Karolina Słowik

Controlling heat flow at the quantum level is a key challenge for next‐generation quantum technologies, including thermal management and quantum information processing. Here, we investigate quantum heat transport in an asymmetrically driven hybrid magnon–photon system in contact with two thermal baths at different temperatures. We demonstrate that external driving of the magnonic subsystem provides a versatile control knob for tailoring steady‐state heat currents and their asymmetry. We identify the mechanisms governing thermal rectification in the hybrid system: we find that strong rectification emerges in the regime of weak magnon–photon hybridization combined with intense magnon driving. In this regime, the external drive enables control over both the magnitude and direction of the heat current, allowing the rectification parameter to be tuned across its entire physically accessible range.

Plasmonic nanostructures enable strong, localized enhancement of electromagnetic fields, making them powerful platforms for mediating light interactions with quantum emitters (QEs) at the nanoscale. However, the spatial inhomogeneity of these fields imposes sub-nanometric precision requirements on emitter placement, limiting the robustness and scalability of quantum photonic systems. Here, we investigate how permanent dipole moments (PDMs) in polar QEs affect their optical response in such environments. In our model, PDMs introduce a nonlinear dependence of coupling strength and spontaneous emission rate on the local field amplitude. Our simulations of QEs near a plasmonic nanosphere show that PDMs can markedly reduce spatial variations in interaction strength and decay rate, yielding stabilized Rabi oscillations and extended coherence times even in strong-field gradients. Exploiting systems with PDMs thus offers a route to more robust QEs in confined optical fields, with potential benefits for quantum control, information processing, and nanophotonic device design.

The dressed atom approach provides a tool to investigate the dynamics of an atom-laser system by fully retaining the quantum nature of the coherent mode. In its standard derivation, the internal atom-laser evolution is described within the rotating-wave approximation, which determines a doublet structure of the spectrum and the peculiar fluorescence triplet in the steady state. However, the rotating wave approximation may fail to apply to atomic systems subject to femtosecond light pulses, light-matter systems in the strong-coupling regime or sustaining permanent dipole moments. This work aims to demonstrate how the general features of the steady-state radiative cascade are affected by the interaction of the dressed atom with propagating radiation modes. Rather than focusing on a specific model, we analyze how these features depend on the parameters characterizing the dressed eigenstates in arbitrary atom-laser dynamics, given that a set of general hypotheses is satisfied. Our findings clarify the general conditions in which a description of the radiative cascade in terms of transition between dressed states is self-consistent. We provide a guideline to determine the properties of photon emission for any atom-laser interaction model, which can be particularly relevant when the model should be tailored to enhance a specific line. We apply the general results to the case in which a permanent dipole moment is a source of low-energy emission, whose frequency is of the order of the Rabi coupling.

We present a hybrid graphene–gold nanoantenna designed to enhance multiphoton absorption signals in molecules. The enhancement process involves two key steps: Firstly, the graphene component of the antenna supports molecular absorption in the mid-infrared and terahertz bands. By applying gate voltage, one can adjust the spectral positions of its resonances and select the desired absorption order, determining the number of photons absorbed in a single transition event. Secondly, gold nanorods with carefully tailored geometrical parameters enhance fluorescent single-photon emission. As a proof of concept, we adjust the geometry parameters of the hybrid antenna to the ATTO 700 dye molecule, taking into account its spectrally resolved emission characteristics. We predict a significant local enhancement of the fluorescence signal indicating the highly nonlinear process of N-photon absorption to exceed 5 orders of magnitude for N = 2 and 13 orders of magnitude for higher nonlinearity orders. Our proposed nanoantenna offers a promising platform for the tunable enhancement of highly nonlinear light–matter interactions.

This paper addresses the challenge of modeling relaxation dynamics in quantum many-body systems, specifically focusing on electrons in graphene nanoflakes. While quantum many-body techniques effectively describe dynamics up to a few particles, these approaches become computationally intractable for large systems. Larger systems may be tackled with a single-particle approach that, however, struggles to incorporate relaxation effects. Existing relaxation models encounter issues such as an inability to capture system complexity and violation of the Pauli principle. In this paper, we propose a single-particle model that accounts for various relaxation effects at the crossroads of quantum optics and solid-state photonics, that overcomes the limitations of previous models. Our approach is rooted in the quantum-optical Lindblad model, where relaxation rates are deactivated once the target levels saturate due to the Pauli principle. This approach is referred to as the saturated-Lindblad model. To validate the predictions of the saturated-Lindblad model, we confront them against phenomenological and many-body physics models in low-dimensional systems, including atomic chains and graphene nanoflakes. Remarkably, the saturated-Lindblad model exhibits excellent agreement with few-body calculations, distinguishing itself from other existing approaches. Moreover, by assigning different relaxation rates to different transitions, we successfully reproduce cascade deexcitation dynamics and predict emission spectra. The saturated-Lindblad model offers the ability to describe dynamics in systems of practical sizes, encompassing a wide range of structures that can be effectively captured within the single-particle description.

Quantum optical coherence tomography (Q-OCT) presents many advantages over its classical counterpart, optical coherence tomography (OCT), provides an increased axial resolution, and is immune to even orders of dispersion. The core of Q-OCT is the quantum interference of negatively correlated entangled photon pairs which, in the Fourier domain, are observed by means of a joint spectrum measurement. In this work, we explore the use of a spectral approach in a novel configuration where classical light pulses are employed instead of entangled photons. The intensity of these light pulses is reduced to a single photon level. We report theoretical analysis along with its experimental validation to show that although such a classical light is much easier to launch into an experimental system, it offers limited benefits compared to Q-OCT based on the entangled light. We analyze the differences in the characteristics of the joint spectrum obtained with entangled photons and with classical optical pulses and point out to the differences’ source: the lack of the advantage-bringing term in the signal.

Birefringence, quantified as the maximum effective index difference of two orthogonally polarized modes, is one of the key characteristics of Lithium Niobate on Insulator (LNOI) waveguide devices. As LNOI optical properties strongly depend on the polarization of the propagating light beam, achieving the zero‐birefringence condition (ZBC) is highly desired. In this work, a rib LNOI waveguide is designed, and the influence of geometrical dimensions on the birefringence is characterized. From the result, the slab's height is found to be the dominant geometrical factor that significantly contributes to the birefringence, with an optimum slab's height found at 50 nm. For this optimized value, the influence of width and wavelength on birefringence are characterized. The analysis is extended to investigate the properties of group indices that can result in the temporal walk‐off phenomenon. This phenomenon can significantly impact the performance of single‐photon devices, as it can cause a polarization mismatch between the incident photon and the polarization properties of the medium, resulting in reduced efficiency of devices such as single‐photon detectors. In the analysis, the zero birefringence and walk‐off are achieved at narrow waveguide width, 𝑤 of 250 nm and wavelength, = 1500 nm, which is within the telecommunication region band.

Fluorescent light emission from atomic quantum systems routinely occurs at the illumination frequency. If the system is polar, an additional fluorescence peak appears at a much lower Rabi frequency, which scales with the illumination field amplitude. This opens the possibility of spectrally controlling the emission, promising tunable coherent radiation sources. However, typically the emission occurs in the MHz to GHz regimes, and its intensity from a single quantum system is relatively low. Here, we propose a hybrid nanoantenna combining noble-metal and graphene elements, exploited for an unusual goal: The silver elements spectrally tune the emission frequency of the molecule and shift it to the THz band, where novel sources of coherent radiation are still desired. Additionally, the graphene elements are used to plasmonically enhance the emission intensity. Their tunability allows for adjustment of the operational frequencies of the device to the illumination conditions and to counteract the fluctuations related to the field modulations in space. All these features are discussed based on the real-life example of a polar molecule of barium monofluoride (BaF).