase-programmable atoms coupled to a spinning WGM resonator, enabling tunable control over photon statistics via atomic phase adjustments.
• The results have practical implications for developing directional nonclassical light sources, programmable quantum photonic networks, and sensitive quantum sensors with relaxed symmetry-breaking requirements.

Quantum Interference Amplification of Weak Chirality in Nonreciprocal Few-Photon Quantum Optics

Motivation and Context

The realization of quantum nonreciprocal photonic devices operating at the few-photon level is foundational for scalable quantum information processing, chiral quantum networks, and high-fidelity nonclassical light sources. Conventional nonreciprocity mechanisms—magneto-optical, chiral coupling, parametric amplification, and non-Hermitian engineering—typically necessitate strong symmetry breaking, such as substantial magnetic fields or pronounced gain-loss imbalance, which limits experimental accessibility and scalability. This paper investigates an alternative approach: leveraging phase-controlled quantum interference to dramatically amplify weak chiral symmetry breaking, specifically the rotation-induced Fizeau splitting in whispering-gallery-mode (WGM) resonators, to achieve giant quantum nonreciprocity in photon statistics and brightness (2605.27447).

Model and Physical Mechanism

The core of the study is a cavity-QED system comprising two phase-programmable atoms coupled to a spinning WGM microresonator supporting clockwise (CW) and counterclockwise (CCW) optical modes. The rotation lifts the degeneracy of these modes via the Sagnac-Fizeau effect, resulting in a weak frequency splitting $\Delta_F$ that constitutes the chiral symmetry breaking. The relative atomic phase $\phi$—determined by the atoms’ spatial separation—manifests as a control parameter for quantum interference among excitation pathways.

The Hamiltonian incorporates the phase-imprinted coupling of each atom to both cavity modes. When $\Delta_F = 0$, photon emission is reciprocal, and the phase $\phi$ modulates interference effects, inducing a crossover between sub-Poissonian (antibunched) and super-Poissonian (bunched) photon statistics, as well as destructive interference-induced dark states. Introduction of a finite $\Delta_F$—even much smaller than typical Kerr nonlinearities or exceptional-point splittings—renders the system highly sensitive to chirality, causing dramatic directional asymmetry in both photon correlations and brightness.

Quantum Statistical Nonreciprocity

Numerical simulations using experimentally realistic parameters for $^{87}$Sr atoms and high-finesse WGM resonators demonstrate that phase-controlled interference amplifies weak Fizeau splitting into pronounced quantum nonreciprocity. The master equation, accounting for cavity and atomic dissipation, reveals:

• Directional photon statistics: For optimal $\phi$, the CW mode exhibits bright, strongly antibunched single-photon emission ($g^{(2)} \ll 1$) while the CCW mode exhibits strongly bunched multiphoton emission ($g^{(2)} \gg 1$), and vice versa depending on the atomic phase and detuning. This is a marked violation of conventional symmetry in few-photon regimes.
• High nonreciprocal isolation: The correlation isolation ratio reaches $65.7$ dB and brightness isolation $\phi$0 dB, with power-law scaling in $\phi$1; specifically, $\phi$2, where the exponents $\phi$3 can exceed $\phi$4 for correlation isolation.
• Contradictory regime: Contrary to photon-blockade intuition, strong antibunching can coexist with high emission brightness in a single spatial direction.

Power-Law Enhancement and Phase Control

A central claim is the power-law amplification of quantum nonreciprocity via phase-controlled interference. For small $\phi$5, the isolation ratios increase nonlinearly with the chiral splitting; the phase $\phi$6 acts as an enhancement knob, with maximal nonreciprocity achieved at $\phi$7. This mechanism enables programmable control over the nature and directionality of single- or multiphoton bundle emission, providing an unprecedented degree of tunability compared to magneto-optical or non-Hermitian approaches.

Experimental Feasibility and Distinction

The proposed scheme operates with Fizeau splitting two orders of magnitude smaller than those required for prior spinning-resonator-based nonreciprocal devices. The atomic phase can be stabilized and tuned experimentally in ring configurations with optical tweezers, avoiding the limitations of mechanical instability or rapid rotation. The approach is fundamentally distinct from Kerr-type nonlinearities, reservoir-engineered devices, or gain-loss-based exceptional point physics, which are typically intensity-dependent or limited in dynamic range.

Implications and Outlook

The results suggest that nonreciprocity can be encoded directly into quantum statistical observables, not just transmission or intensity profiles. This provides:

• Directional nonclassical light sources: Spatial separation and selective addressing of single- and multiphoton states.
• Relaxed requirements for symmetry breaking: Strong quantum nonreciprocity from weak chirality, removing reliance on large Sagnac shifts or strong non-Hermitian design.
• Ultra-sensitive quantum sensing and chiral-molecule detection: Amplification of weak optically-induced shifts into large asymmetries in photon statistics.
• Programmable quantum photonic networks: Phase-controlled interference as a resource for dynamically steerable chiral quantum information protocols.

Future developments may extend the scheme to larger atomic arrays, integrated photonic platforms, programmable quantum routers, and metrological devices exploiting enhanced directional quantum statistical response.

Conclusion

Phase-controlled quantum interference enables the amplification of weak Fizeau-induced chirality into giant quantum nonreciprocity in cavity QED systems. The mechanism produces highly directional photon statistics—bright single-photon emission (antibunching) and strong multiphoton bundle emission (bunching) in opposite directions—governed by power-law scaling in the chiral splitting and tunable by the atomic phase. This approach circumvents the technological barriers imposed by strong symmetry breaking requirements and establishes a versatile pathway toward high-fidelity nonreciprocal quantum photonic sources, programmable chiral networks, and sensitive quantum sensing platforms (2605.27447).