We investigate the scattering processes of two photons in a one-dimensional waveguide coupled to two giant atoms. By adjusting the accumulated phase shifts between the coupling points, we are able to effectively manipulate the characteristics of these scattering photons. Utilizing the Lippmann-Schwinger formalism, we derive analytical expressions for the wave functions describing two-photon interaction in separate, braided, and nested configurations. Based on these wave functions, we also obtain analytical expressions for the incoherent power spectra and second-order correlation functions. In contrast to small atoms, the incoherent spectrum, which is defined by the correlation of the bound state, can exhibit more tunability due to the phase shifts. Additionally, the second-order correlation functions in the transmission and reflection fields could be tuned to exhibit either bunching or antibunching upon resonant driving. These unique features offered by the giant atoms in waveguide QED could benefit the generation of nonclassical itinerant photons in quantum networks.
We investigate the scattering processes of two photons in a one-dimensional waveguide coupled to two giant atoms. By adjusting the accumulated phase shifts between the coupling points, we are able to effectively manipulate the characteristics of these scattering photons. Utilizing the Lippmann-Schwinger formalism, we derive analytical expressions for the wave functions describing two-photon interaction in separate, braided, and nested configurations. Based on these wave functions, we also obtain analytical expressions for the incoherent power spectra and second-order correlation functions. In contrast to small atoms, the incoherent spectrum, which is defined by the correlation of the bound state, can exhibit more tunability due to the phase shifts. Additionally, the second-order correlation functions in the transmission and reflection fields could be tuned to exhibit either bunching or antibunching upon resonant driving. These unique features offered by the giant atoms in waveguide QED could benefit the generation of nonclassical itinerant photons in quantum networks.
We investigate the two-photon scattering processes in a one-dimensional waveguide coupled to either a two-level or three-level giant atom. By manipulating the accumulated phase shift between the two coupling points, we are able to effectively modify the characteristics of these scattering processes. Utilizing the Lippmann-Schwinger formalism, we obtain the exact two-photon interacting scattering wave functions of these two systems. Additionally, analytical expressions for the incoherent power spectra and second-order correlations are derived. The incoherent spectrum, which is defined by the correlation of the bound state, provides valuable insights into photon-photon correlations. It serves as a useful indicator of the degree of photon-photon correlation between scattered photons. Furthermore, the second-order correlation function gives a direct measure of the photon-photon correlation. For photons scattered by the two-level giant atom, manipulating the accumulated phase shift allows for improvement of the photon-photon correlation and adjustment of the evolution of the second-order correlation. In the case of the three-level giant atom, the photon-photon correlation can be substantially increased. The photon-photon interaction and correlation distance of the scattered photons can be further enhanced by tuning the accumulated phase shift. Moreover, the statistical properties can be adjusted by the control field.
We investigate one- and two-photon scattering in a one-dimensional waveguide coupled to a giant atom within the non-Markovian regime using the resolvent approach. The non-Markovian behavior gives rise to an atom-photon bound state that cannot be excited by a single incident photon. However, the bound state can be excited via the two-photon scattering process described by multichannel scattering theory, from which an analytical trapping probability of a photon in the bound state can be achieved. Additionally, we analyze the two-photon scattering process, obtaining the analytical expressions for scattered states. As non-Markovian effects strengthen, two peaks appear in the incoherent power spectrum, attributed to the system behaving as a leaky cavity formed by the giant atom's coupling points. Through the analysis of second-order correlation functions, we observe the bunching behavior for transmitted photons, antibunching behavior for reflected photons, and the distinctive retrieval behavior at the coupling points' separation.
We study the coherent single-photon scattering in a one-dimensional waveguide coupled to a giant artificial molecule consisting of two coupled giant atoms. Since each giant atom couples to the waveguide via two coupling points, the couplings of the molecule with the waveguide have three different coupling configurations: the separated-, braided-, and nested-coupling cases. We obtain the exact expressions of the single-photon transmission and reflection amplitudes with the real-space approach. It is found that the behavior of the scattering spectra depends on the phase shift between two neighboring coupling points, the coupling configuration, and the coupling between the two giant atoms. Concretely, we study the photon scattering in both the Markovian and non-Markovian regimes, in which the photon propagating time between two neighboring coupling points is neglected and considered, respectively. Under the Markovian limit, the asymmetric Fano line shapes in different coupling configurations of the giant-molecule waveguide-QED system can be obtained by choosing proper phase shift, and the transmission window can be adjusted by the coupling strength between the two giant atoms in these three coupling configurations. In particular, multiple reflection peaks and dips in these configurations are revived in the non-Markovian regime. This paper will pave the way for the study of controllable single-photon devices based on the giant-molecule waveguide-QED systems.
Waveguide quantum electrodynamics (WQED) provides a powerful platform for exploring quantum optical phenomena by enhancing atom-photon interactions through photon confinement in a waveguide. Here we investigate the photon-scattering dynamics of a weak coherent pulse incident from the left on a giant atom coupled to a bidirectional waveguide, focusing on effects absent in the small-atom approximation. Using an extended input-output formalism, we calculate the relevant correlation functions and show that the competition between two scattering processes is governed by the ratio of the pulse width to the atomic lifetime, leading to time-dependent switching between bunching and antibunching. In addition, tuning the phase accumulated between the two coupling points of the giant atom allows the photon statistics to be switched among three distinct regimes, each with a finite phase bandwidth. We also discuss the experimental feasibility in superconducting circuits. Our results provide a route toward giant-atom-based control of photon pulses and potential applications in quantum control.
We investigate the single-photon scattering spectra in a giant-molecule waveguide quantum electrodynamics system, where a giant molecule consisting of two coupled giant atoms is connected to two waveguides via two separate sites, respectively. The phases of coupling strengths between the giant molecule and waveguides are considered. Using a real-space method, we obtain the exact analytical expressions for the single-photon scattering coefficients, which are valid both in the Markovian and non-Markovian regimes. It is shown that the transmission spectra strongly depend on phases, including the phases accumulated by photons traveling between coupling points and the phase difference between two coupled channels of the giant-molecule upper (lower) waveguide. Under the Markovian limit, we find that by selecting appropriate accumulated phases and local phase difference, targeted routing and asymmetrical scattering of photons can be achieved without breaking the symmetry in the coupling configuration between the giant molecule and waveguides. Moreover, all the optimal phase combinations for implementing single-photon routing have been summarized. In the non-Markovian regime, the scattering spectra show more complicated line shapes with multiple staggered peaks and dips. More interestingly, the scattering spectrum exhibits periodic changes during the transition from the Markovian to the non-Markovian regime, which makes one obtain a means to control photon routing in addition to phases. Our results may be useful in controlling and manipulating light-matter interactions involving giant atoms at the single-photon level.
Deterministic generation and manipulation of nonclassical light are the central goals of quantum photonics. Waveguide quantum electrodynamics with atomic arrays enable strong photon–photon interactions via collective effects. Recently, giant atoms with multiple coupling points have opened new avenues for engineering photon correlations. In particular, the braided configuration offers unique features; however, most studies on quantum nonlinearity are limited to two atoms. Here, we extend the investigation to arrays of more braided giant atoms and uncover two distinct regimes controlled by phase parameters. One regime corresponds to Lorentzian resonances in the single-excitation subspace, where the system behaves as an effective two-level model at resonance, producing bunched transmitted light and antibunched reflected light. The two-excitation nonlinearity is weaker and tends to bunch photons, and at triple resonance the system loses tunability. The second regime corresponds to the Fano resonance arising from the spectral coalescence of the collective modes. In this regime, we achieve near-unity transmission with strong photon antibunching in transmission, and significant photon bunching in reflection. Our results establish braided giant-atom arrays as a scalable, cavity-free platform for engineering nonclassical photon correlations.