We analyze the differences between relativistic fields with or without quantum degrees of freedom in relativistic quantum information protocols. We classify the regimes where the existence of quantum degrees of freedom is necessary to explain the phenomenology of interacting quantum systems. We also identify the precise regimes where quantum fields can be well approximated by quantum-controlled classical fields in relativistic quantum information protocols. Our results can be useful to discern which features are fundamentally different in classical and quantum field theory.

Photonic graphene is a form of graphene in optic platforms that is important for fabrication of photonic topological insulators, which may lead to novel techniques to realize various types of light manipulation. Among a plethora of schemes to reform photonic graphene to meet the desired specifications, the significance of strain operations has not received sufficient attention. Here, we theoretically and numerically report zero-energy edge states in strained photonic graphene. After applying strain, photonic graphene can be regarded as a stack of Su-Schrieffer-Heeger chains, which can be considered to be a convincing cause of the appearance of zero-energy edge states. In addition, the topological origin is analyzed based on the tight-binding method, and we find that the Zak phase is $\pi$ when there is a zero-energy edge state. In reference to the dispersive nature of zero-energy edge states, the self-action effect of nonlinearity is introduced to balance the dispersive broadening of these states to form both bright and dark zero-energy edge solitons. We believe that the results obtained may provide deeper understanding of the role of strain in two-dimensional lattices and may find potential applications in fabricating future on-chip on-demand photonic devices.

We systematically analyze the various phase transitions of the anisotropic Dicke model that is endowed with both rotating and counterrotating light-matter couplings. In addition to the ground-state quantum phase transition (QPT) from the normal to the superradiant phase, the anisotropic Dicke model also exhibits other transitions, namely, the excited state quantum phase transition (ESQPT), ergodic to nonergodic transition (ENET), and the temperature-dependent phase transition. We show that these phase transitions are profitably studied not only with the standard consecutive level spacing ratio, but also with the aid of various eigenvector quantities such as von Neumann entanglement entropy, the participation ratio, multifractal dimension, and mutual information. For ENET, both the statics and dynamics of the participation ratio offer a consistent and useful picture. An exciting finding from our work is that the ESQPT and the ENET are closely related to each other. We show this with the aid of two characteristic energies in the spectrum corresponding to jumps in von Neumann entropy.

A version of the electric Aharonov-Bohm effect is proposed where the quantum system which picks up the Aharonov-Bohm phase is confined to a Faraday cage with a time-varying spatially uniform scalar potential. The electric and magnetic fields in this region are effectively zero for the entire period of the experiment. The observable consequence of this version of the electric Aharonov-Bohmn effect is to shift the energy levels of the quantum system rather than shift the fringes of the two-slit interference pattern. We show a strong mathematical connection between this version of the scalar electric $AB$ effect and the ac Stark effect.

The quantum wave function of multiple particles provides additional information which is inaccessible to detectors working alone. Here, we introduce the coincidence wave-front sensing (CWS) method to reconstruct the phase of the multiphoton transverse spatial wave function. The spatially resolved coincidence photon counting is involved. Numerical simulations of two-photon cases using the weak measurement wave-front sensor are performed to test its correctness, and the phase information hidden in the correlation is revealed. Our work provides a direct spatial way to characterize multipartite quantum systems and leads to fundamental studies like experimental Bohmian mechanics and applications in quantum optical technologies.

We explore the extent to which two quantum oscillators can exchange their quantum states efficiently through a three-level system which can be spin levels of colored centers in solids. High transition probabilities are obtained using Hamiltonian engineering and quantum control techniques. Starting from a weak-coupling approximation, we derive conditions on the spin-oscillator interaction Hamiltonian that enable a high-fidelity exchange of quanta. We find that these conditions cannot be fulfilled for arbitrary spin-oscillator coupling. To overcome this limitation, we illustrate how a time-dependent control field applied to the three-level system can lead to an effective dynamic that performs the desired exchange of excitation. In the (ultra)strong-coupling regime, an important loss of fidelity is induced by the dispersion of the excitation onto many Fock states of the oscillators. We show that this detrimental effect can be substantially reduced by suitable control fields, which are computed with optimal control numerical algorithms.

We show that the Lamb shift can significantly affect the nature of the interatomic dipole-dipole interaction in structured reservoirs. The effect is explained by the difference in the Lamb shift for atoms located at different positions due to the inhomogeneity of the spatial structure of the modes of the electromagnetic field. Based on the model system involving two atoms in a waveguide, we demonstrate that this difference can be the same order as the natural linewidth of the atomic transition.

Buffer-gas cooling is a universal cooling technique for molecules and used for various purposes. One of its applications is using molecules inside a buffer-gas cell for low-temperature spectroscopy. Although a high-intensity signal is expected in the cell, complex molecular dynamics is a drawback for precise spectroscopy. In this study, we performed high-resolution absorption spectroscopy of low-$J$ transitions in the $\tilde{A}{}^2\mathrm{\Pi }(0,0,0)\text{−}\tilde{X}{}^2{\mathrm{\Sigma }}^{+}(0,0,0)$ band of calcium monohydroxide (CaOH). CaOH molecules were produced by laser ablation in a copper cell and cooled to $\sim 5\mathrm{K}$ using helium buffer gas. We probed the Doppler effects in a buffer-gas cell by injecting counterpropagating lasers inside the cell. The time evolutions of the Doppler width and shift were simulated using a dedicated Monte Carlo simulation and compared with data.

The ultracold and weakly coupled Fermi gas in two spatial dimensions is studied in an effective field theory framework. It has long been observed that universal corrections to the energy density to two orders in the interaction strength do not agree with Monte Carlo simulations in the weak-coupling regime. Here, universal corrections to three orders in the interaction strength are obtained, and are shown to provide agreement between theory and simulation. Special consideration is given to the scale ambiguity associated with the nontrivial renormalization of the singular contact interactions. The isotropic superfluid gap is obtained to next-to-leading order, and nonuniversal contributions to the energy density due to effective range effects, $p$-wave interactions, and three-body forces are computed. Results are compared with precise Monte Carlo simulations of the energy density and the contact in the weakly coupled attractive and repulsive Fermi-liquid regimes. In addition, the known all-orders sum of ladder and ring diagrams is compared with Monte Carlo simulations at weak coupling and beyond.

The ability to convert photons between different electromagnetic modes with low intensity has the potential application in future information technologies. Here, we explore optical-mode-conversion characteristics in a bimodal cavity coupled to a pair of two-level atoms instead of a single atom using experimentally realistic parameters. In our scheme, the two cavity modes, involving a vertically ($V$-) polarized mode and a horizontally ($H$-) polarized mode, are not directly coupled to each other owing to their orthogonal polarizations and the $V$-polarized mode is coherently excited by an external weak pump laser. Via analytical calculations and numerical simulations, we find that optical hyperconversion phenomenon from the $V$-polarized mode to the $H$-polarized mode can be realized in the bad-cavity regime, which is reminiscent of optical hyperradiance introduced in the previous studies. We also provide an approximate analysis which gains deeper understanding into the fundamental reason behind this behavior. Furthermore, we identify the parameter ranges for generating optical-mode subconversion, superconversion, and hyperconversion. The obtained results will deepen the understanding of mode-conversion micromechanisms in the weak-coupling multiatom cavity architectures.

The inevitable noise within one-sided device-independent quantum information tasks may completely break any steering resources. These noisy effects are characterized by the steerability breaking (SB) channels. In this work, we demonstrate the use of a control system that can superpose $N$ copies of the given SB channel to activate SB channels such that the channel becomes a non-SB one. In particular, we analytically and numerically show that the SB depolarizing and amplitude-damping channel can be coherently activated when $N\ge 2$. In addition, for both cases, the channels can preserve more steerability when increasing $N$. Finally, we propose circuit models to implement our proposed approaches and present the simulation results.

We perform a theoretical investigation into how a two-mode squeezed vacuum state, which has undergone photon loss, can be stored and purified using noiseless amplification with a collection of solid-state qubits. The proposed method may be used to probabilistically increase the entanglement between the two parties sharing the state. The proposed amplification step is similar in structure to a set of quantum scissors. However, in this work the amplification step is realized by a state transfer from an optical mode to a set of solid-state qubits, which act as a quantum memory. We explore two different applications, the generation of entangled many-qubit registers and the construction of quantum repeaters for long-distance quantum key distribution.

Quantum memories promise to enable global quantum repeater networks. For field applications, alkali-metal vapors constitute an exceptional storage platform, as neither cryogenics, nor strong magnetic fields are required. We demonstrate a technologically simple, in principle satellite-suited quantum memory based on electromagnetically induced transparency on the cesium $D1$ line, and focus on the tradeoff between end-to-end efficiency and signal-to-noise ratio, both being key parameters in applications. For coherent pulses containing one photon on average, we achieve storage and retrieval with end-to-end efficiencies of ${\eta }_{\text{e2e}}=13\left(2\right)\%$, which correspond to internal memory efficiencies of ${\eta }_{\text{mem}}=33\left(1\right)\%$. Simultaneously, we achieve a noise level corresponding to ${\mu }_1=0.07\left(2\right)$ signal photons. This noise is dominated by spontaneous Raman scattering, with contributions from fluorescence. Four-wave mixing noise is negligible, allowing for further minimization of the total noise level.

We report a comprehensive experimental and theoretical study of the autoionization of core-excited $5p_{1/2}n{\ell }_j$ strontium Rydberg states for intermediate values of ${\ell }_j$ in the range $0\lesssim {\ell }_j\lesssim 5$. For a given value of ${\ell }_j$ autoionization, which results from the energy exchange between the two excited valence electrons, is found to scale as $\sim 1/n^3$, mirroring the decrease in the Rydberg electron probability density in the vicinity of the inner $5p_{1/2}$ electron cloud. In contrast, the ${\ell }_j$ dependence of the autoionization rate is more complex. For the larger values of ${\ell }_j$ $(\ge 4)$, the autoionization rate decreases as ${\ell }_j^{-5}$ due to the centrifugal barrier, which prevents the Rydberg electron from penetrating the core and inner $5p_{1/2}$ electron cloud. For the low-${\ell }_j$ core-penetrating Rydberg states, the scattering phase shift induced by the effective core potential becomes important in determining both the autoionization rates, which no longer show a monotonic variation with ${\ell }_j$, and the quantum defects.

Measured photoionization time delays may exhibit large variations as a function of the emission angles, even for spherically symmetric targets, as shown in recent reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) experiments. The contributions from different pathways to the two-photon quantum channels can explain the observed phase jumps that shape those angular distributions. Here we propose a simple analytical model to describe angularly resolved RABBITT spectra as a function of the relative polarization angle between the ionizing attosecond pulse train and the assisting IR field. We demonstrate that the angular dependences of the measured delays can be analytically predicted and the position of the phase jumps reduced to the analysis of a few relevant parameters.

It is well established that the noise correlations measured by time-of-flight imaging in cold-atom experiments, which correspond to the density-density correlations in the momentum space of trapped atomic gases, can probe the spin structure factor deep in the Mott-insulating regime of SU(2) Hubbard models. We explicitly derive the mathematical relation between the noise correlations and the spin structure factor in the strong-interaction limit of $\mathrm{SU}\left(N\right)$ Hubbard models at any integer filling $\rho$. By calculating the ground states of one-dimensional $\mathrm{SU}\left(N\right)$ Fermi-Hubbard models for $2\le N\le 6$ with use of the density-matrix renormalization-group method, we confirm the relation numerically in the regime of strong interactions $U\gg t$, where $U$ and $t$ denote the on-site interaction and the hopping energy. We show that the deviation between the actual noise correlations and those obtained from the spin structure factor scales as approximately ${(t/U)}^2$ for $\rho =1$ at intermediate and large lattice sizes on the basis of numeric and semianalytic arguments.

The introduction of long-range interaction (LRI) in models that describe topological systems has unveiled previously unknown topological phases and states. Recently, in higher-order topological insulators, consideration of LRI via next-nearest-neighbor (NNN) hopping led to the discovery of unconventional confined corner states; however, their study is still limited in photonic prospects. In this paper, we bring the concept of type-III corner states into the photonic regime and reveal that their existence depends on LRI in photonic lattices, mimicking an extended two-dimensional Su-Schrieffer-Heeger (SSH) model with an asymmetric coupling. The extension of the SSH model is performed by considering a variety of NNN hopping in square lattices, and an asymmetry coupling in the intercellular NNN hopping terms of the Hamiltonian describing the system. Subsequently, we identify type-I, type-II, and type-III corner states in the analytical tight-binding model, revealing that the type-II and type-III corner states are split from the edge states. Based on the obtained results, we propose a photonic model that mimics the asymmetric coupling and find the existence of type-II and type-III corner states in the photonic model. The results presented here extend the knowledge we have about topological corner states in photonic systems and highlight the role played by LRIs in the generation of topological confined modes.