The realization of correlated multiphoton processes usually depends on the interaction between nonlinear media and atoms. However, the nonlinearity of optical materials is generally weak, making it still very challenging to achieve correlated multiphoton dynamics at the few-photon level. Meanwhile, giant atoms, with their capability for multipoint coupling, which is a novel paradigm in quantum optics, mostly focus on the single-photon field. In this work, by using the method described in Phys. Rev. Res. 6, 013279 (2024), we reveal that the ladder-type three-level giant atom spontaneously emits strongly correlated photon pairs with high efficiency by designing and optimizing the target function. In addition, by encoding local phases into the optimal coupling sequence, directional two-photon correlated transfer can be achieved. This method does not require a nonlinear waveguide and can be realized in the conventional environment. We show that the photon pairs emitted in both the bidirectional and the chiral case exhibit strong correlation properties in both time and space. Such correlated photon pairs have great potential applications for quantum information processing. For example, numerical results show that our proposal can realize the two-photon mediated cascaded quantum system.
The realization of correlated multiphoton processes usually depends on the interaction between nonlinear media and atoms. However, the nonlinearity of optical materials is generally weak, making it still very challenging to achieve correlated multiphoton dynamics at the few-photon level. Meanwhile, giant atoms, with their capability for multipoint coupling, which is a novel paradigm in quantum optics, mostly focus on the single-photon field. In this work, by using the method described in Phys. Rev. Res. 6, 013279 (2024), we reveal that the ladder-type three-level giant atom spontaneously emits strongly correlated photon pairs with high efficiency by designing and optimizing the target function. In addition, by encoding local phases into the optimal coupling sequence, directional two-photon correlated transfer can be achieved. This method does not require a nonlinear waveguide and can be realized in the conventional environment. We show that the photon pairs emitted in both the bidirectional and the chiral case exhibit strong correlation properties in both time and space. Such correlated photon pairs have great potential applications for quantum information processing. For example, numerical results show that our proposal can realize the two-photon mediated cascaded quantum system.
Chiral quantum optics is central to developing scalable quantum networks, yet existing approaches rely predominantly on linear single-photon regimes. It remains unclear how to generate directional multiphotons. Here we show that giant emitters coupled to nonlinear quantum optical baths enable tunable directional correlated photons, revealing a mechanism for multiphoton directional emission. We demonstrate that the propagation phases of correlated photons, together with the coupling phases of giant emitters, can generate destructive interference in one direction while enhancing emission in the opposite direction, making directionality fully tunable. Building on this mechanism, we introduce a nonlinear cascaded quantum network paradigm mediated by “correlated flying qubits”, providing a configurable building block enabling distinct many-body applications beyond linear unidirectional setups. These results reveal a rich landscape for engineering multiphoton propagation and correlations through interference in giant emitter-nonlinear bath architectures, offering pathways for quantum networks and strongly correlated light-matter platforms. Chiral quantum optics is a prerequisite for communication protocols in quantum networks, but current methods are limited to the single-photon regime. The authors demonstrate that multiple giant emitters coupled to a nonlinear waveguide enable tunable multiphoton directional emission via interference, establishing a configurable building block for multiphoton cascaded quantum networks.
Strongly correlated multi-photon states are indispensable resources for advanced quantum technologies, yet their deterministic generation remains challenging due to the inherent weak nonlinearity in most optical systems. Here, we propose a scalable architecture for producing correlated few-photon entangled states via cascaded inelastic scattering in a nonlinear waveguide. When a single photon scatters off a far detuned excited two-level emitter, it coherently converts into a propagating doublon, a bound photon pair with anomalous dispersion. This doublon can subsequently scatter off a downstream excited emitter to further convert into a triplon, and so on, thereby establishing a photon-number amplification cascade ∣⋅⟩→∣:⟩→∣∴⟩→... Central to this process is the concept of a pseudo-giant atom, which we introduce here to capture the non-local scattering potential emergent from the wave functions of bound states. By implementing this scheme using a real giant atom with multiple engineered coupling points, we achieve unidirectional and full controllable photon conversion without backscattering. The resulting output state forms a programmable superposition of spatially and temporally isolated photon-number components, automatically sorted by their distinct group velocities. This work opens a new paradigm in quantum state engineering, enabling on-demand generation of complex multi-photon resources for quantum simulation, metrology, and scalable quantum networks.
In recent years, the study of quantum electrodynamics (QED) in light-matter interactions has discovered various interesting phenomenons that orient many applications. However, due to the ambient entanglement among photons and atoms, few-particle dynamics remains challenging to analyze precisely and limits the progress in several fields. In few-particle systems, different number of atoms interacting with the light field generates drastically different results, even when there is only a single photon involved in the system. The interference between individual atom’s spontaneous emission wavefunctions can cooperatively alter the effective atom-light coupling strength. Depending on the spatial distance between individual of atoms and the involved cavities in the interaction progresses, the collective spontaneous emission rate can alter drastically from superradiance, where the emission rate is enhanced to a high value proportional to the atom number, to subradiance, where the excited atom ensemble stays in a dark state with the light trapped forever. Superradiance provides a promising mechanism to create an ultra-strong light-matter coupling environment, which is an important ingredient for the design of ultra-fast optical devices, frequency comb generation and quantum mirrors. The dark state of subradiance, on the other hand, is essential for the quantum memory implementation. Also, nontrivial phenomenons also occur when multiple photons interacting with a single atom simultaneously. When a finite number of photons incidents on an atom, their scattering through an atom is highly nonlinear, and one special eigenstate component of such nonlinear process, namely photonic bound states, exhibits many interesting properties in nonlinear quantum optical systems. The photons in such state are strongly bunched with each other, as if a virtual attractive force is in effect. Also, the frequency of photons are anti-correlated so that their total frequency remains a constant value, no matter how arbitrary the frequency of any individual is. Thesefeatures benefit certain nonlinear quantum processes for quantum computing and quantum imaging. Specially for two-photon excitation imaging, the same-parity two-level fluorophore molecules suffers from extremely low excitation efficiencies due to the far off-resonant intermediate states. However, by replacing the current commonly used ultrashort laser pulse illuminations with two photon bound state photon pairs, namely photonic dimers, the efficiency can be enhanced to orders of magnitude higher. Such exceptionally high excitation efficiency can potentially bring the current in vivo deep brain imaging depth from millimeter level to several centimeters. To obtain a stable source of such photonic dimers, it is found that a laser structure with gain medium of two-photon emission materials is capable of generating the coherent state of photonic dimer and can serve as a promising light source for nonlinear quantum optical devices.
Emitters coupled simultaneously to distant positions of a photonic bath, the so-called giant atoms, represent a new paradigm in quantum optics. When coupled to one-dimensional baths, as recently implemented with transmission lines or SAW waveguides, they lead to striking effects such as chiral emission or decoherence-free atomic interactions. Here, we show how to create giant atoms in dynamical state-dependent optical lattices, which offers the possibility of coupling them to structured baths in arbitrary dimensions. This opens up new avenues to a variety of phenomena and opportunities for quantum simulation. In particular, we show how to engineer unconventional radiation patterns, like multidirectional chiral emission, as well as collective interactions that can be used to simulate nonequilibrium many-body dynamics with no analog in other setups. Additionally, the recipes we provide to harness giant atoms in high dimensions can be exported to other platforms where such nonlocal couplings can be engineered.
A recent experiment [H. S. Han et al., Phys. Rev. Lett. 127, 073604 (2021); see also accompanying online supplemental material] shows that when a three-level V-type atom with two closely lying upper states interacts with the same vacuum radiation field, its excited states enable vacuum-induced coupling (VIC) owing to the quantum interference between the spontaneous emission pathways. Here, we propose a feasible scheme for phase-engineered photon correlations in the presence of the VIC in an optical nanofiber (ONF) cavity quantum electrodynamics (QED) system. Specifically, we show that a phase-dependent strong photon antibunching with high brightness can be generated in the weak-coupling regime of light-atom interactions. This occurs because of the VIC, leading to both the destructive quantum interference between the different pathways for two-photon excitation and the total closed-loop coupling phase. Different types of purely quantum correlations, such as single- and two-photon blockades, can occur by properly tuning the total closed-loop coupling phase adhering on the VIC, and the switch from photon blockade to photon induced tunneling is revealed as well. On the other hand, the strong photon antibunching can be achieved in a broad driving frequency range, which relaxes the requirement for the driving frequency in the ONF cavity QED system. In addition, we compare the analytical and numerical results of the second- and third-order intensity correlation functions, and they are in good agreement. The present study may provide an alternative route to manipulate the few-photon states and have potential applications in single-photon sources and quantum communications.
Abstract Single-photon frequency conversion in the system composed of a Λ -type three-level giant atom chiral couples to a one-dimensional waveguide is investigated. Two configurations, i.e. one of the atomic transitions couples to the waveguide at two separated points while the other one either directly or indirectly couples to the waveguide at a single point, are considered. The scattering amplitudes for representing the frequency conversion in the two kinds of coupling schematics are obtained analytically with the real-space approach. The results show that perfect nonreciprocal transmission with preserving frequency and ideal directional frequency conversion can be realized due to the time-reversal symmetry broken induced by the chiral coupling as well as the self-interference comes from the giant atom. In particular, in the indirect coupling configuration, the scattering spectra exhibit two directional frequency conversion windows due to the atom-cavity coupling, which enables the present system suitable for the simultaneous manipulation of incident photons with different frequencies. These results may have potential applications in various quantum information processing and quantum network engineering.