We develop first-principles theory of kinetic plasma turbulence governed by the Vlasov-Maxwell-Landau equations in the limit of vanishing collision rates. Following an exact renormalization-group approach pioneered by Onsager, we demonstrate the existence of a “collisionless range” of scales (lengths and velocities) in one-particle phase space where the ideal Vlasov-Maxwell equations are satisfied in a “coarse-grained sense.” Entropy conservation may nevertheless be violated in that range by a “dissipative anomaly” due to nonlinear entropy cascade. We derive “$4/5$th-law-type” expressions for the entropy flux, which allow us to characterize the singularities (structure-function scaling exponents) required for its nonvanishing. Conservation laws of mass, momentum, and energy are not afflicted with anomalous transfers in the collisionless limit. In a subsequent limit of small gyroradii, however, anomalous contributions to inertial-range energy balance may appear due to both cascade of bulk energy and turbulent redistribution of internal energy in phase space. In that same limit, the “generalized Ohm’s law” derived from the particle momentum balances reduces to an “ideal Ohm’s law” but only in a coarse-grained sense that does not imply magnetic flux freezing and that permits magnetic reconnection at all inertial-range scales. We compare our results with prior theory based on the gyrokinetic (high-gyrofrequency) limit, with numerical simulations, and with spacecraft measurements of the solar wind and terrestrial magnetosphere.

We solve a minimal model for an ergodic phase in a spatially extended quantum many-body system. The model consists of a chain of sites with nearest-neighbor coupling under Floquet time evolution. Quantum states at each site span a $q$-dimensional Hilbert space, and time evolution for a pair of sites is generated by a $q^2\times q^2$ random unitary matrix. The Floquet operator is specified by a quantum circuit of depth two, in which each site is coupled to its neighbor on one side during the first half of the evolution period and to its neighbor on the other side during the second half of the period. We show how dynamical behavior averaged over realizations of the random matrices can be evaluated using diagrammatic techniques and how this approach leads to exact expressions in the large-$q$ limit. We give results for the spectral form factor, relaxation of local observables, bipartite entanglement growth, and operator spreading.

The realization of a coherent interface between distant charge or spin qubits in semiconductor quantum dots is an open challenge for quantum information processing. Here, we demonstrate both resonant (real) and nonresonant (virtual) photon-mediated coherent interactions between double quantum-dot charge qubits separated by several tens of micrometers. We present clear spectroscopic evidence of the resonant collective enhancement of the coupling of two qubits and the resonator. With both qubits in resonance with each other but detuned from the resonator, we observe exchange coupling between the qubits mediated by virtual photons. In both instances, pronounced bright and dark states governed by the symmetry of the qubit-field interaction are found. Our observations are in excellent quantitative agreement with master-equation simulations. The extracted two-qubit coupling strengths significantly exceed the linewidths of the combined resonator-qubit system, which indicates that this approach is viable for creating photon-mediated two-qubit gates in quantum-dot-based systems.

One of the most controversial phenomena in nonlinear dynamics is the reappearance of initial conditions. Celebrated as the Fermi-Pasta-Ulam-Tsingou problem, the attempt to understand how these recurrences form during the complex evolution that leads to equilibrium has deeply influenced the entire development of nonlinear science. The enigma is rendered even more intriguing by the fact that integrable models predict recurrence as exact solutions, but the difficulties involved in upholding integrability for a sufficiently long dynamic has not allowed a quantitative experimental validation. In natural processes, coupling with the environment rapidly leads to thermalization, and finding nonlinear multimodal systems presenting multiple returns is a long-standing open challenge. Here, we report the observation of more than three Fermi-Pasta-Ulam-Tsingou recurrences for nonlinear optical spatial waves and demonstrate the control of the recurrent behavior through the phase and amplitude of the initial field. The recurrence period and phase shift are found to be in remarkable agreement with the exact recurrent solution of the nonlinear Schrödinger equation, while the recurrent behavior disappears as integrability is lost. These results identify the origin of the recurrence in the integrability of the underlying dynamics and allow us to achieve one of the basic aspirations of nonlinear dynamics: the reconstruction, after several return cycles, of the exact initial condition of the system, ultimately proving that the complex evolution can be accurately predicted in experimental conditions.

Randomness is a defining element of mixing processes in nature and an essential ingredient to many protocols in quantum information. In this work, we investigate how much randomness is required to transform a given quantum state into another one. Specifically, we ask whether there is a gap between the power of a classical source of randomness compared to that of a quantum one. We provide a complete answer to these questions, by identifying provably optimal protocols for both classical and quantum sources of randomness, based on a dephasing construction. We find that in order to implement any noisy transition on a $d$-dimensional quantum system it is necessary and sufficient to have a quantum source of randomness of dimension $\sqrt{d}$ or a classical one of dimension $d$. Interestingly, coherences provided by quantum states in a source of randomness offer a quadratic advantage. The process we construct has the additional features to be robust and catalytic; i.e., the source of randomness can be reused. Building upon this formal framework, we illustrate that this dephasing construction can serve as a useful primitive in both equilibration and quantum information theory: We discuss applications describing the smallest measurement device, capturing the smallest equilibrating environment allowed by quantum mechanics, or forming the basis for a cryptographic private quantum channel. We complement the exact analysis with a discussion of approximate protocols based on quantum expanders deriving from discrete Weyl systems. This gives rise to equilibrating environments of remarkably small dimension. Our results highlight the curious feature of randomness that residual correlations and dimension can be traded against each other.

We construct quantum circuits that exactly encode the spectra of correlated electron models up to errors from rotation synthesis. By invoking these circuits as oracles within the recently introduced “qubitization” framework, one can use quantum phase estimation to sample states in the Hamiltonian eigenbasis with optimal query complexity $\mathcal{O}(\lambda /\epsilon )$, where $\lambda$ is an absolute sum of Hamiltonian coefficients and $\epsilon$ is the target precision. For both the Hubbard model and electronic structure Hamiltonian in a second quantized basis diagonalizing the Coulomb operator, our circuits have T-gate complexity $\mathcal{O}\mathbf{(}N+\log (1/\epsilon )\mathbf{)}$, where $N$ is the number of orbitals in the basis. This scenario enables sampling in the eigenbasis of electronic structure Hamiltonians with T complexity $\mathcal{O}\mathbf{(}{N}^3/\epsilon +N^2\log (1/\epsilon )/\epsilon \mathbf{)}$. Compared to prior approaches, our algorithms are asymptotically more efficient in gate complexity and require fewer T gates near the classically intractable regime. Compiling to surface code fault-tolerant gates and assuming per-gate error rates of one part in a thousand reveals that one can error correct phase estimation on interesting instances of these problems beyond the current capabilities of classical methods using only about a million superconducting qubits in a matter of hours.

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A ${\mathrm{Mn}}_4\mathrm{Ca}$ cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) is used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within $50\mu \mathrm{s}$) changes to the Mn $K\beta$ XES spectrum. However, no oxidation of the ${\mathrm{Mn}}_4\mathrm{Ca}$ core above the all-${\mathrm{Mn}}^{\mathrm{IV}}$ state is detected to precede $\mathrm{O}─\mathrm{O}$ bond formation, and the observed changes are therefore assigned to $\mathrm{O}─\mathrm{O}$ bond-formation dynamics. We propose that $\mathrm{O}─\mathrm{O}$ bond formation occurs prior to the transfer of the final (fourth) electron from the ${\mathrm{Mn}}_4\mathrm{Ca}$ cluster to the oxidized tyrosine ${\mathrm{Tyr}}_Z$ residue. This model resolves the kinetic limitations associated with $\mathrm{O}─\mathrm{O}$ bond formation and suggests an evolutionary adaptation to avoid releasing harmful peroxide species.

Quantum electrodynamics (QED) is one of the most precisely tested theories in the history of science, giving accurate predictions to a wide range of experimental observations. Recent experimental advances allow for the ability to probe physics on extremely short attosecond timescales, enabling ultrafast imaging of quantum dynamics. It is of great interest to extend our understanding of short-time quantum dynamics to QED, where the focus is typically on long-time observables such as $S$ matrices, decay rates, and cross sections. That said, solving the short-time dynamics of the QED Hamiltonian can lead to divergences, making it unclear how to arrive at physical predictions. We present an approach to regularize QED at short times and apply it to the problem of free-electron radiation into a medium, known as Cherenkov radiation. Our regularization method, which can be extended to other QED processes, is performed by subtracting the self-energy in free space from the self-energy calculated in the medium. Surprisingly, we find a number of previously unknown phenomena yielding corrections to the conventional Cherenkov effect that could be observed in current experiments. Specifically, the Cherenkov velocity threshold increases relative to the famous conventional theory. This modification to the conventional theory, which can be non-negligible in realistic scenarios, should result in the suppression of spontaneous emission in readily available experiments. Finally, we reveal a bifurcation process creating radiation into new Cherenkov angles, occurring in the strong-coupling regime, which would be realizable by considering the radiation dynamics of highly charged ions. Our results shed light on QED phenomena at short times and reveal surprising new physics in the Cherenkov effect.

Topological singularities are ubiquitous in many areas of physics. Polarization singularities are locations at which an aspect of the polarization ellipse of light becomes undetermined or degenerate. At C points, the orientation of the ellipse becomes degenerate and light’s electric field vector describes a perfect circle in time. In 2D slices of 3D random fields, the distribution in space of the C points is reminiscent of that of interacting particles. With near-field experiments, we show that when light becomes truly 2D, this has severe consequences for the distribution of C points in space. The most notable change is that the probability of finding two C points with the same topological index at a vanishing distance is enhanced in a 2D field. This case is an unusual finding for any system that exhibits topological singularities, as same-index repulsion is typically observed. All of our experimental findings are supported with theory, and excellent agreement is found between theory and experiment.

The vast majority of network data sets contain errors and omissions, although this fact is rarely incorporated in traditional network analysis. Recently, an increasing effort has been made to fill this methodological gap by developing network-reconstruction approaches based on Bayesian inference. These approaches, however, rely on assumptions of uniform error rates and on direct estimations of the existence of each edge via repeated measurements, something that is currently unavailable for the majority of network data. Here, we develop a Bayesian reconstruction approach that lifts these limitations by allowing for not only heterogeneous errors, but also for single edge measurements without direct error estimates. Our approach works by coupling the inference approach with structured generative network models, which enable the correlations between edges to be used as reliable uncertainty estimates. Although our approach is general, we focus on the stochastic block model as the basic generative process, from which efficient nonparametric inference can be performed and yields a principled method to infer hierarchical community structure from noisy data. We demonstrate the efficacy of our approach with a variety of empirical and artificial networks.

The thermal conductivity $\kappa$ of the cuprate superconductor ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ was measured down to 50 mK in seven crystals with doping from $p=0.12$ to $p=0.24$, both in the superconducting state and in the magnetic field-induced normal state. We obtain the electronic residual linear term ${\kappa }_0/T$ as $T\to 0$ across the pseudogap critical point $p^{\star }=0.23$. In the normal state, we observe an abrupt drop in ${\kappa }_0/T$ upon crossing below $p^{\star }$, consistent with a drop in carrier density $n$ from $1+p$ to $p$, the signature of the pseudogap phase inferred from the Hall coefficient. A similar drop in ${\kappa }_0/T$ is observed at $H=0$, showing that the pseudogap critical point and its signatures are unaffected by the magnetic field. In the normal state, the Wiedemann-Franz law, ${\kappa }_0/T=L_0/\rho (0)$, is obeyed at all dopings, including at the critical point where the electrical resistivity $\rho (T)$ is $T$ linear down to $T\to 0$. We conclude that the nonsuperconducting ground state of the pseudogap phase at $T=0$ is a metal whose fermionic excitations carry heat and charge as conventional electrons do.

We examine the problem of how excited populations of electrons relax after they have been excited by a pump. We include three of the most important relaxation processes: (i) impurity scattering, (ii) Coulomb scattering, and (iii) electron-phonon scattering. The relaxation of an excited population of electrons is one of the most fundamental processes measured in pump-probe experiments, but its interpretation remains under debate. We show how several common assumptions about nonequilibrium relaxation that are pervasive in the field may not hold under quite general conditions. The analysis shows that nonequilibrium relaxation is more complex than previously thought, but it yields to recently developed theoretical methods in nonequilibrium theory. In this work, we show how one can use many-body theory to properly interpret and analyze these complex systems. We focus much of the discussion on implications of these results for experiment.

The method of laser spectroscopy in supersonic gas jets was proposed for high-resolution and high-efficiency in-gas laser ionization and spectroscopy studies of short-lived nuclei. The flow properties of such supersonic gas jets have been characterized under off-line conditions. Planar laser-induced fluorescence spectroscopy of seeded copper atoms has been applied to nonintrusively measure velocity, temperature, and relative density profiles of gas jets formed by different de Laval nozzles. For validation, planar laser-induced fluorescence spectroscopy was applied on supersonic free jets with well-known flow parameters. The performance of the in-gas-jet laser spectroscopy method is determined by the achievable spectral resolution, which requires an optimization and a precise manufacturing of the nozzle inner contour as well as a pressure matching of the background medium at the nozzle exit. Our studies now enable a thorough understanding and quantification of these requirements and a determination of the final performance of the in-gas-jet method. Additionally, a comparison between the experimental results and the numerical calculations was performed for the temperature, velocity, and Mach number profiles of underexpanded and quasiuniform jets formed by a de Laval nozzle.

Quantum coherence, present whenever a quantum system exists in a superposition of multiple classically distinct states, marks one of the fundamental departures from classical physics. Quantum coherence has recently been investigated rigorously within a resource-theoretic formalism. However, the finer-grained notion of multilevel coherence, which explicitly takes into account the number of superposed classical states, has remained relatively unexplored. A comprehensive analysis of multilevel coherence, which acts as the single-party analogue to multipartite entanglement, is essential for understanding natural quantum processes as well as for gauging the performance of quantum technologies. Here, we develop the theoretical and experimental groundwork for characterizing and quantifying multilevel coherence. We prove that nontrivial levels of purity are required for multilevel coherence, as there is a ball of states around the maximally mixed state that do not exhibit multilevel coherence in any basis. We provide a simple, necessary, and sufficient analytical criterion to verify the presence of multilevel coherence, which leads to a complete classification of multilevel coherence for three-level systems. We present the robustness of multilevel coherence, a bona fide quantifier, which we show to be numerically computable via semidefinite programming and experimentally accessible via multilevel coherence witnesses, which we introduce and characterize. We further verify and lower bound the robustness of multilevel coherence by performing a semi-device-independent phase discrimination task, which is implemented experimentally with four-level quantum probes in a photonic setup. Our results contribute to understanding the operational relevance of genuine multilevel coherence, also by demonstrating the key role it plays in enhanced phase discrimination—a primitive for quantum communication and metrology—and suggest new ways to reliably and effectively test the quantum behavior of physical systems.

Restricted Boltzmann machines (RBMs) are energy-based neural networks which are commonly used as the building blocks for deep-architecture neural architectures. In this work, we derive a deterministic framework for the training, evaluation, and use of RBMs based upon the Thouless-Anderson-Palmer (TAP) mean-field approximation of widely connected systems with weak interactions coming from spin-glass theory. While the TAP approach has been extensively studied for fully visible binary spin systems, our construction is generalized to latent-variable models, as well as to arbitrarily distributed real-valued spin systems with bounded support. In our numerical experiments, we demonstrate the effective deterministic training of our proposed models and are able to show interesting features of unsupervised learning which could not be directly observed with sampling. Additionally, we demonstrate how to utilize our TAP-based framework for leveraging trained RBMs as joint priors in denoising problems.

We demonstrate that the pulsed-time structure and high-peak ion intensity provided by the laser-ablation process can be directly combined with the high resolution, high efficiency, and low background offered by collinear resonance ionization spectroscopy. This simple, versatile, and powerful method offers new and unique opportunities for high-precision studies of atomic and molecular structures, impacting fundamental and applied physics research. We show that even for ion beams possessing a relatively large energy spread, high-resolution hyperfine-structure measurements can be achieved by correcting the observed line shapes with the time-of-flight information of the resonantly ionized ions. This approach offers exceptional advantages for performing precision measurements on beams with large energy spreads and allows measurements of atomic parameters of previously inaccessible electronic states. The potential of this experimental method in multidisciplinary research is illustrated by performing, for the first time, hyperfine-structure measurements of selected states in the naturally occurring isotopes of indium, ${}^{113,115}\mathrm{In}$. Ab initio atomic-physics calculations have been performed to highlight the importance of our findings in the development of state-of-the-art atomic many-body methods, nuclear structure, and fundamental-physics studies.

The reflection of waves from rough surfaces is a fundamental process that plays a role in diverse fields such as optics, acoustics, and seismology. While a quantitative understanding of the reflection process has long been established for many types of waves, the precise manner in which thermal phonons of specific wavelengths reflect from atomically rough surfaces remains unclear owing to limited control over terahertz-frequency phonon generation and detection. Knowledge of these processes is critical for many applications, however, and is particularly important for recent attempts to create novel materials by coherently interfering thermal phonons. Here, we report measurements of a key property for these efforts, the phonon-wavelength-dependent specularity parameter, which describes the probability of specular reflections of thermal phonons at a surface. Our experiments show evidence of specular surface reflections of terahertz thermal phonons in our samples around room temperature and indicate a sensitivity of these reflections to surface imperfections on the scale of just 2–3 atomic planes. Our work demonstrates a general route to probe the microscopic interactions of thermal phonons with surfaces that are typically inaccessible with traditional experiments.

We report on an optically pumped laser where photons are simultaneously generated by population inversion and by stimulated Raman scattering in the same active medium, namely crystalline silicon doped by bismuth ($\mathrm{Si}:\mathrm{Bi}$). The medium utilizes three electronic levels: ground state [$|1⟩$: $1s(A_1)$ in $\mathrm{Si}:\mathrm{Bi}$], upper [$|3⟩$: $2p_{\pm }$] and lower [$|2⟩$: $1s(E)$] laser levels. The $|1⟩\leftrightarrow |3⟩$ and $|2⟩\leftrightarrow |3⟩$ transitions are optically allowed and the $|1⟩\leftrightarrow |2⟩$ transition is Raman active. Lasing based on population inversion occurs between the states $|3⟩$ and $|2⟩$, while Raman scattering benefits from the Raman-active transition. At high pump power the inversion-based stimulated emission $|3⟩\to |2⟩$ disappears, because electronic scattering from $|1⟩$ to $|2⟩$ via a virtual state dominates and the electrons are excited into $|2⟩$ rather than into $|3⟩$. Starting as population inversion-based lasing, it ends as stimulated Raman scattering. Our model shows that such a competition occurs on the timescale of the 10-ps-long pump pulse.

The basic physics of sound waves enables ultrasound to visualize biological tissues with high spatial and temporal resolution. Recently, this capability was enhanced with the development of acoustic biomolecules—proteins with physical properties enabling them to scatter sound. The expression of these unique air-filled proteins, known as gas vesicles (GVs), in cells allows ultrasound to image cellular functions such as gene expression in vivo, providing ultrasound with its analog of optical fluorescent proteins. Acoustical methods for the in vivo detection of GVs are now required to maximize the impact of this technology in biology and medicine. We previously engineered GVs exhibiting a nonlinear scattering behavior in response to acoustic pressures above 300 kPa and showed that amplitude-modulated (AM) ultrasound pulse sequences that excite both the linear and nonlinear GV scattering regimes were highly effective at distinguishing GVs from linear scatterers like soft biological tissues. Unfortunately, the in vivo specificity of AM ultrasound imaging is systematically compromised by the nonlinearity added by the GVs to propagating waves, resulting in strong image artifacts from linear scatterers downstream of GV inclusions. To address this issue, we present an imaging paradigm, cross-amplitude modulation (xAM), which relies on cross-propagating plane-wave transmissions of finite aperture X waves to achieve quasi-artifact-free in vivo imaging of GVs. The xAM method derives from counterpropagating wave interaction theory, which predicts that, in media exhibiting quadratic elastic nonlinearity like biological tissue, the nonlinear interaction of counterpropagating acoustic waves is inefficient. By transmitting cross-propagating plane waves, we minimize cumulative nonlinear interaction effects due to collinear wave propagation while generating a transient wave-amplitude modulation at the two plane waves’ intersection. In both simulations and experiments, we show that residual xAM nonlinearity due to wave propagation decreases as the plane-wave cross-propagation angle increases. We demonstrate in tissue-mimicking phantoms that imaging artifacts distal to GV inclusions decrease as the plane-wave cross-propagation angle opens, nearing complete extinction at angles above 16.5 degrees. Finally, we demonstrate that xAM enables highly specific in vivo imaging of GVs located in the gastrointestinal tract, a target of prime interest for future cellular imaging. These results advance the physical facet of the emerging field of biomolecular ultrasound and are also relevant to synthetic ultrasound contrast agents.

We propose a new class of bosonic dark matter (DM) detectors based on resonant absorption onto a gas of small polyatomic molecules. Bosonic DM acts on the molecules as a narrow-band perturbation, like an intense but weakly coupled laser. The excited molecules emit the absorbed energy into fluorescence photons that are picked up by sensitive photodetectors with low dark count rates. This setup is sensitive to any DM candidate that couples to electrons, photons, and nuclei, and may improve on current searches by several orders of magnitude in coupling for DM masses between 0.2 eV and 20 eV. This type of detector has excellent intrinsic energy resolution, along with several control variables—pressure, temperature, external electromagnetic fields, and molecular species or isotopes—that allow for powerful background rejection methods as well as precision studies of a potential DM signal. The proposed experiment does not require usage of novel exotic materials or futuristic technologies, relying instead on the well-established field of molecular spectroscopy and on recent advances in single-photon detection. Cooperative radiation effects, which arise due to the large spatial coherence of the nonrelativistic DM field in certain detector geometries, can tightly focus the DM-induced radiative emission in a direction that depends on the DM’s velocity, possibly permitting a detailed reconstruction of the full 3D velocity distribution in our Galactic neighborhood, as well as further background rejection.