We propose an experimentally feasible method to generate a one-dimensional optical lattice potential in an ultracold Bose gas system that depends on the transverse momentum of the atoms. The optical lattice is induced by the artificial gauge potential generated by a periodically driven multilaser Raman process. We study the many-body Bose-Hubbard model in an effective 1D case and show that the superfluid–Mott-insulator transition can be controlled via tuning the transverse momentum of the atomic gas. Such a feature enables us to control the phase of the quantum gas in the longitudinal direction by changing its transverse motional state. We examine our prediction via a strong-coupling expansion to an effective 1D Bose-Hubbard model and a quantum Monte Carlo calculation and discuss possible applications.

The interplay of biological, social, structural, and random factors makes disease forecasting extraordinarily complex. The course of an epidemic exhibits average growth dynamics determined by features of the pathogen and the population, yet also features significant variability reflecting the stochastic nature of disease spread. In this paper, we reframe a stochastic branching process analysis in terms of probability generating functions and compare it to continuous time epidemic simulations on networks. In doing so, we predict the diversity of emerging epidemic courses on both homogeneous and heterogeneous networks. We show how the challenge of inferring the early course of an epidemic falls on the randomness of disease spread more so than on the heterogeneity of contact patterns. We provide an analysis, which helps quantify, in real time, the probability that an epidemic goes supercritical or conversely, dies stochastically. These probabilities are often assumed to be one and zero, respectively, if the basic reproduction number or $R_0$ is greater than 1, ignoring the heterogeneity and randomness inherent to disease spread. This framework can give more insight into early epidemic spread by weighting standard deterministic models with likelihood to inform pandemic preparedness with probabilistic forecasts.

Ultracold quantum gases are highly controllable and thus capable of simulating difficult quantum many-body problems ranging from condensed matter physics to astrophysics. Although experimental realizations have so far been restricted to flat geometries, recently also curved quantum systems, with the prospect of exploring tunable geometries, have been produced in microgravity facilities as ground-based experiments are technically limited. Here, we analyze bubble-trapped condensates, in which the atoms are confined on the surface of a thin spherically symmetric shell by means of external magnetic fields. A thermally induced proliferation of vorticity yields a vanishing of superfluidity. We describe the occurrence of this topological transition by conceptually extending the theory of Berezinskii, Kosterlitz, and Thouless for infinite uniform systems to such finite-size systems. Unexpectedly, we find universal scaling relations for the mean critical temperature and the finite width of the superfluid transition. Furthermore, we elucidate how they could be experimentally observed in finite-temperature hydrodynamic excitations.

We study classical $J_1-J_2$ models with distinct spin degrees of freedom on a honeycomb lattice. For the XY and Heisenberg spins, the system develops a spiral spin liquid (SSL) that is a thermal cooperative paramagnetic regime with spins fluctuating around the spiral contours in the momentum space, and at low temperatures supports a vector spin-chirality order despite the absence of long-range magnetic order. In a strong contrast, for the Ising moments, the low-temperature spin correlation forms a reciprocal kagome structure in the momentum space that resembles the SSL behaviors and persists for a range of exchange couplings. The unexpected emergence and persistence of the reciprocal kagome structure are attributed to the stiffness of the Ising moments and the frustration. At higher temperatures when the thermal fluctuations are strong and the spin correlation is not fully melted, the reciprocal structures evolve from kagome structure towards the ones demanded by the soft-spin limit. This contrasts strongly with the behaviors of the spiral contours in the SSL regime for the continuous spins. We suggest various experimentally relevant systems including van der Waals magnets such as the transition-metal phosphorus trichalcogenides ${\mathrm{TMPX}}_3, {\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$, the rare-earth chalcohalides (such as HoOF, ErOF, and DyOF), and other isostructural systems to realize the SSL-like behaviors and/or the reciprocal kagome structure.

Run-and-tumble chemotaxis is a representative search strategy for odor sources by sensing its spatial gradients. The optimal ways of sensing and control in run-and-tumble chemotaxis have been theoretically analyzed to elucidate the efficiency of the strategies implemented in organisms. However, because of theoretical difficulties, most attempts have been limited to either linear or deterministic analysis, even though real biological chemotactic systems involve considerable stochasticity and nonlinearity in their sensory processes and controlled responses. In this study, by combining the theories of optimal filtering and Kullback-Leibler control of a partially observed Markov decision process (POMDP), we derive an optimal and fully nonlinear strategy for controlling run-and-tumble motion depending on the noisy sensing of a ligand gradient. The derived optimal strategy comprises optimal filtering dynamics to estimate the run direction from the noisy sensory input and control function to regulate the motor output. Furthermore, we show that this optimal strategy can be naturally associated with a standard biochemical model and experimental data of the chemotaxis of Escherichia coli. Our results demonstrate that our theoretical framework can be used as a basis for analyzing the efficiency and optimality of run-and-tumble chemotaxis.

Fermi-edge singularity and Anderson's orthogonality catastrophe are paradigmatic examples of non-equilibrium many-body physics in conductors, appearing after a quench is created by the sudden change of a localized potential. We investigate if the signal carried by the quench can be used to transmit a long ranged interaction, reminiscent of the RKKY interaction, but with the inclusion of the full many-body propagation over space and time. We calculate the response of a conductor to two quenches induced by localized states at different times and locations. We show that building up and maintaining coherence between the localized states is possible only with finely tuned interaction between the localized states and the conductor. This puts bounds to the use of time controlled RKKY type interactions and may limit the speed at which some quantum gates could operate.

Enhancing the radiative interaction between quantum electronic transitions is of general interest. There are two important properties of radiative interaction: the range and the strength. There has been a trade-off between the range and the strength observed in the literature. Such apparent trade-off arises from the dispersion relation of photonic environments. A general recipe is developed to overcome such trade-off and to simultaneously enhance the range and the strength of radiative interactions.

Neural-network-based algorithms have garnered considerable attention for their ability to learn complex patterns from very-high-dimensional data sets towards classifying complex long-range patterns of entanglement and correlations in many-body quantum systems, and towards processing high-dimensional classical data sets. Small-scale quantum computers are already showing potential gains in learning tasks on large quantum and very large classical data sets. A particularly interesting class of algorithms, the quantum convolutional neural networks (QCNNs) could learn features of a quantum data set by performing a binary classification task on a nontrivial phase of quantum matter. Inspired by this promise, we present a generalization of QCNN, the “branching quantum convolutional neural network,” or bQCNN, with substantially higher expressibility. A key feature of bQCNN is that it leverages midcircuit (intermediate) measurement results, realizable on several current quantum devices, obtained in pooling layers to determine which sets of parameters will be used in the subsequent convolutional layers of the circuit. This results in a “branching” structure, which allows for a greater number of trainable variational parameters in a given circuit depth. This is of particular use in current-day noisy intermediate-scale quantum devices, where circuit depth is limited by gate noise. We present an overview of the Ansatz structure and scaling and provide evidence of its enhanced expressibility compared with QCNN. Using artificially constructed large data sets of training states as a proof of concept, we demonstrate the existence of training tasks in which bQCNN far outperforms an ordinary QCNN. We provide an explicit example of such a task in the recognition of the transition from a symmetry protected topological to a trivial phase induced by multiple, distinct perturbations. Finally, we present future directions where the classical branching structure and increased density of trainable parameters in bQCNN would be particularly valuable.

A close connection between antiferromagnetism and superconductivity is at the core of high-temperature superconductivity. Here, we put forward the projected BCS theory for the unification of antiferromagnetism and strongly correlated superconductivity. Specifically, it is shown that, with the $d$-wave pairing symmetry, the projected BCS theory can provide excellent trial states at general doping for the exact ground states of the $t\text{−}J$ model in the square lattice, generating a unified theory of high-temperature superconductivity as a continuous function of hole concentration. A key to the success of the projected BCS theory is an accurate treatment of the strong correlation between Cooper pairs, which not only causes the breakdown of superconductivity itself at half filling but also defines the precise nature of strongly correlated superconductivity at moderate doping. Also, via the proper incorporation of particle number fluctuations, the projected BCS theory allows direct computation of the superconducting order parameter, shedding important light on the pseudogap phenomenon.

The analogs of optical elements in light-pulse atom interferometers are generated from the interaction of matter waves with light fields. As such, these fields possess quantum properties, which fundamentally lead to a reduced visibility in the observed interference. This loss is a consequence of the encoded information about the atom's path. However, the quantum nature of the atom-optical elements also gives an additional degree of freedom to reduce such effects: We demonstrate that entanglement between all light fields can be used to erase information about the atom's path and by that to partially recover the visibility. Thus, our work highlights the role of complementarity on atom-interferometric experiments.

The treatment protocols of cancerous ocular diseases with proton therapy are well established, and dedicated eye-treatment systems can produce the clinical beam properties that meet the peculiar features required by eye-treatment modalities. However, for general-purpose multiroom systems comprising eye-treatment beamlines and nozzles, the design and commissioning procedures must be optimized to achieve the performances of fully dedicated systems in terms of depth-dose distal falloff, lateral penumbra, and dose rate. This paper presents a realistic start-to-end beam transport and particle-matter interactions model of the ion beam applications Proteus® Plus $(\mathrm{P}+)$ single-scattering eye-treatment room with Beam Delivery SIMulation (bdsim) using Geant4. The model is used to establish optimization patterns in terms of beam optics to achieve a smaller depth-dose distal falloff than the design baseline while maintaining a nominal dose rate and lateral flatness of the dose deposition profile. An alternative design is proposed to increase the dose rate further by up to a factor 3, allowing for delivering a complete hypofractionated treatment session under 60 s. It uses a beam-stopping device to complement the existing scattering features of the nozzle. An in-depth study of the system is performed using bdsim and the numerical simulations are discussed in detail.

We investigate in the framework of quantum noise theory how the striking boundary sensitivity recently discovered in the context of non-Hermitian (NH) topological phases may be harnessed to devise novel quantum sensors. Specifically, we study a quantum-optical setting of coupled modes arranged in an array with broken ring geometry that would realize a NH topological phase in the classical limit. Using methods from quantum-information theory of Gaussian states, we show that a small coupling induced between the ends of the broken ring may be detected with a precision that increases exponentially in the number of coupled modes, e.g., by heterodyne detection of two output modes. While this robust effect only relies on reaching a NH topological regime, we identify a resonance phenomenon without direct classical counterpart that provides an experimental knob for drastically enhancing the aforementioned exponential growth. Our findings pave the way towards designing quantum NH topological sensors that may observe with high precision any physical observable that couples to the boundary conditions of the device.

The two-body scattering amplitude and energy spectrum of confined ultracold atoms are of fundamental importance for both theoretical and experimental studies of ultracold-atom physics. For many systems, one can efficiently calculate these quantities via the zero-range Huang-Yang pseudopotential (HYP), in which the interatomic interaction is characterized by the scattering length $a$. Furthermore, when the scattering length is dependent on the kinetic energy ${\varepsilon }_{\mathrm{r}}$ of two-atom relative motion, i.e., $a=a\left({\varepsilon }_{\mathrm{r}}\right)$, the results are applicable for a broad energy region. However, when the free Hamiltonian of atomic internal state (e.g., the Zeeman Hamiltonian) does not commute with the interatomic interaction, or the center-of-mass (c.m.) motion is coupled to the relative motion, the generalization of this technique is still lacking. In this work we solve this problem and construct a reasonable energy-dependent multichannel HYP, which is characterized by a “scattering length operator” ${\widehat{a}}_{\mathrm{eff}}$, for the above complicated cases. Here, ${\widehat{a}}_{\mathrm{eff}}$ is an operator for atomic internal states and c.m. motion, and depends on both the total two-atom energy and the external field as well as the trapping parameter. The effects from the internal-state or c.m.-relative motion coupling can be self-consistently taken into account by ${\widehat{a}}_{\mathrm{eff}}$. We further show a method based on the quantum defect theory, with which ${\widehat{a}}_{\mathrm{eff}}$ can be analytically derived for systems with van der Waals interatomic interaction. To demonstrate our method, we calculate the spectrum of two ultracold fermionic alkaline-earth-like atoms [in electronic ${}^{1}S_0$ $\left(\right|g\rangle )$ and ${}^{3}P_0$ $\left(\right|e\rangle )$ states, respectively] confined in an optical lattice. By comparing our results with the recent experimental measurements for two ${}^{173}\mathrm{Yb}$ atoms and two ${}^{171}\mathrm{Yb}$ atoms, we calibrate the scattering lengths $a_{\pm }$ with respect to antisymmetric and symmetric nuclear-spin states to be $a_{+}=2012\left(19\right)a_0$ and $a_{-}=193\left(4\right)a_0$ for ${}^{173}\mathrm{Yb}$, and $a_{+}=232\left(3\right)a_0$ and $a_{-}=372\left(1\right)a_0$ for ${}^{171}\mathrm{Yb}$.

We present a systematic inelastic neutron scattering and neutron diffraction study on the magnetic structure of the quasi-one-dimensional spin-$\frac{1}{2}$ magnet ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, where the interchain coupling in the Néel-type antiferromagnetic ground state breaks the static spin lattice into two independent domains. At zero magnetic field, we have observed two new spin excitations with small spectral weights inside the gapped region defined by the spinon bound states. In an external magnetic field along the chain axis, the Néel order gets partially destabilized at ${\mu }_0{H}^{★}=2.0\mathrm{T}$ and completely suppressed at ${\mu }_0{H}_{\mathrm{p}}=3.9\mathrm{T}$, above which a quantum disordered Tomonaga–Luttinger liquid (TLL) prevails. The low-energy spin excitations between ${\mu }_0{H}^{★}$ and ${\mu }_0{H}_{\mathrm{p}}$ are not homogeneous, containing the dispersionless (or weakly dispersive) spinon bound states excited in the Néel phase and the highly dispersive psinon-antipsinon mode characteristic of a TLL. We propose that the two new modes at zero field are spinon excitations inside the domain walls. Since they have a smaller gap than those excited in the Néel domains, the underlying spin chains enter the TLL state via a local quantum phase transition at ${\mu }_0{H}^{★}$, making the Néel/TLL coexistence a stable configuration until the excitation gap in the Néel domains closes at ${\mu }_0{H}_{\mathrm{p}}$.

Ordered atomic arrays with subwavelength spacing have emerged as an efficient and versatile light-matter interface, where collective interactions give rise to sets of super- and subradiant lattice states. Here, we demonstrate that highly subradiant states, so-called lattice dark states, can be individually addressed and manipulated by applying a spatial modulation of the atomic detuning. More specifically, we show that lattice dark states can be used to store and retrieve single photons with near-unit efficiency, as well as to control the temporal, frequency, and spatial degrees of freedom of the emitted electromagnetic field. Furthermore, we demonstrate how to engineer arbitrary coherent interactions between multiple dark states and thereby manipulate information stored in the lattice. These results pave the way towards quantum optics and information processing using atomic arrays.

We study the transport properties of generic out-of-equilibrium quantum systems connected to fermionic reservoirs. We develop a perturbation scheme in the inverse system size, named $1/N$ expansion, to study a large class of out of equilibrium diffusive/ohmic systems. The bare theory is described by a Gaussian action corresponding to a set of independent two level systems at equilibrium. This allows a simple and compact derivation of the diffusive current as a first-order pertubative term. In addition, we obtain exact solutions for a large class of quantum stochastic Hamiltonians (QSHs) with time and space dependent noise, using a self-consistent Born diagrammatic method in the Keldysh representation. We show that these QSHs exhibit diffusive regimes, which are encoded in the Keldysh component of the single particle Green's function. The exact solution for these QSHs models confirms the validity of our system size expansion ansatz, and its efficiency in capturing the transport properties. We consider in particular three fermionic models: (i) a model with local dephasing, (ii) the quantum simple symmetric exclusion process model, and (iii) a model with long-range stochastic hopping. For (i) and (ii) we compute the full temperature and dephasing dependence of the conductance of the system, both for two- and four-points measurements. Our solution gives access to the regime of finite temperature of the reservoirs, which could not be obtained by previous approaches. For (iii), we unveil a ballistic-to-diffusive transition governed by the range and the nature (quantum or classical) of the hopping. As a byproduct, our approach equally describes the mean behavior of quantum systems under continuous measurement.