This experimental work demonstrates multipartite quantum correlation in bright frequency combs out of a microresonator integrated on silicon nitride operating above its oscillation threshold. Multipartite features, going beyond so far reported two-mode correlation, naturally arise due to a cascade of nonlinear optical processes, making a single-color laser pump sufficient to initiate their generation. Our results show the transition from two-mode to multipartite correlation, witnessed by noise reductions as low as $-2.5\mathrm{dB}$ and $-2\mathrm{dB}$, respectively, compared to corresponding classical levels. A constant of the movement of the nonlinear interaction Hamiltonian is identified and used to asses the multipartite behavior. Reported demonstrations pave the way to next generation on-chip multipartite sources for quantum technologies applications.

With the development of any quantum technology comes a need for precise control of quantum systems. Here, we evaluate the impact of control noise on a quantum Otto cycle. While it is postulated that noiseless quantum engines can approach maximal Otto efficiency in finite times, the existence of white noise on the controls is shown to negatively affect average engine performance. Two methods of quantum enhancement, counterdiabatic driving and quantum lubrication, are implemented and found to improve the performance of the noisy cycle only in specified parameter regimes. To gain insight into performance fluctuations, projective energy measurements are used to construct a noise-averaged probability distribution without assuming full thermalization or adiabaticity. From this, the variances in thermodynamic currents are observed to increase as average power and efficiency improve, and are also shown to be consistent with known bounds from thermodynamic uncertainty relations. Lastly, by comparing the average functioning of the unmonitored engine to a projectively measured engine cycle, the role of coherence in work extraction for this quantum engine model is investigated.

We derive the analogues of the Dirac and Pauli equations from a spatially fourth-order Klein-Gordon equation with a universal length scale. Starting from a singularly perturbed variant of Maxwell's equations, we deduce a 32-dimensional variant of the Dirac equation for spin-$1/2$ particles through an algebraic factorization procedure. We illustrate an experimental test of the theory from the split lines of the electron beam in a Stern-Gerlach experiment. This hyperfine splitting leads to four distinct eigenvalues of the spin operator, which can be grouped into two pairs centered around the classic values of $\pm \hslash /2$. The modified electrodynamic framework features an oriented, micropolar spacetime.

We show here that the photonic states emitted by ensembles of multilevel atoms via a superradiance process exhibit entanglement in the modal (frequency) degree of freedom, making this collective emission process a favorable candidate for a fast, bright, and deterministic source of entangled photons. This entanglement is driven by two mechanisms: (i) selective excitation of the atomic ensemble to a superposition state, and (ii) degeneracies of the optical transitions owing to the internal structure of the emitting atoms. The latter induces intricate nonradiative virtual transitions in the ensemble, which create interatomic correlations that are imprinted onto the emitted photons. One of the important outcomes of this complexity is the generation of mode-independent entangled multiphoton states. In addition, we study the dynamics of the correlations of the superradiating multilevel atom ensembles, and demonstrate a case where they exhibit beating in steady state as a result of the aforementioned virtual transitions.

Waveguide quantum electrodynamics (QED) has opened a new frontier in quantum optics, which enables the radiative coupling of distantly located emitters via the spatially extended waveguide mode. This coupling leads to modified emission dynamics and previous work has reported the observation of increased intensity correlations (an antidip) when probing the resonance response of multiple emitters. However, the interference between independent emitters has been shown to lead to a similar response. Here, we directly observe resonant energy transfer between two distant quantum emitters by recording an antidip in the intensity correlations, $g^{\left(2\right)}\left(\tau \right)$, while driving only one of the emitters. Under the condition that only a single emitter is driven, the antidip in photon coincidences is a distinctive signature of emitter-emitter coupling, which enables the transfer of energy from the driven to the undriven emitter. Interestingly, the observed mechanism is a long-range and waveguide-engineered version of resonant Förster transfer, which is responsible for the transport of energy between chlorophylls in the photosynthesis. Building on the established coupling, we demonstrate collective driving of the coupled emitter pair. Specifically, we control the relative driving phase and amplitude of the emitters and apply this collective excitation scheme to selectively populate the long-lived subradiant state. This results in suppressed emission, i.e., the peculiar situation where driving two emitters as opposed to one effectively reduces the probability of photon emission. Our work presents novel emission regimes and excitation schemes for a multiemitter waveguide QED system. These can be exploited to deterministically generate emitter-emitter entanglement and advanced photonic states providing robustness against losses for photonic quantum computation and quantum communication.

We investigate numerically computed Floquet states of a Bose-Hubbard dimer that is subjected to strong, time-periodic forcing with respect to their coherence, invoking a measure for their degree of simplicity previously suggested by Leggett. This serves to ascertain the validity of the mean-field approximation under conditions such that the time-dependent nonlinear Gross-Pitaevskii equation has chaotic solutions. It is shown that for sufficiently large particle numbers the exact $N$-particle Floquet state semiclassically associated with the innermost quantized invariant tube surrounding a stable periodic mean-field orbit represents a macroscopically occupied single-particle state, i.e., a Floquet condensate.

Understanding the universal properties of nonequilibrium phase transitions of spreading processes is a challenging problem. This applies in particular to irregular and dynamically varying networks. We here investigate an experimentally accessible model system for such processes, namely, the absorbing-state phase transition (ASPT) of Rydberg-excitation spreading, known as Rydberg facilitation, in a laser-driven gas of mobile atoms. It occurs on an irregular graph, set by the random atom positions in the gas, and, depending on temperature, changes its character from static to dynamic. By studying the behavior of the order parameter in the work of Brady et al. [Phys. Rev. Lett. 133, 173401 (2024)], we showed numerical evidence for a crossover from directed percolation (DP) universality through various phases of anomalous directed percolation (ADP) to mean-field (MF) behavior when the temperature of the gas is increased. As the behavior of the order parameter is not sufficient to uniquely determine the universality class, we here analyze the distribution of avalanches—characteristic of nonequilibrium critical behavior—to fully characterize the ASPT. Performing extended numerical calculations and experiments on a cold ${}^{87}\mathrm{Rb}$ atom gas, we confirm our earlier numerical findings and our phenomenological model that maps the dynamic network to a static one with power-law tails of the distribution of excitation distances. Furthermore, we discuss the influence of dissipation, present in the experiment and a necessary ingredient for the self-organization of the system to the critical point. In particular, we study the potential modification of the universality class by losses as a function of dissipation strength.

It still remains a significant challenge using the current technology to fabricate devices with customized functions for both electromagnetic (EM) and acoustic waves since the demand of biphysical manipulation, which means that the same device can be used to control EM and acoustic waves simultaneously, difficult is to satisfy in nature. However, the emergence of metasurfaces provides a new way to synchronously control the propagation for both waves. Unfortunately, the independent and continuous wavefront manipulation of multiple physical fields, including acoustic and EM waves, still requires overcoming significant challenges. In this study, we propose and demonstrate EM-acoustic biphysical coding metasurfaces (BCM) which are capable of independently engineering propagation behaviours for both waves. In addition, continuous phase tunability across each domain can be attained. Eight kinds of unit cells with different geometrical parameters which consist of dual-layers are employed to serve as 2-bit reflection-type meta-atoms, thus offering opportunities for almost arbitrary wavefront manipulation of EM and acoustic waves as desired. The simulation results demonstrate that the proposed BCM can perform different functionalities for dual waves, such as multiple-beam generation, focusing, and vortex beam generation, which can be flexibly switched by tuning coding sequences of unit cells. The research results can help to enrich functions of metasurfaces, expand their application scope, and pave the way for the development of biphysical multifunctional devices and applications in the future.

In this work, we theoretically study temporal interfaces between media with strong spatial dispersion and dielectrics. In particular, we consider a temporal discontinuity that transforms a wire medium sample, a metamaterial with resonant spatial dispersion, into a uniaxial dielectric. We show that this transition results in a transformation of the deeply subwavelength spatial spectrum of TEM waves propagating in the wire medium at a certain frequency into a spectrum of plane waves at new frequencies, that are all higher than the initial one. The waves at different frequencies propagate in different directions. Their complex amplitudes and propagation directions are uniquely related to the amplitudes of the spatial harmonics of the field, which existed before the transition. We explain how to implement this transition. The revealed effect may result in a promising method of subwavelength imaging.

Selective excitation of vibrational modes using strong laser pulses has emerged as a powerful material engineering paradigm. However, to have deterministic control over material properties for device applications, an analogous scheme without drive, operating in thermal equilibrium, is desirable. We here propose such an equilibrium analog of the light-driven paradigm, leveraging the strong coupling between lattice degrees of freedom and the quantum fluctuations of the electric field of a THz microcavity. We demonstrate this approach by showing, using ab initio data, how electric field fluctuations can induce a subdominant ferromagnetic order, on top of the dominant zigzag antiferromagnet order, in ${\mathrm{FePS}}_3$ close to its Néel temperature.

Preparation of low-energy quantum many-body states has a wide range of applications in quantum information processing and condensed-matter physics. Quantum cooling algorithms offer a promising alternative to other methods based, for instance, on variational and adiabatic principles, or on dissipative state preparation. In this work, we investigate a set of cooling algorithms in a simple, solvable fermionic model that allows us to identify the mechanisms that underlie the cooling process and, also, those that prevent it. We derive analytical expressions for the cooling dynamics, steady states, and cooling rates in the weak-coupling limit. We find that multifrequency and randomized cycle strategies can significantly enhance the performance of the quantum algorithm and circumvent some of the obstacles. We also analyze the effects of noise and evaluate the conditions under which cooling remains feasible. Furthermore, we present optimized cooling protocols that can significantly enhance cooling performance in the presence of noise. Additionally, we compare cooling and dissipative state preparation and show that, in the model analyzed here, cooling generally achieves lower energies and is more resilient to noise.

Experiments on ADITYA-U tokamak show a marked enhancement in the sawtooth period by application of short gas puffs of fuel that cause a modification of the radial density profile. A consequent suppression of the trapped electron modes then leads to an increase in the core electron temperature. This slows down the heat propagation following a sawtooth crash causing a delay in achieving the critical temperature gradient inside the $q=1$ surface required for the next sawtooth crash to happen. The overall scenario has strong similarities with the behavior of sawtooth under electron cyclotron resonance heating (ECRH). Our findings suggest an alternate technique for sawtooth control that may be usefully employed in small- and/or medium-sized tokamaks that do not have an ECRH or any other auxiliary heating facility.

The modern Hopfield network, proposed by Krotov and Hopfield, is a mathematical generalization of the Hopfield network, which is a basic model of associative memory that employs higher-order interactions. This study introduces an open quantum model for discrete modern Hopfield networks that generalizes the open quantum Hopfield network. Our model integrates dissipative quantum spin systems, governed by quantum master equations, with classical hopping terms and additional quantum effects through a transverse field. We analytically examined the behavior of the stable fixed points and numerically determined the phase diagram. The results demonstrated qualitatively distinct behaviors from the open quantum Hopfield network, showing that the ferromagnetic and limit cycle phases have additional stable fixed points.

The preparation of Fock states, as a fundamental problem in the research of quantum optics and quantum information, still faces challenges in achieving both unconditional preparation and large photon numbers. Here, we study the deterministic generation of large Fock states through the nonlinear interaction in coupled optical-optomechanical cavities. Under the condition that the mechanical frequency is twice the optical coupling strength, and much larger than the optomechanical coupling strength, there occurs a nonlinear interaction among two optical eigenmodes and the mechanical mode, i.e., a photon is annihilated, accompanied by the creation of another photon together with a phonon, and vice versa. It is analytically proved that, through this interaction, Fock states $|n\rangle$ and $|1\rangle$ will generate $|n+1\rangle$ at specific evolution times if the mechanical mode is initially on the ground state. In addition, numerical results show that Fock states $|n\rangle$ and $|p\rangle$ can also generate $|n+p\rangle$ with the fidelities larger than 94% ($1<p\le n\le 40$). The proposed mechanism achieves deterministic generation of large Fock states, which can be applied to quantum metrology and quantum simulation.

Cooling the internal and external degrees of freedom (DOFs) of a massive mechanical system close to the quantum ground state is a prerequisite for the quantum manipulation of macroscopic mechanical devices. In this paper, we propose a approach to simultaneously cool both the internal and external DOFs (phonon and center-of-mass modes) of a levitated micromagnet within a cavity magnomechanical (CMM) system. By driving the cavity mode and magnon mode on their respective red sidebands with microwave fields, the proposed scheme effectively suppresses Stokes scattering and significantly enhances the net cooling rate. Moreover, differing from conventional CMM systems, our proposal can achieve simultaneous cooling of the internal and external DOFs even in the unresolved-sideband regime. Our numerical simulations demonstrate the robustness and effectiveness of the scheme by analyzing average occupancies of the internal and external DOFs, as well as identifying the optimal cooling conditions. The proposed scheme establishes a foundation for probing and manipulating the internal DOFs by controlling the external DOFs, which may open additional possibilities for studying nonequilibrium quantum mesoscopic physics.

A fundamental idea in neuroscience is that cognitive functions—such as perception, learning, memory, and locomotion—are shaped and constrained by the brain's structural organization. Despite significant progress in mapping and analyzing structural connectomes, the principles linking the brain's physical architecture to its functional capabilities remain elusive. Here, we introduce an algebraic quantum model to bridge this theoretical gap, offering insights into the relationship between the connectome and emergent brain functions while connecting structural data to functional predictions. Using the well-mapped C. elegans anatomical and extrasynaptic connectomes, we demonstrate that brain functions, defined as functional networks of a neural system, emerge as thermal equilibrium states of an algebraic quantum system derived from the graph algebra of the underlying directed multigraph. Specifically, these equilibrium states, characterized by the Kubo-Martin-Schwinger formalism, reveal how individual neurons contribute to functional network formation. Our model illuminates the structure-function relationship in neural circuits through two key features: (1) a functional connectome that delineates topologically driven neuronal interactions and (2) an integration capacity index that quantifies how effectively neurons coordinate and modulate diverse information flows. Together these features provide a statistical and mechanistic account of information flow and reveal how the network topology of the connectome predicts cognition and complex behaviors.