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The natural lineshape of an excited two-level atom (TLA) has long been known to be gauge-dependent, with certain experiments in better agreement with the lineshape calculated with the dipole gauge. We show that by using a Coulomb gauge Hamiltonian truncated in a manner consistent with the gauge principle, the correct output spectrum can be obtained. For TLAs undergoing dynamics arising from additional Hamiltonian couplings, we also show that the master equation is gauge-invariant under the same conditions of validity as the Born-Markov approximation, despite different gauges having different spectral densities. These results highlight the importance of using correctly truncated gauge-invariant Hamiltonians in input-output theory for accurate frequency-dependent spectra, even in weak coupling regimes.

In this work, we examine how the structure of system-bath interactions can determine commonly encountered temporal decoherence patterns, such as Gaussian and exponential decay, in molecular qubits coupled to a thermal nuclear environment. The analysis, based on a pure dephasing picture that admits analytical treatment, shows that decoherence, in general, is neither purely Gaussian nor exponential but rather the exponential of oscillatory functions, with periods determined by the environment's frequencies. For initially unentangled qubit-bath states, or for those where the initial entanglement arises due to displacement in the position of the bath states, Gaussian decay is always present at early times. It becomes increasingly dominant with increasing temperature, qubit-environment interaction strength, and the environment's correlation time. By contrast, strict exponential decay arises only in very specific models that we isolate. Still, it becomes dominant for times longer than the environment correlation time or for early times when there is initial entanglement due to momentum displacement of the bath states. Further, we discuss the connection between electronic decoherence dynamics and electronic spectroscopic lineshape theory, where Gaussian spectral peaks correspond to Gaussian coherence decay and Lorentzian peaks correspond to exponential coherence decay. We find that Gaussian spectral peaks, usually associated with inhomogeneous broadening, can emerge from system-bath entanglement even when there is no inhomogeneity in the initial conditions., Comment: 10 pages, 3 figures

In recent years, Virtual Reality (VR) has found its way into different fields besides pure entertainment. One of the topics that can benefit from the immersive experience of VR is education. Furthermore, using game-based approaches in education can increase user motivation and engagement. Accordingly, in this paper, we designed and developed an immersive escape room game in VR to teach building energy simulation topics. In the game, players must solve puzzles like, for instance, assembling walls using different materials. We use a player guidance system that combines educational content, puzzles, and different types of hints to educate the players about parameters that influence energy efficiency, structural resistance, and costs. To improve user onboarding, we implemented a tutorial level to teach players general interactions and locomotion. To assess the user experience, we evaluate both the tutorial and the game with an expert study with gaming and VR experts (n=11). The participants were asked to play both the tutorial level and the escape room level and complete two sets of post-questionnaires, one after the tutorial and one after the puzzle level. The one after the tutorial level consisted of NASA-TLX and SUS questionnaires, while after the escape room level we asked users to complete the NASA-TLX, UESSF, and PXI questionnaires. The results indicate that the onboarding level successfully provided good usability while maintaining a low task load. On the other hand, the escape room level can provide an engaging, visually appealing, and usable learning environment by arousing players' curiosity through the gameplay. This environment can be extended in future development stages with different educational contents from various fields., Comment: 8 pages, 9 figures, to be published in the the conference proceedings of IEEE Conference on Games 2024

By considering a single lossy three-dimensional cavity mode and using a quasinormal mode expansion for the photonic medium, we show that the frequency-dependence of the coupling between the cavity and its reservoir is dependent on both the cavity contents and the gauge used. For the case of coupling to a single quantum dipole, we identify the form of the spectral density, revealing a $\sim \omega^{-1}$ prefactor scaling, as well as a spatially-dependent contribution. We thus establish the correct quantum form for the cavity-reservoir interaction and show its significant impact on broadband strong coupling.

Fully-processed SiC power metal-oxide-semiconductor field-effect transistors (MOSFETs) emit light during switching of the gate terminal, while both drain and source terminals are grounded. The emitted photons are caused by defect-assisted recombination of electrons and holes at the 4H-SiC/SiO$_2$ interface and can be detected through the SiC substrate. Here, we present time-gated spectroscopic characterization of these interfacial point defects. Unlike in previous studies, the devices were opened in such a way that the drain-contact remained electrically active. A separate examination of the photons emitted at the rising and falling transitions of the gate-source voltage enabled the extraction of two different spectral components. One of these components consists of a single transition with phonon replicas of a local vibrational mode (LVM) with an astonishingly high energy of 220 meV $\unicode{x2013}$ well above the highest phonon modes in 4H-SiC and SiO$_2$ of 120 meV and 137 meV, respectively. Based on a quantum mechanical model, we successfully fitted its emission spectrum and assigned it to donor-acceptor pair recombination involving a carbon cluster-like defect. Other transitions were assigned to EH$_{6/7}$-assisted, EK$_2$-D, and nitrogen-aluminum donor-acceptor pair recombination. Due to the relevance of these defects in the operation of SiC MOSFETs, these novel insights will contribute to improved reliability and performance of these devices., Comment: 18 pages, 12 figures

In broadband quantum optical systems, nonlinear interactions among a large number of frequency components induce complex dynamics that may defy heuristic analysis. In this work we introduce a perturbative framework for factoring out reservoir degrees of freedom and establishing a concise effective model (effective field theory) for the remaining system. Our approach combines approximate diagonalization of judiciously partitioned subsystems with master equation techniques. We consider cascaded optical $\chi^{(2)}$ (quadratic) nonlinearities as an example and show that the dynamics can be construed (to leading order) as self-phase modulations of dressed fundamental modes plus cross-phase modulations of dressed fundamental and second-harmonic modes. We then formally eliminate the second-harmonic degrees of freedom and identify emergent features of the fundamental wave dynamics, such as two-photon loss channels, and examine conditions for accuracy of the reduced model in dispersive and dissipative parameter regimes. Our results highlight the utility of system-reservoir methods for deriving accurate, intuitive reduced models for complex dynamics in broadband nonlinear quantum photonics., Comment: 25 pages, 8 figures

Focusing on the widely adopted Hopfield model with cavity dissipation, we show how the linear spectrum of an ultrastrongly coupled cavity and a dipole can be described either classically or quantum mechanically, but only when the quantum model includes (i) corrections to maintain gauge invariance, and (ii) a specific type of cavity bath coupling. We also show the impact of this bath model on the quantum Rabi model, which has no classical analogue in ultrastrong coupling.

Establishing the fundamental chemical principles that govern molecular electronic quantum decoherence has remained an outstanding challenge. Fundamental questions such as how solvent and intramolecular vibrations or chemical functionalization contribute to the decoherence remain unanswered and are beyond the reach of state-of-the-art theoretical and experimental approaches. Here we address this challenge by developing a strategy to isolate electronic decoherence pathways for molecular chromophores immersed in condensed phase environments that enables elucidating how electronic quantum coherence is lost. For this, we first identify resonance Raman spectroscopy as a general experimental method to reconstruct molecular spectral densities with full chemical complexity at room temperature, in solvent, and for fluorescent and non-fluorescent molecules. We then show how to quantitatively capture the decoherence dynamics from the spectral density and identify decoherence pathways by decomposing the overall coherence loss into contributions due to individual molecular vibrations and solvent modes. We illustrate the utility of the strategy by analyzing the electronic decoherence pathways of the DNA base thymine in water. Its electronic coherences decay in ~ 30 fs. The early-time decoherence is determined by intramolecular vibrations while the overall decay by solvent. Chemical substitution of thymine modulates the decoherence with hydrogen-bond interactions of the thymine ring with water leading to the fastest decoherence. Increasing temperature leads to faster decoherence as it enhances the importance of solvent contributions but leaves the early-time decoherence dynamics intact. The developed strategy opens key opportunities to establish the connection between molecular structure and quantum decoherence as needed to develop chemical strategies to rationally modulate it., Comment: Main text 8 pages, 6 figures (Figures 5 and 6 have been changed) SI 9 pages, 5 figures, 6 tables

The coherent interaction between a two-level system and electromagnetic fields serves as a foundation for fundamental quantum physics and modern photonic quantum technology. A profound example is resonance fluorescence, where the non-classical photon emission appears in the form of a Mollow-triplet when a two-level system is continuously driven by a resonant laser. Pushing resonance fluorescence from a static to dynamic regime by using short optical pulses generates on-demand emissions of highly coherent single photons. Further increasing the driving strength in the dynamical regime enables the pursuit of exotic non-classical light emission in photon number superposition, photon number entanglement, and photon bundle states. However, the long-sought-after spectrum beyond the Mollow-triplet, a characteristic of dynamic resonance fluorescence under strong driving strength, has not been observed yet. Here we report the direct observation and systematic investigations of dynamic resonance fluorescence spectra beyond the Mollow-triplet in a solid-state cavity quantum electrodynamic system. The dynamic resonance fluorescence spectra with up to five pairs of side peaks, excitation detuning induced spectral asymmetry, and cavity filtering effect are observed and quantitatively modeled by a full quantum model with phonon scattering included. Time-resolved measurements further reveal that the multiple side peaks originate from interference of the emission associated with different temporal positions of the excitation pulses. Our work facilitates the generation of a variety of exotic quantum states of light with dynamic driving of two-level systems., Comment: Manuscript submitted on 19th May 2023

Robotic grasping in highly noisy environments presents complex challenges, especially with limited prior knowledge about the scene. In particular, identifying good grasping poses with Bayesian inference becomes difficult due to two reasons: i) generating data from uninformative priors proves to be inefficient, and ii) the posterior often entails a complex distribution defined on a Riemannian manifold. In this study, we explore the use of implicit representations to construct scene-dependent priors, thereby enabling the application of efficient simulation-based Bayesian inference algorithms for determining successful grasp poses in unstructured environments. Results from both simulation and physical benchmarks showcase the high success rate and promising potential of this approach., Comment: 4 pages, 5 figures, submitted to the workshop Geometric Representations at ICRA 2023

Quantum simulation of quantum field theories offers a new way to investigate properties of the fundamental constituents of matter. We develop quantum simulation algorithms based on the light-front formulation of relativistic field theories. The process of quantizing the system in light-cone coordinates will be explained for a Hamiltonian formulation, which becomes block diagonal, each block approximating the Fock space with a certain harmonic resolution K. We analyze a QCD theory in 2+1D. We compute the analogue of parton distribution functions, the generalized parton distribution functions for mesonic composite particles, like hadrons, in these theories. The dependence of such analyses on the scaling of the number of qubits is compared with other schemes and conventional computations. There is a notable advantage to the light-front formulation., Comment: 6 pages

The loss of gauge invariance in models of light-matter interaction which arises from material and photonic space truncation can pose significant challenges to conventional quantum optical models when matter and light strongly hybridize. In structured photonic environments, necessary in practice to achieve strong light-matter coupling, a rigorous model of field quantization within the medium is also needed. Here, we use the framework of macroscopic QED by quantizing the fields in an arbitrary material system, with a spatially-dependent dispersive and absorptive dielectric, starting from a fundamental light-matter action. We truncate the material and mode degrees of freedom while respecting the gauge principle by imposing a partial gauge fixing constraint during canonical quantization, which admits a large number of gauges including the Coulomb and multipolar gauges. We also consider gauge conditions with explicit time-dependence, enabling us to unambiguously introduce phenomenologically time-dependent light-matter interactions in any gauge. Our results allow one to derive rigorous models of ultrastrong light-matter interactions in structured photonic environments with no gauge ambiguity. Results for two-level systems and the dipole approximation are discussed, as well as how to go beyond the dipole approximation for effective single-particle models. By comparing with the limiting case of an inhomogeneous dielectric, where dispersion and absorption can be neglected and the fields can be expanded in terms of the generalized transverse eigenfunctions of the dielectric, we show how lossy systems can introduce an additional gauge ambiguity, which we resolve and predict to have fundamental implications for open quantum system models. Finally, we show how observables in mode-truncated systems can be calculated without ambiguity by using a simple gauge-invariant photodetection model., Comment: Note the argument regarding ambiguities due to intrinsic mode truncation in open quantum systems has been reworked and clarified compared to the previous version

Recent studies demonstrate the use of a two-stage supervised framework to generate images that depict human perception to visual stimuli from EEG, referring to EEG-visual reconstruction. They are, however, unable to reproduce the exact visual stimulus, since it is the human-specified annotation of images, not their data, that determines what the synthesized images are. Moreover, synthesized images often suffer from noisy EEG encodings and unstable training of generative models, making them hard to recognize. Instead, we present a single-stage EEG-visual retrieval paradigm where data of two modalities are correlated, as opposed to their annotations, allowing us to recover the exact visual stimulus for an EEG clip. We maximize the mutual information between the EEG encoding and associated visual stimulus through optimization of a contrastive self-supervised objective, leading to two additional benefits. One, it enables EEG encodings to handle visual classes beyond seen ones during training, since learning is not directed at class annotations. In addition, the model is no longer required to generate every detail of the visual stimulus, but rather focuses on cross-modal alignment and retrieves images at the instance level, ensuring distinguishable model output. Empirical studies are conducted on the largest single-subject EEG dataset that measures brain activities evoked by image stimuli. We demonstrate the proposed approach completes an instance-level EEG-visual retrieval task which existing methods cannot. We also examine the implications of a range of EEG and visual encoder structures. Furthermore, for a mostly studied semantic-level EEG-visual classification task, despite not using class annotations, the proposed method outperforms state-of-the-art supervised EEG-visual reconstruction approaches, particularly on the capability of open class recognition.

We describe how a coherent optical drive that is near-resonant with the upper rungs of a three-level ladder system, in conjunction with a short pulse excitation, can be used to provide a frequency-tunable source of on-demand single photons. Using an intuitive master equation model, we identify two distinct regimes of device operation: (i) for a resonant drive, the source operates using the Autler-Townes effect, and (ii) for an off-resonant drive, the source exploits the ac Stark effect. The former regime allows for a large frequency tuning range but coherence suffers from timing jitter effects, while the latter allows for high indistinguishability and efficiency, but with a restricted tuning bandwidth due to high required drive strengths and detunings. We show how both these negative effects can be mitigated by using an optical cavity to increase the collection rate of the desired photons. We apply our general theory to semiconductor quantum dots, which have proven to be excellent single-photon sources, and find that scattering of acoustic phonons leads to excitation-induced dephasing and increased population of the higher energy level which limits the bandwidth of frequency tuning achievable while retaining high indistinguishability. Despite this, for realistic cavity and quantum dot parameters, indistinguishabilities of over $90\%$ are achievable for energy shifts of up to hundreds of $\mu$eV, and near-unity indistinguishabilities for energy shifts up to tens of $\mu$eV. Additionally, we clarify the often-overlooked differences between an idealized Hong-Ou-Mandel two-photon interference experiment and its usual implementation with an unbalanced Mach-Zehnder interferometer, pointing out the subtle differences in the single-photon visibility associated with these different setups., Comment: 27 pages, 12 figures

Resonance fluorescence of two-level quantum systems has emerged as a powerful tool in quantum information processing. Extension of this approach to higher-level systems provides new opportunities for quantum optics applications. Here we introduce a coherent driving scheme of a three-level ladder system utilizing Autler-Townes and ac Stark effects by resonant excitation with two laser fields. We propose theoretically and demonstrate experimentally the feasibility of this approach towards all-optical spectral tuning of quantum dot-based single-photon sources and investigate photon indistinguishability and purity levels. Our tuning technique allows for fast optical control of the quantum emitter spectrum which paves the way towards temporal and spectral shaping of the single photons, formation of topological Floquet states or generation of high-dimensional frequency-encoded quantum states of light.

The prototypical system constituted by a two-level atom interacting with a quantized single-mode electromagnetic field is described by the quantum Rabi model (QRM). The QRM is potentially valid at any light-matter interaction regime, ranging from the weak (where the decay rates exceeds the coupling rate) to the deep strong coupling (where the interaction rate exceeds the bare transition frequencies of the subsystems). However, when reaching the ultrastrong coupling regime, several theoretical issues may prevent the correct description of the observable dynamics of such a system: (i) the standard quantum optics master equation fails to correctly describe the interaction of this system with the reservoirs; (ii) the correct output photon rate is no longer proportional to the intracavity photon number; and (iii) the appears to violate gauge invariance. Here, we study the photon flux emission rate of this system under the incoherent excitation of the two-level atom for any light-matter interaction strength, and consider different effective temperatures. The dependence of the emission spectra on the coupling strength is the result of the interplay between energy levels, matrix elements of the observables, and the density of states of the reservoirs. Within this approach, we also study the occurence of light-matter decoupling in the deep strong coupling regime, and show how all of the obtained results are gauge invariant., Comment: 25 pages, 21 figures