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Condensates of excitons have been observed in the quantum Hall regime, but evidence for their existence at low magnetic fields remains controversial. Now evidence of coherence between optically pumped interlayer excitons in MoS₂ marks a step towards confirming exciton condensation at low magnetic fields.

In strongly correlated systems, how magnetic excitations are renormalized by charge carriers remains an open question. An experiment now reports the observation of magnon-polarons—magnons dressed by doped holes—in a Fermi–Hubbard quantum simulator.

Topological phases are unusual states of matter whose properties are robust against small perturbations. Using a photonic quantum walk system, Kitagawaet al. simulate one-dimensional topological phases and reveal novel topological phenomena far from the static or adiabatic regimes.

The robustness of edge states against external influence is a phenomenon that has been successfully applied to electron transport. A study now predicts that the same concept can also lead to improved optical devices. Topological protection might, for example, reduce the deleterious influence of disorder on coupled-resonator optical waveguides.

A method for analysing STM data enables the recovery of information about quasiparticle scattering in the form of holographic maps. The approach is verified for superconducting cuprates, but may find applications in heavy-fermion materials research.

The transport measurements of an interacting fermionic quantum gas in an optical lattice provide a direct experimental realization of the Hubbard model—one of the central models for interacting electrons in solids—and give insights into the transport properties of many-body phases in condensed-matter physics.

In the band theory of solids, the topological properties of Bloch bands are characterized by geometric phases. For cold atoms moving in a one-dimensional optical potential the geometric phase can be measured directly using Bloch oscillations and Ramsey interferometry.

Quantum critical points in many-body systems are characterized by the appearance of long-range entanglement. These subtle quantum correlations are known to be extremely fragile with respect to thermal noise. But theoretical work now shows that, unexpectedly, another classical disturbance, the ubiquitous 1/f noise, does preserve the critical correlations.

Fast particles propagating through a classical medium give rise to shock waves. Calculations now uncover the surprising behaviour of particles in one-dimensional quantum fluids: a fast particle will never come to a full stop, and a supersonic particle will propagate through the medium undergoing long-lived oscillations.

Currently available quantum hardware is limited by noise, so practical implementations often involve a combination with classical approaches. Sels et al. identify a promising application for such a quantum–classic hybrid approach, namely inferring molecular structure from NMR spectra, by employing a range of machine learning tools in combination with a quantum simulator.

All-electrical excitation of the hyperbolic phonon polaritons in hexagonal boron nitride by drifting charge carriers in nearby graphene results in electroluminescence at mid-infrared frequencies.

Are the rules that determine relaxation to equilibrium the same in the classical and quantum worlds? Recent experiments supported the idea that they are — but an investigation with ultracold atoms now contradicts that.

Competition between different possible ground states of strongly correlated electron systems can lead to the emergence of mixed states called microemulsions. Now this phenomenon is reported at the melting transition of a Wigner crystal.

The microscopic pairing mechanism in high-temperature superconductors remains debated. Here, the authors offer a new perspective on this problem by proposing that the strong pairing in Fermi-Hubbard type models relevant to cuprates is driven by a Feshbach resonance, which enhances interactions between doped holes.

The presence of small thermal regions in a many-body localized system could lead to its delocalization. An experiment with cold atoms now monitors the delocalization induced by the coupling of a many-body localized region with a thermal bath.

The signature of a Wigner crystal—the analogue of a solid phase for electrons—is observed via the optical reflection spectrum in a monolayer transition metal dichalcogenide.

Optical signatures reveal correlated insulating Wigner crystals—electron solids—in a bilayer of a two-dimensional transition metal dichalcogenide, MoSe₂, with hexagonal boron nitride between the layers.

Employing nonlinear, time-resolved terahertz spectroscopy to study condensate dynamics on Ta₂NiSe₅—a narrow-bandgap semiconductor and putative excitonic insulator—the authors reveal enhanced terahertz reflectivity upon photoexcitation and condensation-like temperature dependence below the structural transition critical temperature.

Quantum gas microscopes provide high-resolution real-space snapshots of quantum many-body systems. Now machine-learning techniques are used in choosing theoretical descriptions according to the consistency of their predictions with these snapshots.

Cavity spectroscopy measurements elucidate the Fermi polaron nature of the optical excitations in monolayer transition metal dichalcogenides.

The complex interactions arising in mixtures of bosons and fermions make it hard to understand their coupled dynamics. The collective oscillations of fermions embedded in a boson condensate have now been characterized in an ultracold-atom experiment.

A triangular-lattice Hubbard system realized with ultracold atoms is used to directly image spin polarons, revealing ferromagnetic correlations around a charge dopant, a manifestation of the Nagaoka effect.

Photonic time crystal refers to a material whose dielectric properties oscillate in time. Here the authors theoretically show such behaviour in the excitonic insulator candidate Ta2NiSe5 under optical excitation and use it to explain the enhanced THz reflectivity recently observed in pump-probe experiments

The authors theoretically propose a simple microscopic mechanism for light-induced superconductivity based on a boson coupled to an electronic interband transition. The electron-electron attraction needed for the superconductivity can be resonantly amplified when the boson’s frequency is close to the energy difference between the two electronic bands. The model can be engineered using a 2D heterostructure.

Emergence of Nagaoka polarons and kinetic magnetism is observed in a Hubbard system realized with strongly interacting fermions trapped in a triangular optical lattice.

The scaling of entanglement entropy and mutual information is key for the understanding of correlated states of matter. An experiment now reports the measurement of von Neumann entropy and mutual information in a quantum field simulator.

The direct observation of hole pairing in a doped Hubbard model is demonstrated using ultracold atoms in a quantum gas microscope setting by engineering mixed-dimensional fermionic ladders.

Studies of unconventional pairing mechanisms in cold atoms require ultralow temperatures. Large-scale numerics show that certain bilayer models allow for deeply bound and highly mobile pairs of charges at more accessible temperatures.

Physical principles underlying machine learning analysis of quantum gas microscopy data are not well understood. Here the authors develop a neural network based approach to classify image data in terms of multi-site correlation functions and reveal the role of fourth-order correlations in the Fermi-Hubbard model.

An effective Hamiltonian exhibiting \({\Bbb Z}_2\) symmetry has been engineered by implementing a Floquet-based method on ultracold bosons in an optical lattice, providing a first step towards quantum simulation of \({\Bbb Z}_2\) lattice gauge theories with ultracold matter.

A proposal describes how to detect topologically ordered states of ultracold matter in an optical lattice, and shows how these exotic states, which strongly correlated quantum systems can exhibit, could be harnessed for practical applications, such as robust quantum computation.

Spin transport far from equilibrium is studied in a Heisenberg model with adjustable anisotropy realized with coupled ultracold ⁷Li atoms, and different dynamical regimes are found for positive and negative anisotropies.

The Kondo effect is a prototypical strongly correlated phenomenon, in which a strong, localized repulsion gives rise to a many body resonance that controls the low-energy physics of a metal. Here, the authors show that the same effect can be induced by purely dissipative means through localized two body losses, which provides a nontrivial–and experimentally relevant– application of nonlinear dissipation.

Nitrogen vacancy centre quantum sensors are quantitative, non-invasive and physically robust probes of condensed matter systems that offer nanoscale resolution across a wide range of temperatures. This Technical Review discusses the connections between NV measurements and important physical characteristics in condensed matter.

Magnetic polarons are imaged with single-site spin and density resolution in the low-doping regime of the atomic Fermi–Hubbard model, showing that mobile delocalized doublons are necessary for polaron formation.

A ‘Higgs’ mode, signifying the breaking of a continuous symmetry in a model with emergent Lorentz invariance, is found close to the transition to the Mott insulating phase in a two-dimensional neutral superfluid.

An antiferromagnet with a correlation length that encompasses the whole system is created with the aid of quantum gas microscopy of cold atoms in an optical lattice.

Discrete time-crystalline order is observed in a driven, disordered ensemble of about one million dipolar spin impurities in diamond at room temperature, and is shown to be very stable to perturbations.

The robust implementation of gauge fields coupled to dynamical matter in large-scale quantum simulators is limited by the ever-present gauge-breaking errors. The authors propose an experimentally suitable scheme combining two-body interactions with weak fields, demonstrating its robustness against gauge breaking errors and its flexibility in the study of various models with Z2 gauge symmetry.

Quantum simulation of fermion-boson systems is significant in material applications, while limited by the unbounded boson states. By merging variational non-Gaussian transformations and variational quantum Eigensolvers, the authors design a hybrid quantum-classical algorithm suitable for the simulation of strongly correlated electrons coupled to phonons.

Light-induced superconductivity is a recent phenomenon where many aspects of the underlying physics are still to be fully understood. Here, the authors analyse coupled Ginzburg-Landau and Maxwell equations to investigate the dynamics of the quantum states when a superconductor is irradiated by a laser pulse.