After a quench in a quantum many-body system, expectation values tend to relax towards long-time averages. However, temporal fluctuations remain in the long-time limit, and it is crucial to study the suppression of these fluctuations with increasing system size. The particularly important case of nonintegrable models has been addressed so far only by numerics and conjectures based on analytical bounds. In this work, we are able to derive analytical predictions for the temporal fluctuations in a nonintegrable model (the transverse Ising chain with extra terms). Our results are based on identifying a dynamical regime of “many-particle dephasing,” where quasiparticles do not yet relax but fluctuations are nonetheless suppressed exponentially by weak integrability breaking.

Closed string theory exhibits an $O(D,D)$ duality symmetry on tori, which in double field theory is manifest before compactification. I prove that to first order in ${\alpha }^{\prime }$ there is no manifestly background independent and duality invariant formulation of bosonic string theory in terms of a metric, $b$ field, and dilaton. To this end I use $O(D,D)$ invariant second order perturbation theory around flat space to show that the unique background independent candidate expression for the gauge algebra at order ${\alpha }^{\prime }$ is inconsistent with the Jacobi identity. A background independent formulation exists instead for frame variables subject to ${\alpha }^{\prime }$-deformed frame transformations (generalized Green-Schwarz transformations). Potential applications for curved backgrounds, as in cosmology, are discussed.

The achievement of optimum conversion efficiency in conventional spontaneous parametric down-conversion requires consideration of quantum processes that entail multisite electrodynamic coupling, actively taking place within the conversion material. The physical mechanism, which operates through virtual photon propagation, provides for photon pairs to be emitted from spatially separated sites of photon interaction; occasionally pairs are produced in which each photon emerges from a different point in space. The extent of such nonlocalized generation is influenced by individual variations in both distance and phase correlation. Mathematical analysis of the global contributions from this mechanism provides a quantitative measure for a degree of positional uncertainty in the origin of down-converted emission.

We propose a novel type of optomechanical coupling which enables a tripartite interaction between a quantum emitter, an optical mode, and a macroscopic mechanical oscillator. The interaction uses a mechanism we term mode field coupling: a mechanical displacement modifies the spatial distribution of the optical mode field, which, in turn, modulates the emitter-photon coupling rate. In properly designed multimode optomechanical systems, we can achieve situations in which mode field coupling is the only possible interaction pathway for the system. This enables, for example, swapping of a single excitation between emitter and phonon, creation of nonclassical states of motion, and mechanical ground-state cooling in the bad-cavity regime. Importantly, the emitter-phonon coupling rate can be enhanced through an optical drive field, allowing active control of the emitter-phonon coupling for realistic experimental parameters.

We present narrow-band self-emission x-ray images from a titanium tracer layer placed at the fuel-shell interface in 60-laser-beam implosion experiments at the OMEGA facility. The images are acquired during deceleration with inferred convergences of $\sim 9–14.$ Novel here is that a systematically observed asymmetry of the emission is linked, using full sphere 3D implosion modeling, to performance-limiting low mode asymmetry of the drive.

We predict, formulate, and observe experimentally a generalized version of the Talbot effect that allows one to create diffraction-induced self-images of a periodic two-dimensional (2D) waveform with arbitrary control of the image spatial periods. Through the proposed scheme, the periods of the output self-image are multiples of the input ones by any desired integer or fractional factor, and they can be controlled independently across each of the two wave dimensions. The concept involves conditioning the phase profile of the input periodic wave before free-space diffraction. The wave energy is fundamentally preserved through the self-imaging process, enabling, for instance, the possibility of the passive amplification of the periodic patterns in the wave by a purely diffractive effect, without the use of any active gain.

Quantum field theory predicts that a spatially homogeneous but temporally varying medium will excite photon pairs out of the vacuum state. However, this important theoretical prediction lacks experimental verification due to the difficulty in attaining the required nonadiabatic and large amplitude changes in the medium. Recent work has shown that in epsilon-near-zero (ENZ) materials it is possible to optically induce changes of the refractive index of the order of unity, in femtosecond time scales. By studying the quantum field theory of a spatially homogeneous, time-varying ENZ medium, we theoretically predict photon-pair production that is up to several orders of magnitude larger than in non-ENZ time-varying materials. We also find that while in standard materials the emission spectrum depends on the time scale of the perturbation, in ENZ materials the emission is always peaked at the ENZ wavelength. These studies pave the way to technologically feasible observation of photon-pair emission from a time-varying background with implications for quantum field theories beyond condensed matter systems and with potential applications as a new source of entangled light.

Tangles of a quantized vortex line of initial density $\mathcal{L}(0)\sim 6\times 10^3{\mathrm{cm}}^{-2}$ and a variable amplitude of fluctuations of flow velocity $U(0)$ at the largest length scale are generated in superfluid ${}^4\mathrm{He}$ at $T=0.17\mathrm{K}$, and their free decay $\mathcal{L}(t)$ is measured. If $U(0)$ is small, the excess random component of the vortex line length first decays as $\mathcal{L}\propto t^{-1}$ until it becomes comparable with the structured component responsible for the classical velocity field, and the decay changes to $\mathcal{L}\propto t^{-3/2}$. The latter regime always ultimately prevails, provided the classical description of $U$ holds. A quantitative model of coexisting cascades of quantum and classical energies describes all regimes of the decay.

Dirac and Weyl semimetals display a host of novel properties. In ${\mathrm{Cd}}_3{\mathrm{As}}_2$, the Dirac nodes lead to a protection mechanism that strongly suppresses backscattering in a zero magnetic field, resulting in ultrahigh mobility ($\sim {10}^7{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$). In an applied magnetic field, an anomalous Nernst effect is predicted to arise from the Berry curvature associated with the Weyl nodes. We report the observation of a large anomalous Nernst effect in ${\mathrm{Cd}}_3{\mathrm{As}}_2$. Both the anomalous Nernst signal and transport relaxation time ${\tau }_{\mathrm{tr}}$ begin to increase rapidly at $\sim 50\mathrm{K}$. This suggests a close relation between the protection mechanism and the anomalous Nernst effect. In a field, the quantum oscillations of bulk states display a beating effect, suggesting that the Dirac nodes split into Weyl states, allowing the Berry curvature to be observed as an anomalous Nernst effect.

Antiferromagnets may exhibit spin superfluidity since the dipole interaction is weak. We seek to establish that this phenomenon occurs in insulators such as NiO, which is a good spin conductor according to previous studies. We investigate nonlocal spin transport in a planar antiferromagnetic insulator with a weak uniaxial anisotropy. The anisotropy hinders spin superfluidity by creating a substantial threshold that the current must overcome. Nevertheless, we show that applying a high magnetic field removes this obstacle near the spin-flop transition of the antiferromagnet. Importantly, the spin superfluidity can then persist across many micrometers, even in dirty samples.

Quantum mechanics postulates random outcomes. However, a model making the same output predictions but in a deterministic manner would be, in principle, experimentally indistinguishable from quantum theory. In this work we consider such models in the context of nonlocality on a device-independent scenario. That is, we study pairs of nonlocal boxes that produce their outputs deterministically. It is known that, for these boxes to be nonlocal, at least one of the boxes’ outputs has to depend on the other party’s input via some kind of hidden signaling. We prove that, if the deterministic mechanism is also algorithmic, there is a protocol that, with the sole knowledge of any upper bound on the time complexity of such an algorithm, extracts that hidden signaling and uses it for the communication of information.

We report correlation measurements on two ${{}^9\mathrm{Be}}^{+}$ ions that violate a chained Bell inequality obeyed by any local-realistic theory. The correlations can be modeled as derived from a mixture of a local-realistic probabilistic distribution and a distribution that violates the inequality. A statistical framework is formulated to quantify the local-realistic fraction allowable in the observed distribution without the fair-sampling or independent-and-identical-distributions assumptions. We exclude models of our experiment whose local-realistic fraction is above 0.327 at the 95% confidence level. This bound is significantly lower than 0.586, the minimum fraction derived from a perfect Clauser-Horne-Shimony-Holt inequality experiment. Furthermore, our data provide a device-independent certification of the deterministically created Bell states.

A boson-sampling device is a quantum machine expected to perform tasks intractable for a classical computer, yet requiring minimal nonclassical resources as compared to full-scale quantum computers. Photonic implementations to date employed sources based on inefficient processes that only simulate heralded single-photon statistics when strongly reducing emission probabilities. Boson sampling with only single-photon input has thus never been realized. Here, we report on a boson-sampling device operated with a bright solid-state source of single-photon Fock states with high photon-number purity: the emission from an efficient and deterministic quantum dot-micropillar system is demultiplexed into three partially indistinguishable single photons, with a single-photon purity $1-g^{(2)}(0)$ of $0.990\pm 0.001$, interfering in a linear optics network. Our demultiplexed source is between 1 and 2 orders of magnitude more efficient than current heralded multiphoton sources based on spontaneous parametric down-conversion, allowing us to complete the boson-sampling experiment faster than previous equivalent implementations.

Photoelectron spectra and ionization rates encompassing relativistic intensities and hydrogenlike ions with relativistic binding energies are obtained using a quasiclassical $S$-matrix approach. These results, along with those based on the imaginary time method, are compared with three-dimensional, $½$-period ab initio simulations for a wide range of ionization potentials and electric field amplitudes. Significant differences between the three results are demonstrated. Time-dependent simulations indicate that the peak ionization current can occur before the peak of the electric field.

We introduce universally robust sequences for dynamical decoupling, which simultaneously compensate pulse imperfections and the detrimental effect of a dephasing environment to an arbitrary order, work with any pulse shape, and improve performance for any initial condition. Moreover, the number of pulses in a sequence grows only linearly with the order of error compensation. Our sequences outperform the state-of-the-art robust sequences for dynamical decoupling. Beyond the theoretical proposal, we also present convincing experimental data for dynamical decoupling of atomic coherences in a solid-state optical memory.