We present evidence of non-excimer-based secondary scintillation in gaseous xenon, obtained using both the NEXT-White time projection chamber (TPC) and a dedicated setup. Detailed comparison with first-principle calculations allows us to assign this scintillation mechanism to neutral bremsstrahlung (NBrS), a process that is postulated to exist in xenon that has been largely overlooked. For photon emission below 1000 nm, the NBrS yield increases from about ${10}^{-2}\mathrm{photon}/e^{-}{\mathrm{cm}}^{-1}{\mathrm{bar}}^{-1}$ at pressure-reduced electric field values of $50\mathrm{V}{\mathrm{cm}}^{-1}{\mathrm{bar}}^{-1}$ to above $3\times {10}^{-1}\mathrm{photon}/e^{-}{\mathrm{cm}}^{-1}{\mathrm{bar}}^{-1}$ at $500\mathrm{V}{\mathrm{cm}}^{-1}{\mathrm{bar}}^{-1}$. Above $1.5\mathrm{kV}{\mathrm{cm}}^{-1}{\mathrm{bar}}^{-1}$, values that are typically employed for electroluminescence, it is estimated that NBrS is present with an intensity around $1\mathrm{photon}/e^{-}{\mathrm{cm}}^{-1}{\mathrm{bar}}^{-1}$, which is about 2 orders of magnitude lower than conventional, excimer-based electroluminescence. Despite being fainter than its excimeric counterpart, our calculations reveal that NBrS causes luminous backgrounds that can interfere, in either gas or liquid phase, with the ability to distinguish and/or to precisely measure low primary-scintillation signals (S1). In particular, we show this to be the case in the “buffer” region, where keeping the electric field below the electroluminescence threshold does not suffice to extinguish secondary scintillation. The electric field leakage in this region should be mitigated to avoid intolerable levels of NBrS emission. Furthermore, we show that this new source of light emission opens up a viable path toward obtaining S2 signals for discrimination purposes in future single-phase liquid TPCs for neutrino and dark matter physics, with estimated yields up to $20–50\mathrm{photons}/e^{-}{\mathrm{cm}}^{-1}$.

Stochastic processes play a key role for modeling a huge variety of transport problems out of equilibrium, with manifold applications throughout the natural and social sciences. To formulate models of stochastic dynamics, the conventional approach consists in superimposing random fluctuations on a suitable deterministic evolution. These fluctuations are sampled from probability distributions that are prescribed a priori, most commonly as Gaussian or Lévy. While these distributions are motivated by (generalized) central limit theorems, they are nevertheless unbounded, meaning that arbitrarily large fluctuations can be obtained with finite probability. This property implies the violation of fundamental physical principles such as special relativity and may yield divergencies for basic physical quantities like energy. Here, we solve the fundamental problem of unbounded random fluctuations by constructing a comprehensive theoretical framework of stochastic processes possessing physically realistic finite propagation velocity. Our approach is motivated by the theory of Lévy walks, which we embed into an extension of conventional Poisson-Kac processes. The resulting extended theory employs generalized transition rates to model subtle microscopic dynamics, which reproduces nontrivial spatiotemporal correlations on macroscopic scales. It thus enables the modeling of many different kinds of dynamical features, as we demonstrate by three physically and biologically motivated examples. The corresponding stochastic models capture the whole spectrum of diffusive dynamics from normal to anomalous diffusion, including the striking “Brownian yet non-Gaussian” diffusion, and more sophisticated phenomena such as senescence. Extended Poisson-Kac theory can, therefore, be used to model a wide range of finite-velocity dynamical phenomena that are observed experimentally.

Layered transition-metal dichalcogenides (TMDCs) host a plethora of interesting physical phenomena ranging from charge order to superconductivity. By introducing magnetic ions into $2H\text{−}T{A}_2$ ($T=\mathrm{Nb}$, Ta; $A=\mathrm{S}$, Se), the material forms a family of magnetic intercalated TMDCs $M_{x}T{A}_2$ ($M=3d$ transition metal). Recently, ${\mathrm{Fe}}_{1/3+\delta }{\mathrm{NbS}}_2$ has been found to possess intriguing resistance switching and magnetic memory effects coupled to the Néel temperature of $T_N\sim 45\mathrm{K}$ [Maniv et al., Nat. Phys. 17, 525 (2021); Sci. Adv. 7, eabd8452 (2021)]. We present comprehensive single crystal neutron diffraction measurements on underintercalated ($\delta \sim -0.01$), stoichiometric, and overintercalated ($\delta \sim 0.01$) samples. Magnetic defects are usually considered to suppress magnetic correlations and, concomitantly, transition temperatures. Instead, we observe highly tunable magnetic long-ranged states as the Fe concentration is varied from underintercalated to overintercalated, that is, from Fe vacancies to Fe interstitials. The under- and overintercalated samples reveal distinct antiferromagnetic stripe and zigzag orders, associated with wave vectors $k_1=(0.5,0,0)$ and $k_2=(0.25,0.5,0)$, respectively. The stoichiometric sample shows two successive magnetic phase transitions for these two wave vectors with an unusual rise-and-fall feature in the intensities connected to $k_1$. We ascribe this sensitive tunability to the competing next-nearest neighbor exchange interactions and the oscillatory nature of the Ruderman-Kittel-Kasuya-Yosida mechanism. We discuss experimental observations that relate to the observed intriguing switching resistance behaviors. Our discovery of a magnetic defect tuning of the magnetic structure in bulk crystals ${\mathrm{Fe}}_{1/3+\delta }{\mathrm{NbS}}_2$ provides a possible new avenue to implement controllable antiferromagnetic spintronic devices.

Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. Almost all these circuits operate in the regime where quantum phase fluctuations are small—the associated flux is smaller than the superconducting flux quantum—although entering the regime of large fluctuations would have profound implications for metrology and qubit protection. The difficulty arises from the apparent need for circuit impedances vastly exceeding the resistance quantum. Independently, exotic circuit elements that require Cooper pairs to form pairs in order to tunnel have been developed to encode and topologically protect quantum information. In this work, we demonstrate that pairing Cooper pairs magnifies the phase fluctuations of the circuit ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy only, implying a twofold increase in the vacuum phase fluctuations and showing that the ground state is delocalized over several Josephson wells.

We numerically study the zero-temperature relaxation dynamics of several glass-forming models to their inherent structures, following quenches from equilibrium configurations sampled across a wide range of initial temperatures. In a mean-field Mari-Kurchan model, we find that relaxation changes from a power law to an exponential decay below a well-defined temperature, consistent with recent findings in mean-field $p$-spin models. By contrast, for finite-dimensional systems, the relaxation is always algebraic, with a nontrivial universal exponent at high temperatures crossing over to a harmonic value at low temperatures. We demonstrate that this apparent evolution is controlled by a temperature-dependent population of localized glassy excitations. Our work unifies several recent lines of studies aiming at a detailed characterization of the complex potential energy landscape of glass formers, and challenges both mean-field and real space descriptions of glasses.

A single-hole ground state Ansatz for the two-dimensional $t\text{−}J$ model has been recently studied by the variational Monte Carlo (VMC) method. Such a doped hole behaves like a “twisted” non-Landau quasiparticle characterized by an emergent quantum number in agreement with exact numerics. In this work, we further investigate the ground state of two holes by VMC. It is found that the two holes strongly attract each other to form a pairing state with a new quantum number the same as obtained by the numerical exact diagonalization and density matrix renormalization group (DMRG) calculations. A unique feature of this pairing state is a dichotomy in the pairing symmetry, i.e., a d wave in terms of the electron $c$ operators and an s wave in terms of the new quasiparticles, as explicitly illustrated in the ground state wave function. A similar VMC study of a two-hole wave function for the $t\text{−}J$ two-leg ladder also yields a good agreement with the DMRG result. We demonstrate that the pairing mechanism responsible for the strong binding here is not due to the long-range antiferromagnetic order nor the resonating-valence-bound pairing in the spin background but is the consequence of the quantum phase strings created by the hopping of holes. The resulting spin-current pattern mediating the pairing force is explicitly illustrated in the VMC calculation. Physical implications to superconductivity at finite doping are also discussed.

The rich phenomenology of twisted bilayer graphene (TBG) near the magic angle is believed to arise from electron correlations in topological flat bands. An unbiased approach to this problem is highly desirable, but also particularly challenging, given the multiple electron flavors, the topological obstruction to defining tight-binding models, and the long-ranged Coulomb interactions. While numerical simulations of realistic models have thus far been confined to zero temperature, typically excluding some spin or valley species, analytic progress has relied on fixed point models away from the realistic limit. Here, we present unbiased Monte Carlo simulations of realistic models of magic-angle TBG at charge neutrality. We establish the absence of a sign problem for this model in a momentum-space approach and describe a computationally tractable formulation that applies even on breaking chiral symmetry and including band dispersion. Our results include (i) the emergence of an insulating Kramers intervalley coherent ground state in competition with a correlated semimetal phase, (ii) detailed temperature evolution of order parameters and electronic spectral functions that reveal a “pseudogap” regime, in which gap features are established at a higher temperature than the onset of order, and (iii) predictions for electronic tunneling spectra and their evolution with temperature. Our results pave the way towards uncovering the physics of magic-angle graphene through exact simulations of over a hundred electrons across a wide temperature range.

We study the collective dynamics of two distant magnets coherently coupled by acoustic phonons that are transmitted through a nonmagnetic spacer. By tuning the ferromagnetic resonances of the two magnets to an acoustic resonance of the intermediate, we control a coherent three-level system. We show that the parity of the phonon mode governs the indirect coupling between the magnets: the resonances with odd (even) phonon modes correspond to out-of-phase (in-phase) lattice displacements at the interfaces, leading to bright (dark) states in response to uniform microwave magnetic fields, respectively. The experimental sample is a trilayer garnet consisting of two thin magnetic films epitaxially grown on both sides of a half-millimeter-thick nonmagnetic single crystal. In spite of the relatively weak magnetoelastic interaction, the long lifetimes of the magnon and phonon modes are the key to unveil strong coupling over a macroscopic distance, establishing the value of garnets as a platform to study multipartite hybridization processes at microwave frequencies.

Model bias is an inherent limitation of the current dominant approach to optimal quantum control, which relies on a system simulation for optimization of control policies. To overcome this limitation, we propose a circuit-based approach for training a reinforcement learning agent on quantum control tasks in a model-free way. Given a continuously parametrized control circuit, the agent learns its parameters through trial-and-error interaction with the quantum system, using measurement outcomes as the only source of information about the quantum state. Focusing on control of a harmonic oscillator coupled to an ancilla qubit, we show how to reward the learning agent with measurements of experimentally available observables. We train the agent to prepare various nonclassical states via both unitary control and control with adaptive measurement-based quantum feedback, and to execute logical gates on encoded qubits. The agent does not rely on averaging for state tomography or fidelity estimation, and significantly outperforms widely used model-free methods in terms of sample efficiency. Our numerical work is of immediate relevance to superconducting circuits and trapped ions platforms where such training can be implemented in experiment, allowing complete elimination of model bias and the adaptation of quantum control policies to the specific system in which they are deployed.

Recent rapid progress in neutron-star (NS) observations offers great potential to constrain the properties of strongly interacting matter under the most extreme conditions. In order to fully exploit the current observational inputs and to study the impact of future observations of NS masses, radii, and tidal deformabilities, we analyze a large ensemble of randomly generated viable NS-matter equations of state (EOSs) and the corresponding rotating stellar structures. We discuss the compatibility and impact of various hypotheses and measurements on the EOS, including those involving the merger product of the gravitational-wave (GW) event GW170817, the binary-merger components in GW190814, and radius measurements of the pulsar PSR $\mathrm{J}0740+6620$. We obtain an upper limit for the dimensionless spin of a rigidly rotating NS $|\chi |<0.81$, an upper limit for the compactness of a NS $GM/(R{c}^2)<0.33$, and find that the conservative hypothesis that the remnant in GW170817 ultimately collapsed to a black hole strongly constrains the EOS and the maximal mass of NSs, implying $M_{\mathrm{TOV}}<2.53M_{\odot }$ (or $M_{\mathrm{TOV}}<2.19M_{\odot }$ if we assume that a hypermassive NS was created). Additionally, we derive a novel lower limit for the tidal deformability as a function of the NS mass and provide fitting formulas that can be used to set priors for parameter estimation and to discern whether neutron stars or other compact objects are involved in future low-mass GW events. Finally, we find that the recent NICER results for the radius of the massive NS PSR $\mathrm{J}0740+6620$ place strong constraints for the behavior of the EOS and that the indicated radius values $R(2M_{\odot })\gtrsim 11\mathrm{km}$ are compatible with moderate speeds of sound in NS matter and thus with the existence of quark-matter cores in massive NSs.

The light-matter interaction in bulk semiconductors is in the strong-coupling regime with hybrid eigenstates, the so-called exciton polaritons and phonon polaritons. In two-dimensional (2D) systems, the translational invariance is broken in the direction perpendicular to the plane of the 2D system. The light-matter interaction switches to the weak-coupling regime with a finite radiative lifetime of the matter excitations in 2D. Radiative phenomena have been extensively studied for 2D excitons in quantum wells and 2D crystals, but their counterpart has never been addressed for optical phonons in 2D. Here we present a parallel study of the exciton and phonon radiative linewidths in atomically thin layers of hexagonal boron nitride ($h$-BN), epitaxially grown on graphite. Reflectivity experiments are performed either in the deep ultraviolet for the excitonic resonance or in the midinfrared for the phononic one. A quantitative interpretation is implemented in the framework of a transfer matrix approach generalized to the case of monolayers with the inclusion of Breit-Wigner resonances of either excitonic or phononic nature. For the exciton we find a giant radiative broadening in comparison to other 2D crystals, with a value of $\sim 25\mathrm{meV}$ related to the strong excitonic effects in $h$-BN. For the phonon we provide the first estimation of the radiative linewidth of a 2D phonon, with a value of $\sim 0.2\mathrm{meV}$ in monolayer $h$-BN. Our results are found to be in good agreement with first-principles calculations. Our study unravels the existence of radiative states for optical phonons in 2D, with numerous perspectives for fundamental physics, optoelectronic applications in the midinfrared spectral range, and advanced thermal management, and $h$-BN is emerging as a model system in this novel topic.

Nuclear spins interact weakly with their environment and therefore exhibit long coherence times. This has led to their use as memory qubits in quantum information platforms, where they are controlled via electromagnetic waves. Scaling up such platforms comes with challenges in terms of power efficiency, as well as cross-talk between devices. Here, we demonstrate coherent control of a single nuclear spin using surface acoustic waves. We use mechanically driven Ramsey and spin-echo sequences to show that the nuclear spin retains its excellent coherence properties. We estimate that this approach requires 2–3 orders of magnitude less power than more conventional control methods. Furthermore, this technique is scalable because of the possibility of guiding acoustic waves and reduced cross-talk between different acoustic channels. This work demonstrates the use of mechanical waves for complex quantum control sequences, offers an advantageous alternative to the standard electromagnetic control of nuclear spins, and opens prospects for incorporating nuclear spins in mechanically interfaced hybrid quantum architectures.

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O $K$-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ and a prototypical cuprate ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ ($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer–Mott-Hubbard character with the Coulomb repulsion $U$ of similar size to the charge-transfer energy $\mathrm{\Delta }$. Nevertheless, the transition-metal-oxygen hopping is larger in ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ than in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ despite its larger $\mathrm{\Delta }$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

We investigate the collective decay dynamics of atoms with a generic multilevel structure (angular momenta $F\leftrightarrow F^{\prime }$) coupled to two light modes of different polarization inside a cavity. In contrast to two-level atoms, we find that multilevel atoms can harbor eigenstates that are perfectly dark to cavity decay even within the subspace of permutationally symmetric states (collective Dicke manifold). The dark states arise from destructive interference between different internal transitions and are shown to be entangled. Remarkably, the superradiant decay of multilevel atoms can end up stuck in one of these dark states, where a macroscopic fraction of the atoms remains excited. This opens the door to the preparation of entangled dark states of matter through collective dissipation that are useful for quantum sensing and quantum simulation. Our predictions should be readily observable in current optical cavity experiments with alkaline-earth atoms or Raman-dressed transitions.

Cladding layers of waveguides prevent interchannel interference yet are unfavorable for the integration of photonic circuits. Here, we report the realization of ultracompact waveguide arrays, bends, and circuits with essentially zero interchannel separation. This supercompactness is achieved via arrays of waveguides with shifted spatial dispersions, where each waveguide functions as both the transmission channel and an effective “cladding layer” of its neighboring waveguides, and has been experimentally realized in low-loss all-dielectric photonic crystals. We show that the zero-spacing transmission array possesses the remarkable features of negligible crosstalk, high-efficiency sharp bending, and ultracompact photonic routing where the light can traverse the entire physical space. This finding opens a new avenue for extreme space utilization efficiency in waveguide physics and integrated photonics.

Self-interacting random walks are endowed with long-range memory effects that emerge from the interaction of the random walker at time $t$ with the territory that it has visited at earlier times $t^{\prime }<t$. This class of non-Markovian random walks has applications in a broad range of examples, from insects to living cells, where a random walker locally modifies its environment—leaving behind footprints along its path and, in turn, responding to its own footprints. Because of their inherent non-Markovian nature, the exploration properties of self-interacting random walks have remained elusive. Here, we show that long-range memory effects can have deep consequences on the dynamics of generic self-interacting random walks; they can induce aging and nontrivial persistence and transience exponents, which we determine quantitatively, in both infinite and confined geometries. Based on this analysis, we quantify the search kinetics of self-interacting random walkers and show that the distribution of the first-passage time to a target site in a confined domain takes universal scaling forms in the large-domain size limit, which we characterize quantitatively. We argue that memory abilities induced by attractive self-interactions provide a decisive advantage for local space exploration, while repulsive self-interactions can significantly accelerate the global exploration of large domains.

Protein concentration in a living cell fluctuates over time due to noise in growth and division processes. In the high expression regime, variance of the protein concentration in a cell is found to scale with the square of the mean, which belongs to a general phenomenon called Taylor’s law (TL). To understand the origin for these fluctuations, we measure protein concentration dynamics in single Escherichia coli cells from a set of strains with a variable expression of fluorescent proteins. The protein expression is controlled by a set of constitutive promoters with different strength, which allows one to change the expression level over 2 orders of magnitude without introducing noise from fluctuations in transcription regulators. Our data confirm the square TL, but the prefactor has a cell-to-cell variation independent of the promoter strength. Furthermore, distributions of the normalized protein concentration for different promoters collapse onto the same curve. To explain these observations, we use a minimal mechanistic model to describe the stochastic growth and division processes in a single cell with a feedback mechanism for regulating cell division. In the high expression regime where extrinsic noise dominates, the model reproduces our experimental results quantitatively. By using the mean-field approximation in the minimal model, we show that the stochastic dynamics of protein concentration is described by a Langevin equation with multiplicative noise. The Langevin equation has a scale invariance which is responsible for the square TL. By solving the Langevin equation, we obtain an analytical solution for the protein concentration distribution function that agrees with experiments. The solution shows explicitly how the prefactor depends on strength of different noise sources, which explains its cell-to-cell variability. By using this approach to analyze our single-cell data, we find that the noise in production rate dominates the noise from cell division. The deviation from the square TL in the low expression regime can also be captured in our model by including intrinsic noise in the production rate.

We study the phenomenon of Hilbert space fragmentation in isolated Hamiltonian and Floquet quantum systems using the language of commutant algebras, the algebra of all operators that commute with each local term that appears in the Hamiltonian or each local gate of the circuit. We provide a precise definition of Hilbert space fragmentation in this formalism as the case where the dimension of the commutant algebra grows exponentially with the system size. Fragmentation can, hence, be distinguished from systems with conventional symmetries such as U(1) or SU(2), where the dimension of the commutant algebra grows polynomially with the system size. Furthermore, the commutant algebra language also helps distinguish between “classical” and “quantum” Hilbert space fragmentation, where the former refers to fragmentation in the product state basis. We explicitly construct the commutant algebra in several systems exhibiting classical fragmentation, including the $t-J_z$ model and the spin-1 dipole-conserving model, and we illustrate the connection to previously studied “statistically localized integrals of motion.” We also revisit the Temperley-Lieb spin chains, including the spin-1 biquadratic chain widely studied in the literature, and show that they exhibit quantum Hilbert space fragmentation. Finally, we study the contribution of the full commutant algebra to the Mazur bounds in various cases. In fragmented systems, we use expressions for the commutant to analytically obtain new or improved Mazur bounds for autocorrelation functions of local operators that agree with previous numerical results. In addition, we are able to rigorously show the localization of the on-site spin operator in the spin-1 dipole-conserving model.

A thorough understanding of molecular scattering in the ultralow temperature regime is crucial for realizing long coherence times and enabling tunable interactions in molecular gases, systems which offer many opportunities in quantum simulation, quantum information, and precision measurement. Of particular importance is the nature of the long-lived intermediate complexes which may be formed in ultracold molecular collisions, as such complexes can dramatically affect the stability of molecular gases, even when exothermic reaction channels are not present. Here, we investigate collisional loss in an ultracold mixture of ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ molecules and ${}^{87}\mathrm{Rb}$ atoms, where chemical reactions between the two species are energetically forbidden. Through direct detection of the ${{\mathrm{KRb}}_2}^{*}$ intermediate complexes formed from atom-molecule collisions, we show that a 1064 nm laser source used for optical trapping of the sample can efficiently deplete the complex population via photoexcitation, an effect which can explain the strong two-body loss observed in the mixture. By monitoring the time evolution of the ${{\mathrm{KRb}}_2}^{*}$ population after a sudden reduction in the 1064 nm laser intensity, we measure the lifetime of the complex [0.39(6) ms], as well as the photoexcitation rate for 1064 nm light [$0.50(3)\mu {\mathrm{s}}^{-1}(\mathrm{kW}/{\mathrm{cm}}^2{)}^{-1}$]. The observed lifetime, which is $\sim {10}^5$ times longer than recent estimates based on the Rice-Ramsperger-Kassel-Marcus statistical theory, calls for new theoretical insight to explain its origin.

Understanding and protecting the coherence of individual quantum systems is a central challenge in quantum science and technology. Over the past decades, a rich variety of methods to extend coherence have been developed. A complementary approach is to look for naturally occurring systems that are inherently protected against decoherence. Here, we show that pairs of identical nuclear spins in solids form intrinsically long-lived qubits. We study three carbon-13 pairs in diamond and realize high-fidelity measurements of their quantum states using a single nitrogen-vacancy center in their vicinity. We then reveal that the spin pairs are robust to external perturbations due to a combination of three phenomena: a decoherence-free subspace, a clock transition, and a variant on motional narrowing. The resulting inhomogeneous dephasing time is $T_2^{*}=1.9(3)\min$, the longest reported for individually controlled qubits. Finally, we develop complete control and realize an entangled state between two spin pairs through projective parity measurements. These long-lived qubits are abundantly present in diamond and other solids and provide new opportunities for ancilla-enhanced quantum sensing and for robust memory qubits for quantum networks.