Room-temperature ionic liquids (RTILs) are molten salts which exhibit unique physical and chemical properties, commonly harnessed for lubrication and energy applications. The pure ionic nature of RTIL leads to strong electrostatic interactions among the liquid, furthermore exalted in the presence of interfaces and confinement. In this work, we use a tuning-fork-based dynamic surface force tribometer, which allows probing both the rheological and the tribological properties of RTIL films confined between a millimetric sphere and a surface, over a wide range of confinements. When the RTIL is confined between metallic surfaces, we see evidence of an abrupt change of its rheological properties below a threshold confinement. This is reminiscent of a recently reported confinement-induced capillary freezing, here observed with a wide contact area. In parallel, we probe the tribological response of the film under imposed nanometric shear deformation and unveil a yielding behavior of the interfacial solid phase below this threshold confinement. This is characterized by a transition from an elastic to a plastic regime, exhibiting striking similarities with the response of glassy materials. This transition to yielding of the RTIL in metallic confinement leads overall to a reduction in friction and offers a self-healing protection of the surfaces avoiding direct contact, with obvious applications in tribology.

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with millimeter-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range, where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, an investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically lined waveguides powered by temporally long, narrow-band, multicycle THz pulses that copropagate with the electrons. This geometry offers centimeter-scale single-stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression, and focusing in two sequential ${\mathrm{Al}}_2{\mathrm{O}}_3$-based dielectric capillary stages powered by the same multicycle THz pulse. Since the multicycle THz energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, recycling the THz pulses provides a key factor for reaching relativistic energies with existing sources and paves the way for applications in future ultrafast electron diffraction and free-electron lasers.

Diverse physical systems are characterized by their response to small perturbations. Near thermodynamic equilibrium, the fluctuation-dissipation theorem provides a powerful theoretical and experimental tool to determine the nature of response by observing spontaneous equilibrium fluctuations. In this spirit, we derive here a collection of equalities and inequalities valid arbitrarily far from equilibrium that constrain the response of nonequilibrium steady states in terms of the strength of nonequilibrium driving. Our work opens new avenues for characterizing nonequilibrium response. As illustrations, we show how our results rationalize the energetic requirements of common biochemical motifs.

Molecular iodine is photoexcited by a strong 800-nm laser driving several channels of multiphoton excitation. The motion following photoexcitation is probed using time-resolved x-ray scattering, which produces a scattering map $S(Q,\tau )$. Temporal Fourier-transform methods are employed to obtain a frequency-resolved x-ray-scattering signal $\tilde{S}(Q,\omega )$. Taken together, $S(Q,\tau )$ and $\tilde{S}(Q,\omega )$ separate different modes of motion so that mode-specific nuclear oscillatory positions, oscillation amplitudes, directions of motion, and times may be measured accurately. Molecular dissociations likewise have a distinct signature, which may be used to identify both velocities and dissociation time shifts and also can reveal laser-induced couplings among the molecular potentials.

Sufficiently large electric current applied to metallic nanostructures can bring them far out of equilibrium, resulting in non-Ohmic behaviors characterized by current-dependent resistance. We experimentally demonstrate a linear dependence of resistance on current in microscopic thin-film metallic wires at cryogenic temperatures, and show that our results are inconsistent with common non-Ohmic mechanisms such as Joule heating. As the temperature is increased, the linear dependence becomes smoothed out, resulting in the crossover to behaviors consistent with Joule heating. A plausible explanation for the observed behaviors is the strongly nonequilibrium distribution of phonons generated by the current. Analysis based on this interpretation suggests that the observed anomalous current-dependent resistance can provide information about phonon transport and electron-phonon interaction at nanoscale. The ability to control the properties of phonons generated by current can lead to new routes for the optimization of thermal properties of electronic nanodevices.

Measuring the magnetic response of matter relies acutely on the degree to which a magnetic field source’s amplitude, spatial, and temporal character can be tailored. Magnetic fields are inseparable from light-matter interaction, yet due to the dominance of electric-field-induced effects in many systems, laser pulses have heretofore provided comparatively limited insight into the high-frequency magnetic response of matter. Conductors or superconductors arranged in a solenoidal configuration embody the state-of-the-art apparatus for generating spatially isolated magnetic fields, but the reliance on electrical circuitry limits the field amplitude, pulse brevity, and absolute timing of the generated fields. We transfer the concept of solenoidal currents commonly leveraged in electromagnets to photo-ionized electrons driven by moderately intense vector laser beams, in a scheme that does not require the laser mode to carry orbital angular momentum. We predict that this all-optical approach will enable magnetic fields exceeding 8 Tesla to be turned on within 50 femtoseconds using moderate laser intensities, an unprecedented combination of parameters that will open the possibility for ultrafast metrological techniques to be combined with intense, spatially isolated, magnetic fields.

The discovery of massless Dirac electrons in graphene and topological Dirac-Weyl materials has prompted a broad search for bosonic analogues of such Dirac particles. Recent experiments have found evidence for Dirac magnons above an Ising-like ferromagnetic ground state in a two-dimensional (2D) kagome lattice magnet and in the van der Waals layered honeycomb crystal ${\mathrm{CrI}}_3$, and in a 3D Heisenberg magnet ${\mathrm{Cu}}_3{\mathrm{TeO}}_6$. Here, we report our inelastic neutron scattering investigation on a large single crystal of a stacked honeycomb lattice magnet ${\mathrm{CoTiO}}_3$, which is part of a broad family of ilmenite materials. The magnetically ordered ground state of ${\mathrm{CoTiO}}_3$ features ferromagnetic layers of ${\mathrm{Co}}^{2+}$, stacked antiferromagnetically along the $c$ axis. The magnon dispersion relation is described very well with a simple magnetic Hamiltonian with strong easy-plane exchange anisotropy. Importantly, a magnon Dirac cone is found along the edge of the 3D Brillouin zone. Our results establish ${\mathrm{CoTiO}}_3$ as a model pseudospin-$1/2$ material to study interacting Dirac bosons in a 3D quantum $\mathit{XY}$ magnet.

Betatron x-ray sources from laser-plasma accelerators reproduce the principle of a synchrotron at the millimeter scale. They combine compactness, femtosecond pulse duration, broadband spectrum, and micron source size. However, when produced with terawatt class femtosecond lasers, their energy and flux are not sufficient to compete with synchrotron sources, thus limiting their dissemination and its possible applications. Here we present a simple method to enhance the energy and the flux of betatron sources without increasing the laser energy. The orbits of the relativistic electrons emitting the radiation were controlled using density tailored plasmas so that the energetic efficiency of the betatron source is increased by more than one order of magnitude.

The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise. During the past decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit. However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood. We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by high-frequency charge noise rather than the quasistatic noise inherent to any semiconductor device. We employ a feedback-control technique to actively suppress the latter, demonstrating a $\pi$-flip gate fidelity as high as $99.04\pm 0.23\%$ in a gallium arsenide quantum dot. We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the $1/f$ noise observed in isotopically purified silicon qubits.

We propose a system that exploits the fundamental features of topological photonics and synthetic dimensions to force many semiconductor laser resonators to synchronize, mutually lock, and under suitable modulation emit a train of transform-limited mode-locked pulses. These lasers exploit the Floquet topological edge states in a 1D array of ring resonators, which corresponds to a 2D topological system with one spatial dimension and one synthetic frequency dimension. We show that the lasing state of the multielement laser system possesses the distinct characteristics of spatial topological edge states while exhibiting topologically protected transport. The topological synthetic-space edge mode imposes a constant-phase difference between the multifrequency modes on the edges, and together with modulation of the individual elements forces the ensemble of resonators to mode lock and emit short pulses, robust to disorder in the multiresonator system. Our results offer a proof-of-concept mechanism to actively mode lock a laser diode array of many lasing elements, which is otherwise extremely difficult due to the presence of many spatial modes of the array. The topological synthetic-space concepts proposed here offer an avenue to overcome this major technological challenge and open new opportunities in laser physics.

Bosonic rotation codes, introduced here, are a broad class of bosonic error-correcting codes based on phase-space rotation symmetry. We present a universal quantum computing scheme applicable to a subset of this class—number-phase codes—which includes the well-known cat and binomial codes, among many others. The entangling gate in our scheme is code agnostic and can be used to interface different rotation-symmetric encodings. In addition to a universal set of operations, we propose a teleportation-based error-correction scheme that allows recoveries to be tracked entirely in software. Focusing on cat and binomial codes as examples, we compute average gate fidelities for error correction under simultaneous loss and dephasing noise and show numerically that the error-correction scheme is close to optimal for error-free ancillae and ideal measurements. Finally, we present a scheme for fault-tolerant, universal quantum computing based on the concatenation of number-phase codes and Bacon-Shor subsystem codes.

Gradient-descent-based algorithms and their stochastic versions have widespread applications in machine learning and statistical inference. In this work, we carry out an analytic study of the performance of the algorithm most commonly considered in physics, the Langevin algorithm, in the context of noisy high-dimensional inference. We employ the Langevin algorithm to sample the posterior probability measure for the spiked mixed matrix-tensor model. The typical behavior of this algorithm is described by a system of integrodifferential equations that we call the Langevin state evolution, whose solution is compared with the one of the state evolution of approximate message passing (AMP). Our results show that, remarkably, the algorithmic threshold of the Langevin algorithm is suboptimal with respect to the one given by AMP. This phenomenon is due to the residual glassiness present in that region of parameters. We also present a simple heuristic expression of the transition line, which appears to be in agreement with the numerical results.

Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photoinduced nonequilibrium response in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. We find that upon photoexcitations, the Josephson plasma edge in the superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at a frequency lower than the static plasma edge within a short time. In the normal state, an enhancement or weaker edgelike shape is indeed induced by pump pulses in the reflectance spectrum accompanied by simultaneous rises in both real and imaginary parts of conductivity. We compare the pump-induced effects between near- and midinfrared excitations and exclude phonon pumping as a scenario for the photoinduced effects above. We further elaborate that the transient responses in the normal state are unlikely to be explained by photoinduced superconductivity.

The relaxation of few-body quantum systems can strongly depend on the initial state when the system’s semiclassical phase space is mixed; i.e., regions of chaotic motion coexist with regular islands. In recent years, there has been much effort to understand the process of thermalization in strongly interacting quantum systems that often lack an obvious semiclassical limit. The time-dependent variational principle (TDVP) allows one to systematically derive an effective classical (nonlinear) dynamical system by projecting unitary many-body dynamics onto a manifold of weakly entangled variational states. We demonstrate that such dynamical systems generally possess mixed phase space. When TDVP errors are small, the mixed phase space leaves a footprint on the exact dynamics of the quantum model. For example, when the system is initialized in a state belonging to a stable periodic orbit or the surrounding regular region, it exhibits persistent many-body quantum revivals. As a proof of principle, we identify new types of “quantum many-body scars,” i.e., initial states that lead to long-time oscillations in a model of interacting Rydberg atoms in one and two dimensions. Intriguingly, the initial states that give rise to most robust revivals are typically entangled states. On the other hand, even when TDVP errors are large, as in the thermalizing tilted-field Ising model, initializing the system in a regular region of phase space leads to a surprising slowdown of thermalization. Our work establishes TDVP as a method for identifying interacting quantum systems with anomalous dynamics in arbitrary dimensions. Moreover, the mixed phase space classical variational equations allow one to find slowly thermalizing initial conditions in interacting models. Our results shed light on a link between classical and quantum chaos, pointing toward possible extensions of the classical Kolmogorov-Arnold-Moser theorem to quantum systems.

Generalized hydrodynamics is a recent theory that describes large-scale transport properties of one-dimensional integrable models. It is built on the (typically infinitely many) local conservation laws present in these systems and leads to a generalized Euler-type hydrodynamic equation. Despite the successes of the theory, one of its cornerstones, namely, a conjectured expression for the currents of the conserved charges in local equilibrium, has not yet been proven for interacting lattice models. Here, we fill this gap and compute an exact result for the mean values of current operators in Bethe ansatz solvable systems valid in arbitrary finite volume. Our exact formula has a simple semiclassical interpretation: The currents can be computed by summing over the charge eigenvalues carried by the individual bare particles, multiplied with an effective velocity describing their propagation in the presence of the other particles. Remarkably, the semiclassical formula remains exact in the interacting quantum theory for any finite number of particles and also in the thermodynamic limit. Our proof is built on a form-factor expansion, and it is applicable to a large class of quantum integrable models.

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature $T_C$, as evidenced by the terahertz-frequency optical properties in the nonequilibrium state. In ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$, this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$, similar effects were reported also for excitation at near-infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date, it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency-dependent photosusceptibility for this phenomenon. Here, we make use of a newly developed nonlinear optical device, which generates widely tunable high-intensity femtosecond pulses, to excite ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ throughout the entire optical spectrum (3–750 THz). In the far-infrared region (3–24 THz), signatures of nonequilibrium superconductivity are induced only for excitation of the 16.4- and 19.2-THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25–750 THz), a second resonance is observed around the charge transfer band edge at approximately 350 THz. These findings highlight the importance of coupling to the electronic structure of the ${\mathrm{CuO}}_2$ planes, mediated either by a phonon or by charge transfer.

Flexible mechanical metamaterials possess repeating structural motifs that imbue them with novel, exciting properties including programmability, anomalous elastic moduli, and nonlinear and robust response. We address such structures via micromorphic continuum elasticity, which allows highly nonuniform deformations (missed in conventional elasticity) within unit cells that nevertheless vary smoothly between cells. We show that the bulk microstructure gives rise to boundary elastic terms. Discrete lattice theories have shown that critically coordinated structures possess a topological invariant that determines the placement of low-energy modes on edges of such a system. We show that in continuum systems, a new topological invariant emerges, which relates the difference in the number of such modes between two opposing edges. Guided by the continuum limit of the lattice structures, we identify macroscopic experimental observables for these topological properties that may be observed independently on a new length scale above that of the microstructure.

Monitoring the interactions between electronic and vibrational degrees of freedom in molecules is critical to our understanding of their structural dynamics. This is typically hampered by the lack of spectroscopic probes able to detect different energy scales with high temporal and frequency resolution. Coherent Raman spectroscopy can combine the capabilities of multidimensional spectroscopy with structural sensitivity at ultrafast timescales. Here, we develop a three-color-based 2D impulsive stimulated Raman technique that can selectively probe vibrational mode couplings between different active sites in molecules by taking advantage of resonance Raman enhancement. Three temporally delayed pulses generate nuclear wave packets whose evolution reports on the underlying potential energy surface, which we decipher using a diagrammatic approach enabling us to assign the origin of the spectroscopic signatures. We benchmark the method by revealing vibronic couplings in the ultrafast dynamics following photoexcitation of the green fluorescent protein.

Thin films promise new opportunities for the manipulation of surface states of topological semimetals with the potential to realize new states that cannot be obtained in bulk materials. Here, we report transport studies of gated Hall bar structures fabricated from approximately 50-nm-thick, (001)-oriented epitaxial films of cadmium arsenide, a prototype three-dimensional Dirac semimetal, in magnetic fields up to 45 T. The films exhibit a quantized Hall effect with pronounced odd-integer plateaus that is strikingly different from that of the more widely studied (112)-oriented films. We show that the unusual quantum Hall effect is a consequence of the inverted bulk band structure of cadmium arsenide that creates topological-insulator-like states at the bottom and top interfaces, each exhibiting a half-integer quantum Hall effect. A small potential offset between the two surfaces results in the crossing of the Landau levels and gives rise to the filling factor sequences observed in the experiments. Moreover, at large negative values of gate bias, the filling factor $\nu =1$ is abruptly preempted by an insulating state that is accompanied by the collapse of the well-developed quantum Hall effect. We suggest that this new phase cannot be explained within a single-particle picture and discuss the role of Coulomb interactions between spatially separated surface states.

We show that the combination of charge and dipole conservation—characteristic of fracton systems—leads to an extensive fragmentation of the Hilbert space, which, in turn, can lead to a breakdown of thermalization. As a concrete example, we investigate the out-of-equilibrium dynamics of one-dimensional spin-1 models that conserve charge (total $S^z$) and its associated dipole moment. First, we consider a minimal model including only three-site terms and find that the infinite temperature autocorrelation saturates to a finite value—showcasing nonthermal behavior. The absence of thermalization is identified as a consequence of the strong fragmentation of the Hilbert space into exponentially many invariant subspaces in the local $S^z$ basis, arising from the interplay of dipole conservation and local interactions. Second, we extend the model by including four-site terms and find that this perturbation leads to a weak fragmentation: The system still has exponentially many invariant subspaces, but they are no longer sufficient to avoid thermalization for typical initial states. More generally, for any finite range of interactions, the system still exhibits nonthermal eigenstates appearing throughout the entire spectrum. We compare our results to charge and dipole moment-conserving random unitary circuit models for which we reach identical conclusions.