We study, experimentally and theoretically, temporal correlations between the polarization of photon pairs emitted during the biexciton-exciton radiative cascade from a single semiconductor quantum dot, optically excited by a continuous-wave light source. The system is modeled by a Lindbladian coupled to two Markovian baths: One bath represents the continuous light source, and a second represents the emitted radiation. Very good agreement is obtained between the theoretical model that we constructed and a set of 36 different time-resolved, polarization correlation measurements between cascading photon pairs.

We examine the interplay between electron correlations and phonons in an Anderson-Holstein impurity model with an Einstein phonon. When the phonons are slow compared to charge fluctuations (frequency ${\omega }_0\ll U/2$, the onsite Coulomb scale $U/2$), we demonstrate analytically that the expected phonon-mediated reduction of interactions is completely suppressed, even in the strong-coupling regime. This suppression arises from the oscillator's inability to respond to rapid charge fluctuations, manifested as a compensation effect between the polaronic cloud and the excited-state phonons associated with valence fluctuations. We identify a frozen mixed valence phase, above a threshold dimensionless electron-phonon coupling ${\alpha }^{*}$ when the phonons are slow, where the static phonon cloud locks the impurity into specific valence configurations, potentially explaining the puzzling coexistence of mixed valence behavior and insulating properties in materials like rust. Conversely, when the phonon is fast (${\omega }_0\gtrsim U/2$), the system exhibits conventional polaronic behavior with renormalized onsite interactions effectively $U_{\text{eff}}$ due to phonon-mediated attraction, with additional satellite features in the local spectral function due to phonon excitations. Using numerical renormalization group calculations, a fully dynamic renormalization technique, we confirm these behaviors in both regimes. These findings have important implications for strongly correlated systems where phonon energy scales may be comparable to the Coulomb scale, such as in twisted bilayer graphene, necessitating careful consideration of interaction renormalizations in theoretical models.

Rare-earth spin ensembles are a promising platform for microwave quantum memory applications due to their hyperfine transitions, which can exhibit exceptionally long coherence times when using an operation point with zero first-order Zeeman (ZEFOZ) shift. In this work, we use broadband electron paramagnetic resonance (EPR) spectroscopy on ${}^{167}\mathrm{Er}$:${}^7\mathrm{LiYF}_4$ single crystals at subkelvin temperatures. By fitting the spin Hamiltonian to the zero-field spectrum, we obtain refined parameters of the magnetic field-independent interactions, such as the hyperfine and quadrupole interaction. We also study the influence of the quadrupole interaction on the hyperfine splitting in the zero and low magnetic field range by analyzing EPR spectra between $0\mathrm{m}\mathrm{T}$ and $50\mathrm{m}\mathrm{T}$. Our findings highlight the broadband EPR spectroscopy approach as a powerful tool for the precise determination of the spin Hamiltonian parameters and for the characterization of hyperfine transitions in terms of their selection rules and linewidth.

The transition dynamics of two-state systems with time-dependent energy levels, first considered by Landau, Zener, Majorana, and Stückelberg, is one of the basic models in quantum physics and has been used to describe various physical systems. We propose here a generalization of the Landau-Zener (LZ) problem characterized by distinct paths of the instantaneous eigenstates as the system evolves in time while keeping the instantaneous eigenenergies exactly as in the standard LZ model. We show that these paths play an essential role in the transition probability $P$ between the two states, and can lead to a substantial reduction of $P$, being possible even to achieve $P=0$ in an instructive extreme case, and also to large $P$ even in the absence of any anticrossing point. The partial LZ model can describe valley transition dynamics during charge and spin shuttling in semiconductor quantum dots.

Heat transfer in quantum systems is a current topic of interest due to emerging quantum technologies that attempt to miniaturize engines and examine fundamental aspects of thermodynamics. In this work we consider heat transfer between two thermal reservoirs in which a central spin degree of freedom mediates the process. Our objective is to identify the system-bath coupling operators that maximize heat transfer at arbitrary system-bath coupling strengths. By employing a Markovian embedding method in the form of the reaction-coordinate mapping, we study numerically heat transfer at arbitrary system-bath coupling energy and for general system-bath coupling operators between the baths and the central qubit system. We find a stark contrast in the conditions required for optimal heat transfer depending on whether the system is weakly or strongly coupled to the heat baths. In the weak-coupling regime, optimal heat transfer requires identical coupling operators that facilitate maximum sequential transport, resonant with the central qubit. In contrast, in the strong-coupling regime, noncommuting system-bath coupling operators between the hot and cold reservoirs are necessary to achieve optimal heat transfer. We further employ the Effective Hamiltonian theory and gain partial analytical insights into the observed phenomena. We discuss the limitations of this approximate method in capturing the behavior of the heat current for noncommuting coupling operators, calling for its future extensions to capture transport properties in systems with general interaction Hamiltonians.

Hybrid superconductor-semiconductor systems have received a great deal of attention in the past few years because of their potential for quantum engineering, including novel qubits and topological devices. The proximity effect, the process by which the semiconductor inherits superconducting correlations, is an essential physical mechanism of such hybrids. Recent experiments have demonstrated the proximity effect in hole-based semiconductors, but, in contrast to electrons, the precise mechanism by which the hole bands acquire superconducting correlations remains an open question. In addition, hole spins exhibit a complex strong spin-orbit interaction, with largely anisotropic responses to electric and magnetic fields, further emphasizing the importance of understanding the interplay between such effects and the proximity effect. In this work, we analyze this physics with a focus on germanium-based two-dimensional gases. Specifically, we develop an effective theory supported by full numerics, allowing us to extract various analytical expressions and predict different types of superconducting correlations including nonstandard forms of singlet and triplet pairing mechanisms with nontrivial momentum dependence, as well as different Zeeman and Rashba spin-orbit contributions. This, together with their precise dependence on electric and magnetic fields, allows us to make specific experimental predictions, including the emergence of f-type superconductivity, Bogoliubov Fermi surfaces, and gapless regimes caused by large in-plane magnetic fields.

Technology involving hybrid superconductor–semiconductor materials is a promising avenue for engineering quantum devices for information storage, manipulation, and transmission. Proximity-induced superconducting correlations are an essential part of such devices. While the proximity effect in the conduction band of common semiconductors is well understood, its manifestation in confined hole gases, realized for instance in germanium, is an active area of research. Lower-dimensional hole-based systems, particularly in germanium, are emerging as an attractive platform for a variety of solid-state quantum devices, due to their combination of efficient spin and charge control and long coherence times. The recent experimental realization of the proximity effect in germanium thus calls for a theoretical description that is tailored to hole gases. In this work, we propose a simple model to describe proximity-induced superconductivity in two-dimensional hole gases, incorporating both the heavy-hole (HH) and light-hole (LH) bands. We start from the Luttinger–Kohn model, introduce three parameters that characterize hopping across the superconductor–semiconductor interface, and derive explicit intraband and interband effective pairing terms for the HH and LH bands. Unlike previous approaches, our theory provides a quantitative relationship between induced pairings and interface properties. Restricting our general model to an experimentally relevant case where only the HH band crosses the chemical potential, we predict the coexistence of $s$-wave and $d$-wave singlet pairings, along with triplet-type pairings, and modified Zeeman and Rashba spin–orbit couplings. Our results thus present a starting point for theoretical modeling of quantum devices based on proximitized hole gases, fueling further progress in quantum technology.

The antiferromagnetic Ising model on a triangular lattice (AFIT) serves as a paradigmatic example of frustration, stemming from its geometry that precludes simultaneous satisfaction of all interactions. Here, we present an experimental platform to investigate AFIT by utilizing magnets arranged in vertical cavities of a triangular lattice, which stabilize at either the bottom or top of the cavity, mimicking the bistability of Ising spins. The strong magnet interactions and the unique growth process allow the frustrated behavior and its ground state configurations to be directly observed. An effective thermalization process is developed to reduce metastable energy barriers via external perturbations, facilitating evolution toward the ground state dominated by dipolar interactions. Theoretical simulations are performed to reveal the evolution of various magnetic states under effective thermal fluctuations, which are remarkably consistent with experimental results. Our system offers a unique approach to studying frustrated systems and paves the way for future research in complex geometries.

Details of the electronic band structure in unconventional superconductors are key to the understanding of their fundamental ground state. The potential spin-triplet superconductor ${\mathrm{UTe}}_2$, with $T_{\mathrm{c}}\approx 2.1\mathrm{K}$, has attracted attention recently. Its main Fermi surface consists of weakly corrugated, two-dimensional Fermi-surface cylinders that run along the crystallographic $c$ axis. In addition, there is evidence for the presence of an additional small three-dimensional band. This has been discussed controversially as it may be essential for the realization of superconductivity in ${\mathrm{UTe}}_2$. Here, we investigate the angle-resolved magnetoresistance and Hall effect in bulk crystalline samples with current along the $c$ axis in fields up to $60\mathrm{T}$. We observe low-frequency magnetic quantum oscillations with light effective masses that are most pronounced for magnetic field applied along the $a$ axis. Two distinct frequencies indicate two separate changes in the Fermi-surface topology, likely connected with Lifshitz transitions. We discuss the origin of these oscillations in terms of magnetic breakdown, quantum interference, and other potential mechanisms.

We present a comprehensive theoretical investigation of the excitonic properties of uniaxially strained transition-metal dichalcogenide monolayers (TMD-MLs) by solving the Bethe-Salpeter equation (BSE) in the Wannier tight-binding scheme established on the base of first-principles. The presence of a uniaxial strain in a ${\mathrm{MoS}}_2$-ML is shown to impact most the spin-forbidden exciton states, i.e., gray exciton (GX) and dark exciton (DX) states. As a consequence of strain-induced valley drift and electron-hole exchange interaction, imposing a uniaxial strain onto a ${\mathrm{MoS}}_2$-ML dramatically reshapes the band dispersion of DX from a parabola to a Mexican-hat-like profile, accompanying with unusual sign-reversal of the exciton effective mass and slight strain-activated brightness. Contrarily, the band dispersion of spin-forbidden GX yet remains parabolic but is characterized by a drastically reduced effective mass by an imposed strain. The first-principles-based studies further predict the strain-induced diversification of the exciton diffusivities, transition dipoles, and angle-resolved optical patterns of bright excitons, GXs, and DXs in uniaxially strained TMD-MLs, suggesting the feasibility of spatially resolving spin-allowed and -forbidden excitons in exciton transport and optical spectroscopy experiments.

The development of spectroscopic techniques in the extreme ultraviolet (XUV) spectral range has significantly advanced the understanding of ultrafast interactions in magnetic systems triggered by optical excitation. In this work, we introduce a previously missing geometry that facilitates the observation of the ultrafast magnetization dynamics of magnetic systems with an out-of-plane magnetization grown on XUV opaque substrates. This approach to probing ultrafast magnetization dynamics combines the magneto-optical Kerr effect with the strong dependence of a sample's reflectance near its Brewster angle. It therefore works with linearly polarized light and does not require any additional polarizing optics. We provide a comprehensive analysis of the technique by presenting both simulations and experimental data as a function of the energy and the polarization of the XUV probe radiation as well as of the delay time after optical excitation.

We report an extensive study of the noncentrosymmetric half-Heusler superconductor YPtBi, revealing an unusual relation between bulk superconductivity and the possible appearance of surface superconductivity on the (111) oriented surface, at temperatures up to three times the bulk transition temperature. Transport measurements confirmed the low carrier density of the material and its bulk superconducting transition, which was also observed in ac susceptibility through mutual inductance (MI) measurements. However, a weak signature of superconductivity in the MI measurements appeared much above the bulk transition temperature, which was further observed in scanning tunneling spectroscopy, pointing to a possible surface superconducting state. Polar Kerr effect measurements suggest that while the bulk superconductor may exhibit an unusual nodal superconducting state, only the surface state breaks time reversal symmetry. Complementary tunneling measurements on LuPtBi are used to establish the observations on YPtBi, while density-functional theory calculations may shed light on the origin of this unusual surface state.

Improving the efficiency and stability of organic light-emitting diodes (OLEDs) will further expand their present success in display applications. Triplet exciton-polaron quenching (TPQ) is an important cause of limited efficiency and stability in modern phosphorescent OLEDs, where triplet excitons are the emitting species. Lack of understanding of the TPQ mechanism in these OLEDs impedes the development of more efficient and stable OLEDs. We investigate the TPQ mechanism for triplet excitons on a phosphorescent guest interacting with hole polarons on a host. Our quantum-chemical calculations show that at distances relevant for TPQ the Förster approximation for the TPQ rate fails and that dipole-quadrupole coupling is dominant. This resolves a discrepancy between estimates of the TPQ rate obtained from an OLED device study and from the overlap between the emission spectrum of the emitter and absorption spectrum of the charged host. Equivalently to the Förster radius for dipole-dipole TPQ, the dipole-quadrupole TPQ rate can be quantified by a dipole-quadrupole radius obtained from the overlap between the emission spectrum of the emitter and the quadrupolar absorption spectrum of the charged host. The findings of this work are expected to have a broad relevance and to be useful in developing phosphorescent emitter-host combinations with reduced TPQ.

Altermagnetic superconductors offer the possibility of exploring unconventional superconductivity, including topological states and finite-momentum superconductivity, with promising applications in spintronics and quantum information. However, a direct experimental confirmation of their existence remains elusive. In this work, we propose nonmagnetic impurities as probes of the interplay between altermagnetism and superconductivity. These impurities induce spin-polarized subgap states whose spatial extension reflects the magnetic order of the substrate material. Depending on whether or not the impurity respects the bulk symmetries, these states form spin-degenerate or spin-split doublets. An external magnetic field aligned with the Néel vector can further control the doublet splitting. We further demonstrate that coupling between impurity states leads to a position-dependent, spin-sensitive hybridization, enabling another approach for in situ control of atomic-size quantum devices. These findings provide unambiguous experimental signatures of altermagnetic superconductivity accessible via local measurements such as scanning tunneling microscopy and open unexplored pathways for designing tunable quantum devices.

We investigate the exciton dynamics in an exactly solvable two-band model for semiconductors. The model incorporates light-matter, electron-electron, and electron-phonon interactions, and captures exciton formation as well as the transition from the coherent to the incoherent regime. We analyze excitonic polarization, populations, and coherences, with special focus on their impact in time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). For nonresonant pumping with below-gap photon energies, TR-ARPES spectra reveal distinct excitonic replica and quantum beats persisting in the incoherent regime. These are due to a coherence between different species of incoherent excitons. Such type of coherence is resistant to phonon dephasing, indicating that it follows different dynamics than those governing the coherences considered so far.

We formulate and investigate a quantum state, the chiral vortex-line liquid (CVLL), emerging in three-dimensional interacting Bose systems exhibiting moat-band dispersions. Such dispersions feature extensive degeneracy along closed manifolds in momentum space, significantly amplifying quantum fluctuations that suppress conventional Bose-Einstein condensation. By extending the two-dimensional Chern-Simons (CS) flux-attachment transformation to three dimensions through a combination of planar CS phases and Jordan-Wigner fermionization along vortex lines, we construct the CVLL state, characterized by preserved rotational $SO\left(2\right)$ symmetry, broken time-reversal symmetry, nontrivial vortex-line excitations, and topological gapless edge surface states. We construct the associated field theory in a curved spatial geometry and analyze the low-energy effective theory of the CVLL state, demonstrating its topological nature. Using Monte Carlo simulations, we numerically determine the scaling of the chemical potential of the CVLL ground state as a function of boson density for interacting bosons in a cylindrical moat-band geometry and demonstrate that the CVLL phase energetically outcompetes traditional condensate phases at low densities, highlighting its relevance to experimental platforms including frustrated quantum magnets, ultracold atomic gases, excitonic systems, the physics of rotons in ${}^4\mathrm{He}$, and moat regimes in heavy-ion collisions.

Two-dimensional (2D) massless Dirac fermions (MDF), which represent a type of quasiparticles with linear energy-momentum dispersions only in 2D momentum space, provide a fertile ground for realizing novel quantum phenomena. However, 2D MDF were seldom observed in the superconducting bulk states of 3D materials. Furthermore, as a cornerstone for accurately tuning the quantum phenomena based on 2D MDF, a quantitative relationship between 2D MDF and a structural parameter has rarely been revealed so far. Here, we report magnetoinfrared spectroscopy studies of the iron-arsenide-superconductor systems NaFeAs and $A{\mathrm{Fe}}_2{\mathrm{As}}_2(A=\mathrm{Ca},\mathrm{Ba})$ at temperature $T\sim 4.2$ K and at magnetic fields ($B$) up to 17.5 T. Our results demonstrate the existence of 2D MDF in the superconducting bulk state of NaFeAs. Moreover, the 2D-MDF Fermi velocities in NaFeAs and $A{\mathrm{Fe}}_2{\mathrm{As}}_2(A=\mathrm{Ca},\mathrm{Ba})$, which are extracted from the slopes of the linear $\sqrt{B}$ dependences of the Landau-level transition energies, scale linearly with the Fe-As bond lengths. The linear scaling between the 2D-MDF Fermi velocities and the Fe-As bond lengths is supported by (i) the linear relationship between the square root of the effective mass of the $d_{xy}$ electrons and the Fe-As bond length and (ii) the linear dependence of the square root of the calculated tight-binding hopping energy on the Fe-As bond length. Our results open up avenues for exploring and tuning quantum phenomena based on 2D MDF in the superconducting bulk states of 3D materials.

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are currently limited by their operational stability and a decreased efficiency at high brightness, both of which are strongly related to the interactions between triplet excitons and other quasiparticles. Here, an expression for the lifetime of triplet excitons in TADF OLEDs is derived, which correlates with the efficiency roll-off. The triplet lifetime in a TADF OLED intriguingly can be well approximated by the delayed-fluorescence lifetime of the TADF emitter, enabling facile quantification of the triplet population in a working OLED for investigating triplet-related loss processes, such as the efficiency roll-off and degradation.

Changes in the wave function's phase during propagation in a random Hermitian lattice, a process known as dephasing, results in diffusion rather than Anderson localization. However, when non-Hermiticity is introduced, the wave behavior changes drastically. In particular, we demonstrate that in weakly disordered non-Hermitian lattices, dephasing enhances eigenmode localization, which results in abrupt jumps between spatially distant regions. These jumps, which are absent under purely coherent conditions, emerge from the interplay between complex disorder and dephasing.

The competition between quantum many-particle dynamics and continuous monitoring can lead to measurement-induced phase transitions (MIPTs). So far, MIPTs have been extensively explored in fermionic or spin systems. To examine the possibility of an MIPT in bosonic systems, we study the entanglement structure in continuously monitored free bosons with long-range couplings. When the measurement is local, we find that no MIPTs occur because the substantial entanglement generated by the long-range coupling overcomes the entanglement destruction due to the measurement. In contrast, we show that the nonlocal measurement can efficiently suppress the entanglement generation, leading to an MIPT where the bipartite entanglement entropy exhibits the subvolume-to-area law phase transition as the measurement strength is increased. Our numerical results indicate that the critical point should be described by a certain conformal field theory, while the transition does not belong to a conventional universality class such as Berezinskii-Kosterlitz-Thouless class.