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arXiv:2607.26898

Revival of transport reciprocity via quantum interference in asymmetric nonlinear devices

Rupak Bag, Dibyendu Roy

quant-phcond-mat.mes-hallphysics.opticsSubmitted Jul 29, 2026• Announced Jul 30, 2026• 0 citations
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Abstract

Structural asymmetry combined with optical nonlinearity often leads to nonreciprocal light transport. We explore the mechanism by which 1-photon interference effects can revive reciprocity in such nonlinear models. To this end, we study correlated 2-photon scattering where an artificial atom is asymmetrically (a) side-coupled to an infinite waveguide at two spatially separated points, and (b) direct-coupled to two semi-infinite waveguides. The setup (a) gives robust reciprocal transport for the two photons. However, the setup (b) shows a transition from a nonreciprocal to a reciprocal regime by tuning the interference effect via an additional tunneling path for photons between the two waveguides.

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Structural asymmetry combined with optical nonlinearity often leads to nonreciprocal light transport. We explore the mechanism by which 1-photon interference effects can revive reciprocity in such nonlinear models. To this end, we study correlated 2-photon scattering where an artificial atom is asymmetrically (a) side-coupled to an infinite waveguide at two spatially separated points, and (b) direct-coupled to two semi-infinite waveguides. The setup (a) gives robust reciprocal transport for the two photons. However, the setup (b) shows a transition from a nonreciprocal to a reciprocal regime by tuning the interference effect via an additional tunneling path for photons between the two waveguides.
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Light propagation in a nonlinear optical medium is nonreciprocal for spatially asymmetric linear permittivity. We examine the physical mechanism and properties of such nonreciprocity (NR). For this, we calculate the transmission of light through two models of a nonlinear optical isolator consisting of (a) a two-level atom and (b) a driven $\mathrm{\ensuremath{\Lambda}}$-type three-level atom coupled asymmetrically to light inside open waveguides. We find a higher NR in the model (b) than in the model (a) due to a stronger optical nonlinearity in the former. We determine the critical intensity of incident light for maximum NR and a dependence of the corresponding NR on asymmetry in the coupling. Surprisingly, we find that it is mainly coherent elastic scattering compared to incoherent scattering of incident light which causes maximum NR near the critical intensity. We also show a higher NR of an incident light in the presence of an additional weak light at the opposite port.
In this paper, we employ two two-level atoms and a $\mathrm{\ensuremath{\Lambda}}$-type atom to connect two one-dimensional semi-infinite coupled resonator waveguides, respectively. The first configuration is a chiral setup, where incident photons undergo an elastic scattering process. We investigate the influence on the single-photon transfer rate of spatial coupling points between the small or giant atom and the waveguides. Our numerical simulations demonstrate that additional coupling points in the giant atom system will modify the transmission rule observed in the small atom system due to their unique interference effects. The second configuration is a nonreciprocal setup, where the conduction direction depends entirely on the initial state of the $\mathrm{\ensuremath{\Lambda}}$-type atom. The incident photon undergoes an inelastic scattering process accompanied by frequency conversion.
We propose and theoretically investigate a model to realize cascaded optical nonlinearity with few atoms and photons in one-dimension (1D). The optical nonlinearity in our system is mediated by resonant interactions of photons with two-level emitters, such as atoms or quantum dots in a 1D photonic waveguide. Multi-photon transmission in the waveguide is nonreciprocal when the emitters have different transition energies. Our theory provides a clear physical understanding of the origin of nonreciprocity in the presence of cascaded nonlinearity. We show how various two-photon nonlinear effects including spatial attraction and repulsion between photons, background fluorescence can be tuned by changing the number of emitters and the coupling between emitters (controlled by the separation).
Nonreciprocal devices are a key element for signal routing and noise isolation. Rapid development of quantum technologies has boosted the demand for a new generation of miniaturized and low-loss nonreciprocal components. Here, we use a pair of tunable superconducting artificial atoms in a 1D waveguide to experimentally realize a minimal passive nonreciprocal device. Taking advantage of the quantum nonlinear behavior of artificial atoms, we achieve nonreciprocal transmission through the waveguide in a wide range of powers. Our results are consistent with theoretical modeling showing that nonreciprocity is associated with the population of the two-qubit nonlocal entangled quasidark state, which responds asymmetrically to incident fields from opposing directions. Our experiment highlights the role of quantum correlations in enabling nonreciprocal behavior and opens a path to building passive quantum nonreciprocal devices without magnetic fields.
We study the coherent transport of one or two photons in a one-dimensional waveguide chirally coupled to a nonlinear resonator. Analytic solutions of the one-photon and two-photon scattering is derived. Although the resonator acts as a non-reciprocal phase shifter, light transmission is reciprocal at one-photon level. However, the forward and reverse transmitted probabilities for two photons incident from either the left side or the right side of the nonlinear resonator are nonreciprocal due to the energy redistribution of the two-photon bound state. Hence, the nonlinear resonator acts as an optical diode at two-photon level.