When Carnot and Stirling initially conceptualized heat engines, temperature unambiguously represented our everyday perception of cold and hot. However, as this energy scale is expanded to measure the strength of noise in general nonequilibrium heat baths, such as those consisting of bacteria or active particles, it takes on different definitions and connotations. This raises a fundamental question of whether and how thermodynamic conclusions beyond a unique definition of temperature would deviate from our conventional understanding. To address this inquiry, we investigate a colloidal Stirling engine governed by a large number of stochastic dynamical systems. Within experimentally accessible parameter values, we discover certain exceptional active engines that can outperform their passive counterpart, as notably claimed in a recent experiment involving a bacterial bath. Our analysis shows that such heightened performance can be attributed to either a restoring effect in noise or a significant dissipation kernel. The revealed influence of active baths on Stirling efficiency provides further insights into their impact on maximum power output, Carnot efficiency, and Curzon-Alhborn efficiency. The finding elucidates the origins of exceptional performance in stochastic heat engines, offers strategies for harvesting energies from active noises, and sheds light on the effects of nonequilibrium temperature in stochastic thermodynamics.

We present a deep neural network (DNN) -based model (HubbardNet) to variationally find the ground-state and excited-state wave functions of the one-dimensional and two-dimensional Bose-Hubbard model. Using this model for a square lattice with $M$ sites, we obtain the energy spectrum as an analytical function of the on-site Coulomb repulsion, $U$, and the total number of particles, $N$, from a single training. This approach bypasses the need to solve a new Hamiltonian for each different set of values $(U,N)$ and generalizes well even for out-of-distribution $(U,N)$. Using HubbardNet, we identify the two ground-state phases of the Bose-Hubbard model (Mott insulator and superfluid). We show that the DNN-parametrized solutions are in excellent agreement with results from the exact diagonalization of the Hamiltonian, and it outperforms exact diagonalization in terms of computational scaling. These advantages suggest that our model is promising for efficient and accurate computation of exact phase diagrams of many-body lattice Hamiltonians.

A frozen, solid helium core, dubbed a snowball, is typically observed around cations in liquid helium. Here we discover, using path integral simulations, that around a cationic molecular impurity, protonated methane, the ${}^4\mathrm{He}$ atoms are indeed strongly localized akin to snowballs but still participate in vivid bosonic exchange induced by the rovibrational motion of the impurity. Such a combination of solidlike order with pronounced superfluid response in the first helium shell indicates that manifestations of local supersolid behavior of ${}^4\mathrm{He}$ can be induced by charged molecules.

We introduce two infinite sequences of entanglement monotones, which are constructed from expectation values of polynomials in the modular Hamiltonian. These monotones yield infinite sequences of inequalities that must be satisfied in majorizing state transitions. We demonstrate this for information erasure, deriving an infinite sequence of “Landauer inequalities” for the work cost, bounded by linear combinations of expectation values of powers of the modular Hamiltonian. These inequalities give improved lower bounds for the work cost in finite-dimensional systems, and depend on more details of the erased state than just on its entropy and variance of modular Hamiltonian. Similarly one can derive lower bounds for marginal entropy production for a system coupled to an environment. These infinite sequences of entanglement monotones also give rise to relative quantifiers that are monotonic in more general processes, namely those involving so-called $\sigma$ majorization with respect to a fixed point full rank state $\sigma$; such quantifiers are called resource monotones. As an application to thermodynamics, one can use them to derive finite-dimension corrections to the Clausius inequality. Finally, in order to gain some intuition for what (if anything) plays the role of majorization in field theory, we compare pairs of states in discretized theories at criticality and study how majorization depends on the size of the bipartition with respect to the size of the entire chain.

By combining experiments and numerical simulations which model the dynamics of shaken atomic Bose-Einstein condensates, we reveal the surprising nature of quantum turbulence in these systems. Unlike the tangles of vortex lines described in the superfluid helium literature, we find that our turbulent atomic condensate contains a mixture of strong fragmented density fluctuations and small random vortex loops which are not homogeneously distributed. This unusual form of turbulence, with its own properties and scaling behavior, which we call strong quantum turbulence, is significantly different from the turbulence which is observed in either classical or other quantum systems, thus posing a new challenge in turbulence research.

We consider a configuration of three stacked graphene monolayers with commensurate twist angles ${\theta }_{12}/{\theta }_{23}=p/q$, where $p$ and $q$ are coprime integers with $0<p<\left|q\right|$ and $q$ can be positive or negative. We study this system using the continuum model in the chiral limit when interlayer coupling terms between ${\text{AA}}_{12}$ and ${\text{AA}}_{23}$ sites of the moiré patterns 12 and 23 are neglected. There are only three inequivalent displacements between the moiré patterns 12 and 23, at which the three monolayers' Dirac zero modes are protected. Remarkably, for these displacements and an arbitrary $p/q$ we discover exactly flat bands at an infinite set of twist angles (magic angles). We provide theoretical explanation and classification of all possible configurations and topologies of the flat bands.

Quantum phase transitions between different topologically ordered phases exhibit rich structures and are generically challenging to study in microscopic lattice models. In this paper, we propose a tensor-network solvable model that allows us to tune between different symmetry enriched topological (SET) phases. Concretely, we consider a decorated two-dimensional toric code model for which the ground state can be expressed as a two-dimensional tensor-network state with bond dimension $D=3$ and two tunable parameters. We find that the time-reversal (TR) symmetric system exhibits three distinct phases: (i) an SET toric code phase in which anyons transform nontrivially under TR, (ii) a toric code phase in which TR does not fractionalize, and (iii) a topologically trivial phase that is adiabatically connected to a product state. We characterize the different phases using the topological entanglement entropy and a membrane order parameter that distinguishes the two SET phases. Along the phase boundary between the SET toric code phase and the toric code phase, the model has an enhanced $U\left(1\right)$ symmetry and the ground state is a quantum critical loop gas wavefunction whose squared norm is equivalent to the partition function of the classical $O\left(2\right)$ model. By duality transformations, this tensor-network solvable model can also be used to describe transitions between SET double-semion phases and between ${\mathbb{Z}}_2\times {\mathbb{Z}}_2^T$ symmetry protected topological phases in two dimensions.

Ultrafast pump-probe experiments can reveal the physics of complex materials by triggering a reaction with short laser pulses and then looking for the response. However, the repetition rate in such experiments is limited because the sample needs to cool down back to the initial state between subsequent laser excitations. Here we investigate by theory and experiment how this cooling rate depends on the shape and dimensionality of the material. Using $\mathrm{V}{\mathrm{O}}_2$ as the example, a strongly correlated material with an ultrafast phase transition with large latent heat, we report more than six orders of magnitude of change between the cooling times of freestanding thin films, thin films on a substrate, and bulk materials. A numerical latent-heat model reproduces all results and can therefore be generally used to predict the speed of the back reaction in almost all kinds of phase-change materials.

Skyrmions are topological solitons in two-dimensional systems and have been observed in various physical systems. Generating and controlling skyrmions in artificial resonator arrays lead to novel acoustic, photonic, and electric devices, but it is a challenge to implement a vector variable with the chiral exchange interaction. Here, we propose to use quadrature variables, where their parametric coupling enables skyrmions to be stabilized. A finite-element simulation indicates that an acoustic skyrmion would exist in a realistic structure consisting of a piezoelectric membrane array.

Inspired by path integral molecular dynamics, we build a spin model, in terms of spin coherent states, from which we can compute the quantum expectation values of a spin in a constant magnetic field, at finite temperature. This formulation facilitates the description of a discrete quantum spin system in terms of a continuous classical model and recasts the quantum spin effects within the framework of path integrals in a double $1/s$ and $\hslash s$ expansion, where $s$ is the magnitude of the spin. In particular, it allows for a much more direct path to the low- and high-temperature limits of the quantum system and to the definition of effective classical Hamiltonians that describe both thermal and quantum fluctuations. In this formalism, the quantum properties of the spins emerge as an effective anisotropy. We use atomistic spin dynamics to sample the path integral, calculate thermodynamic observables, and show that our effective classical models can reproduce the thermal expectation values of the quantum system within temperature ranges relevant for studying magnetic ordering.

Using first-principles calculations we model the out-of-plane switching of local dipoles in ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ (CIPS) that are largely induced by Cu off-centering. Previously, a coherent switching of polarization via a quadruple-well potential was proposed for these materials. In the supercells we considered, we find multiple structures with similar energies but with different local polar order. Our results suggest that the individual dipoles are weakly coupled in-plane, and under an electric field at very low temperatures these dipoles in CIPS should undergo incoherent disordered switching. The barrier for switching is determined by the single Cu-ion switching barrier. This in turn suggests a scale-free polarization with a switching barrier of $\sim 203.6$–258.0 meV, a factor of five smaller than that of ${\mathrm{HfO}}_2$ (1380 meV), a prototypical scale-free ferroelectric. The mechanism of polarization switching in CIPS is mediated by the switching of each weakly interacting dipole rather than the macroscopic polarization itself as previously hypothesized. These findings reconcile prior observations of a quadruple well with sloping hysteresis loops, large ionic conductivity even at 250 K, well below the Curie temperature (315 K), and a significant wake-up effect where the macroscopic polarization is slow to order and sets in under an applied electric field. We also find that computed piezoelectric response and the polarization show a linear dependence on the local dipolar order. This is consistent with having scale-free polarization and other polarization-dependent properties and opens doors for engineering tunable metastability by design in CIPS (and related families of materials) for neuromorphic applications.

Green's functions of non-Hermitian systems play a fundamental role in various dynamical processes. Because non-Hermitian systems are sensitive to boundary conditions due to the non-Hermitian skin effect, open-boundary Green's functions are closely related to the non-Bloch band theory. While the exact formula of open-boundary Green's functions in single-band non-Hermitian systems proves to be an integral along the generalized Brillouin zone (GBZ), the proper generalization in generic multiband systems remains unclear. In this paper, we derive a formula of open-boundary Green's functions in multiband non-Hermitian systems by viewing the multiband GBZ on the Riemann surface. This formula can be applied to describe directional amplification in multiband systems, which can be verified at various experimental platforms.

Noncovalent interactions are a key ingredient to determine the structure, stability, and dynamics of materials, molecules, and biological complexes. However, accurately capturing these interactions is a complex quantum many-body problem, with no efficient solution available on classical computers. A widely used model to accurately and efficiently model noncovalent interactions is the Coulomb-coupled quantum Drude oscillator (cQDO) many-body Hamiltonian, for which no exact solution is known. We show that the cQDO model lends itself naturally to simulation on a photonic quantum computer, and we calculate the binding energy curve of diatomic systems by leveraging Xanadu's strawberry fields photonics library. Our study substantially extends the applicability of quantum computing to atomistic modeling by showing a proof-of-concept application to noncovalent interactions, beyond the standard electronic-structure problem of small molecules. Remarkably, we find that two coupled bosonic QDOs exhibit a stable bond. In addition, our study suggests efficient functional forms for cQDO wave functions that can be optimized on classical computers, and capture the bonded-to-noncovalent transition for increasing interatomic distances.

Quantum state transfer by propagating wave packets of electromagnetic radiation requires tunable couplings between the sending and receiving quantum systems and the propagation channel or waveguide. The highest fidelity of state transfer in experimental demonstrations so far has been in superconducting circuits. Here, the tunability always comes together with nonlinear interactions, arising from the same Josephson junctions that enable the tunability. The resulting nonlinear dynamics correlates the photon number and spatiotemporal degrees of freedom and leads to a multimode output state, for any multiphoton state. In this work, we study as a generic example the release of complex quantum states from a superconducting resonator, employing a flux tunable coupler to engineer and control the release process. We quantify the multimode character of the output state and discuss how to optimize the fidelity of a quantum state transfer process with this in mind.

Molecular predissociation, the spontaneous nonradiative bond-breaking process, can limit the ability to scatter a large number of photons required to reach the ultracold regime in laser cooling. Unlike rovibrational branching, predissociation is irreversible since the fragments fly apart with high kinetic energy. Of particular interest is the simple diatomic molecule CaH, for which the two lowest electronically excited states used in laser cooling, $A{}^2{\mathrm{\Pi }}_{1/2}$ and $B{}^2{\mathrm{\Sigma }}^{+}$, lie above the dissociation threshold of the ground potential. In this work, we present measurements and calculations that quantify the predissociation probabilities $P_{\mathrm{pd}}$ affecting the cooling cycle. For the lowest vibrational levels, we find $P_{\mathrm{pd}}$ of $\sim {10}^{-6}$ for $A(v^{\prime }=0)$ and $\sim {10}^{-3}$ for $B(v^{\prime }=0)$. The results allow us to design a laser-cooling scheme that will enable the creation of an ultracold and optically trapped cloud of CaH molecules. In addition, we use the results to propose a two-photon pathway to controlled dissociation of the molecules in order to gain access to their ultracold fragments, including hydrogen.

We investigate the phase diagram at the boundary of an infinite two-dimensional cluster state subject to bulk measurements using tensor network methods. The state is subjected to uniform measurements $M=cos\theta Z+sin\theta X$ on the lower boundary qubits and in all bulk qubits. Our results show that the boundary of the system exhibits volume-law entanglement at the measurement angle $\theta =\pi /2$ and area-law entanglement for any $\theta <\pi /2$. Within the area-law phase, a phase transition occurs at ${\theta }_c=1.371$. The phase with $\theta \in ({\theta }_c,\pi /2)$ is characterized by a noninjective matrix product state, which cannot be realized as the unique ground state of a one-dimensional local, gapped Hamiltonian. Instead, it resembles a cat state with spontaneous symmetry breaking. These findings demonstrate that the phase diagram of the boundary of a two-dimensional system can be more intricate than that of a standard one-dimensional system.

The thermal activation process by which a system passes from one local energy minimum to another is a recurring motif in physics, chemistry, and biology. For instance, biopolymer chains are typically modeled in terms of energy landscapes, with folded and unfolded conformations represented by two distinct wells separated by a barrier. The rate of transfer between wells depends primarily on the height of the barrier, but it also depends on the details of the shape of the landscape along the trajectory. We consider the case of bias due to an external force, analogous to the pulling force applied in optical tweezer experiments on biopolymers. Away from the Arrhenius-law limit and well out of equilibrium, somewhat idiosyncratic behavior might be expected. Instead, we identify universal behavior of the biased activated-barrier-crossing process and demonstrate that data collapse on a universal curve can be achieved for simulated data over a wide variety of energy landscapes having barriers of different height and shape and for loading rates spanning many orders of magnitude.

We introduce a class of stochastic engines in which the regime of units operating synchronously can boost the performance. Our approach encompasses a minimal setup composed of $N$ interacting units placed in contact with two thermal baths and subjected to a constant driving worksource. The interplay between unit synchronization and interaction leads to an efficiency at maximum power between the Carnot ${\eta }_c$ and the Curzon-Ahlborn bound ${\eta }_{CA}$. Moreover, these limits can be respectively saturated maximizing the efficiency, and by simultaneous optimization of power and efficiency. We show that the interplay between Ising-like interactions and a collective ordered regime is crucial to operate as a heat engine. The main system features are investigated by means of a linear analysis near equilibrium, and developing an effective discrete-state model that captures the effects of the synchronous phase. The robustness of our findings extends beyond the all-to-all interactions and paves the way for the building of promising nonequilibrium thermal machines based on ordered structures.