Session N54: Nonlinear Systems and Interactions

Revealing Optical Bistability and Multistability in a cavity including two coupled quantum Wells

We investigate the bistability and multistability within a cavity containing two coupled quantum wells

pumped by a squeezed light. Our findings highlight the importance of frequency detunings relative to

direct and indirect excitons in generating bistability, which are predominantly impacted by Kerr effects.

Furthermore, we show that the presence of multistable behavior in excitonic and photonic intensities is

due to the interplay of second- and third-order nonlinearities.

In addition, we analyze the controllability of transitioning between bistability and multistability.

The amplitude of the squeezed light or the frequency detuning between the external coherent drive and

the cavity can be utilized to adjust this transition. We notice that the tunneling phenomenon has a dual

effect, promoting or inhibiting the establishment of bistability and multistability. The outcome is

determined on whether direct or indirect exciton nonlinearities are considered.

The transition between bistability and multistability can be controlled  by changing the tunneling rate

between the wells which is dependent on the distance separating the two quantum wells inside the

cavity.   

Emergent system-reservoir methods for broadband nonlinear quantum optics: a case study on cascaded chi(2)

In broadband quantum optical systems, nonlinear interactions among a large number of frequency components induce highly rich but complicated dynamics, for which we crucially lack intuitive understanding on the phenomenology. In this work, we introduce a perturbative framework in which to factor out effective ``reservoir'' degrees of freedom and establish concise effective models for the remaining system. Our approach relies on a partitioning of the intra-system degrees of freedom into primary and auxiliary sectors, followed by approximate diagonalization of subsystems by means of unitary transformation, as well as master equation techniques. We apply the method to the case of cascaded optical $chi^{(2)}$ nonlinearities, and show that the cascaded process can be summarized as interactions among dressed fundamental and second harmonic modes, where the former experience self-phase modulations to leading order, and these states interact via cross-phase modulations. Using this approach, we eliminate the second harmonic degree of freedom, and identify features of the fundamental wave dynamics beyond the conventional single-mode cascaded nonlinearity, such as emergent dissipative two-photon loss channels. Our results highlight the utility of system-reservoir methods to concisely understand the complicated dynamics of multimode quantum photonic systems, which we expect to be a powerful theoretical toolbox for quantum engineering with photons.   

Quantum Walks of Two-Photon Wannier States in Nonlinear Waveguide Lattices with Floquet-Modulated Coupling

Thouless pumping has been realized both theoretically and experimentally in photonic waveguide lattices with Floquet-modulated coupling, including demonstrations of topological phenomena like integer and fractional quantized motion of solitons in nonlinear lattices. However, these demonstrations were with classical coherent light, which is similar to the dynamics of a single particle in such lattices. On the other hand, two-particle evolution has been observed in atomic and photonic lattices with constant coupling, demonstrating nontrivial quantum correlations, interference, and entanglement. Here we study the dynamics of two-photon states in nonlinear waveguide lattices with Floquet-modulated coupling. We predict a band-dependent evolution and localization of two-photon Wannier states that cannot be explained in the classical or single-photon regime. Our work shows that the transport of Wannier states through nonlinear waveguide lattices with Floquet-modulated coupling exhibits uniquely different features compared to both classical transport through nonlinear lattices and two-photon quantum transport through constant-coupling and linear lattices. These results have applications for analog quantum simulation and engineering topological systems using photonic devices.   

Nonlinear Quantum Photonics from Reflecting Coherent Light off a Tin-Vacancy Center in a Diamond Waveguide

Strong interaction between single photons and material qubit systems is a key element for quantum information science and technology, and in particular for photon-mediated entanglement generation between remote quantum systems. Diamond Group-IV-vacancy color centers are one of the leading platforms for realizing efficient spin qubit-photon interfaces in nanophotonic devices thanks to their protection from charge noise by inversion symmetry. In particular, the Tin-Vacancy center (SnV) has emerged as a high potential candidate towards scalable diamond integrated devices. The SnV offers excellent spin coherence at temperatures around 1K and high quantum efficiency which can provide efficient coupling to light with lower requirements on the nanophotonic device properties and operating temperature than the more established diamond SiV center.

 

Here we present our work on SnV centers integrated in nanophotonic diamond waveguides. We show high stability of the optical transition and efficient coupling of a single SnV to the waveguide optical mode, leading to a coherent extinction of the transmission of up to 25%. We demonstrate tuneable interference of the reflected single-photon field with a coherent state. Finally, we investigate how a single SnV alters the photon statistics of a coherent input state, both in reflection and transmission, proving the strong nonlinearity at the single photon level.   

Collective and nonlinear dynamics in spatially structured and spectrally inhomogeneous emitter ensembles

Ordered emitters coupled to photonic baths realise uniquely cooperative light-matter platforms with applications to analogue quantum simulation and photonic state generation. On the other hand, spectral inhomogeneity in spatially homogeneous systems can enable novel technologies such as atomic frequency combs and quantum memories. However, the difficulty of theoretically studying large spatially and spectrally inhomogeneous spin systems has hindered our understanding and ability to exploit them in tandem. Here we advance the theory of and shed light on the optical response and dynamics of spatially ordered systems of solid-state emitters featuring spectral inhomogeneity. We first show at the single-photon level how such lattices can exhibit huge optical response and broad cooperative resonances that exceed the inhomogeneous line in experimentally relevant scenarios. We then move to investigate nonlinear dynamics under initial population inversion and determine generic conditions on spectral homogeneity that permit superradiant bursts via formation of macroscopic coherence overcoming spectral broadening. Our results reconcile spectral inhomogeneity with the rich world of ordered emitter arrays and open the path for exploring their combined potential applications.   

Fully Non-Linear Neuromorphic Computing with Linear Wave Scattering

The increasing complexity of neural networks and the energy consumption associated with training and inference create a need for alternative neuromorphic approaches, e.g., using optics. Current proposals and implementations rely on physical non-linearities or opto-electronic conversion to realise the required non-linear activation function. However, there are significant challenges with these approaches related to power levels, control, energy-efficiency, and delays.

                             

In this talk, we present a scheme for a neuromorphic system that relies on linear wave scattering and yet achieves non-linear processing with a high expressivity [1]. The key idea is to inject the input via physical parameters that affect the scattering processes. Moreover, we show that gradients needed for training can be directly measured in scattering experiments.  We predict classification accuracies on par with results obtained by standard artificial neural networks. Our proposal can be readily implemented with existing state-of-the-art, scalable platforms, e.g., in optics, microwave and electrical circuits, and we propose an integrated-photonics implementation based on racetrack resonators that achieves high connectivity with a minimal number of waveguide crossings.

 

[1] C.C. Wanjura, F. Marquardt. arXiv:2308.16181 (2023).   

Quantum Multiphoton Rabi Oscillations in Waveguide QED

Chip-scale nanophotonics, with waveguide QED as a crucial component, remains instrumental in advancing the field of quantum information processing and communication. A key facet of photonic technologies revolves around the scattering of coherent light across a two-level system which leads to Rabi oscillations in the semiclassical, strong-field limit. We analytically investigate the scattering dynamics of multiphoton Fock states as they encounter a two-level emitter and examine both the weak-field and strong-field regimes that exhibit widely disparate dynamical features. The excitation amplitude of the qubit features a linear superposition of various independent scattering events originating from the possibility of sequential photon absorptions and emissions. In the strong pumping limit characterized by asymptotically large photon numbers, all scattering amplitudes attain comparable significance, and signatures of Rabi-like oscillatory dynamics are recovered, closely mirroring semiclassical predictions. Hence, our formulation affords a robust theoretical foundation for understanding the transition from finite to asymptotically large photon numbers. We also explore the scattering dynamics of pulsed wave packets, unveiling strongly enhanced excitation efficiency even in scenarios involving a few photons. This has practical implications for quantum computing protocols based on multiphoton states of light and for waveguide-integrated photonic technologies.   

Purcell enhanced emission and saturable absorption of cavity-coupled CsPbBr3 quantum dots

Colloidal perovskite quantum dots are promising quantum emitters because of their bright emission, color tunable optical properties, low-cost chemical synthesis, scalability and ease of nano-photonics integration. Their coherence time is a considerable fraction of their short radiative lifetime suggesting that moderately Purcell-enhanced nanophotonic cavity integration has the potential to achieve transform-limited photon coherence. In addition, cavities enhance the light-matter interaction , and the interplay between the system's absorption characteristics and the confinement of photon modes within the cavity can lead to pronounced nonlinear effects. We integrate zwitterionic CsPbBr3 quantum dots to a polarization degenerate Bullseye cavity mode exhibiting higher Q than reported before. We achieve an average of around 10 times improvement in the brightness of the cavity coupled dots compared to the bulk. From time-resolved lifetime measurement, we calculate a factor of 8 times Purcell enhancement for the cavity coupled dots. We also observe saturable absorption behavior of the perovskites inside the cavity where the cavity quality factor increases by around 20% with quantum dot saturation. These findings are an important step towards utilizing cavity coupled perovskites as efficient light sources and for strong optical nonlinearity.