We study the notion of work fluctuations in quantum field theory, highlighting that the most common definitions used in finite-dimensional quantum systems cannot be applied to quantum field theory (QFT). Then we propose work distributions that are compatible with QFT, and we show that they satisfy the first law of thermodynamics up to second moments. We also show how these distributions satisfy the Crooks theorem and provide a fully nonperturbative thermodynamic analysis of spacetime localized unitary processes on a quantum field.

We investigate the effects of static electric and magnetic fields on the differential ac Stark shifts for microwave transitions in ultracold bosonic ${}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}$ molecules, for light of wavelength $\lambda =1064\mathrm{nm}$. Near this wavelength we observe unexpected two-photon transitions that may cause trap loss. We measure the ac Stark effect in external magnetic and electric fields, using microwave spectroscopy of the first rotational transition. We quantify the isotropic and anisotropic parts of the molecular polarizability at this wavelength. We demonstrate that a modest electric field can decouple the nuclear spins from the rotational angular momentum, greatly simplifying the ac Stark effect. We use this simplification to control the ac Stark shift using the polarization angle of the trapping laser.

Nonlocality, one of the most puzzling features of multipartite quantum correlation, has been identified as a useful resource for device-independent quantum information processing. Motivated by the resource theory of quantum entanglement recently an operational framework has been proposed by R. Gallego et al. [Phys. Rev. Lett. 109, 070401 (2012)] and J.-D. Bancal et al. [Phys. Rev. A 88, 014102 (2013)] that characterizes the nonlocal resource present in multipartite quantum correlations. While bipartite no-signaling correlations allow a dichotomous classification—local vs nonlocal—in a multipartite scenario the authors show the existence of several types of nonlocality that are inequivalent under the proposed operational framework. In this work we present a finer characterization of multipartite no-signaling correlations based on the same operational framework. We also clarify a statement in Gallego et al.'s work that could be misinterpreted and fine-tune the conclusions of that work here.

In this work, we propose a unified model to evaluate processes of electronic interactions of charged particles with hot and dense matter, including energy losses, mean free paths, and thermalization ranges of protons or other light ions. To formulate this method, we introduce modifications to the extended-wave-packet method, which allows one to describe and evaluate the effects of ionization as well as changes in target density and temperature. The ionization of the target leads to the formation of a dense surrounding plasma with distinct energy-absorption properties. We use this unified method to evaluate the contributions of inner shells and free electrons (produced by the target ionization) to the energy loss of protons in Si, C, and Fe targets, on an extensive range of parameters that include low, intermediate, and high energies, with densities and temperatures going from normal laboratory conditions to very high values, such as those of interest for inertial fusion and astrophysical studies.

Cold atoms in an optical lattice provide an ideal platform for studying Bloch oscillations. Here we extend Bloch oscillations to two superposed optical lattices that are accelerated away from one another, and show that these symmetric Bloch oscillations can split, reflect, and recombine matter waves coherently. Using the momentum parity symmetry of the Hamiltonian, we map out the energy band structure of the process and show that superpositions of momentum states are created by adiabatically following the ground state of the Hamiltonian. The relative phase and velocity of the two lattices completely determines the trajectories of different branches of the matter wave. Experimentally we demonstrate symmetric Bloch oscillations using cold cesium atoms where we form interferometers with up to $240\hslash k$ momentum splitting, one of the largest coherent momentum splittings achieved to date. This work has applications in macroscopic tests of quantum mechanics, measurements of fundamental constants, and searches for new physics.

The dynamics of systems with higher-order dispersion are rapidly advancing to the center of modern hydrodynamics research. From natural shallow water waves and nonlinear optics to manufactured microcavity resonators, these systems offer many surprises that motivate both fundamental insights as well as new device paradigms. An extreme regime of hydrodynamics is the formation of shock waves where nonlinearities of the system further enhance the phenomenology. Higher-order dispersion can lead to novel dispersive shocks structures whose precise modeling is challenging current mathematical concepts. Here we present a seminal paper demonstrating, experimentally and numerically, the dynamics in an interacting superfluid with higher-order dispersion. Raman dressing, a technique which over recent years has emerged as a flexible tool to modify the dispersion, is used to induce spin-orbit coupling that features a region of negative effective mass. Intriguingly, the breaking of Galilean invariance by the spin-orbit coupling allows two different types of shock structures to emerge simultaneously in a single system. Furthermore, we describe an interplay between vortices and shock fronts leading to a surprising stability of one shock, which we attribute to reduced turbulence in regions of higher-order dispersion. Our paper suggests that spin-orbit coupling can be used as a powerful means to tune the effective viscosity in cold-atom experiments serving as quantum simulators of turbulent hydrodynamics with implications for quantum metrology, quantum information, photonic applications, and quantum simulations of neutron stars.

We study a twocomponent Bose-Einstein condensate in the presence of an inhomogeneous artificial gauge field. In response to this field, the condensate forms a localized vortex lattice structure that leads to a nontrivial symmetry breaking in the phase separated regime. The underlying physical mechanism can be understood by considering the energy landscape and we present a simplified model that is capable of reproducing the main features of the phase separation transition. The intuition gained by numerically solving this simplified model is then corroborated using an analytical solution found within the Thomas-Fermi limit.

We study the classical simulation complexity, in both the weak and strong senses, of matchgate (MG) computations supplemented with all combinations of settings involving inclusion of intermediate adaptive or nonadaptive computational basis measurements, product state or magic and general entangled state inputs, and single- or multiple-line outputs. We find a striking parallel to known results for Clifford circuits, after some rebranding of resources. We also give bounds on the amount of classical simulation effort required in the case of limited access to intermediate measurements and entangled inputs. In further settings we show that adaptive MG circuits remain classically efficiently simulable if arbitrary two-qubit entangled input states on consecutive lines are allowed, but become quantum universal for three or more lines. And if adaptive measurements in noncomputational bases are allowed, even with just computational basis inputs, we get quantum universal power again.

Two-dimensional crystals of ions stored in Penning traps are a leading platform for quantum simulation and sensing experiments. For small amplitudes, the out-of-plane motion of such crystals can be described by a discrete set of normal modes called the drumhead modes, which can be used to implement a range of quantum information protocols. However, experimental observations of crystals with Doppler-cooled and even near-ground-state-cooled drumhead modes reveal an unresolved drumhead-mode spectrum. In this work, we establish in-plane thermal fluctuations in ion positions as a major contributor to the broadening of the drumhead-mode spectrum. In the process, we demonstrate how the confining magnetic field leads to unconventional in-plane normal modes, whose average potential and kinetic energies are not equal. This property, in turn, has implications for the sampling procedure required to choose the in-plane initial conditions for molecular-dynamics simulations. For current operating conditions of the NIST Penning trap, our study suggests that the two-dimensional crystals produced in this trap undergo in-plane potential-energy fluctuations of the order of $10\mathrm{mK}$. Our study therefore motivates the need for designing improved techniques to cool the in-plane degrees of freedom.

We present an experimental method to detect lower bounds to the quantum capacity of two-qubit communication channels. We consider an implementation with polarization degrees of freedom of two photons and report on the efficiency of such a method in the presence of correlated noise for varying values of the correlation strength. The procedure is based on the generation of separable states of two qubits and local measurements at the output. We also compare the performance of the correlated two-qubit channel with the single-qubit channels corresponding to the partial trace on each of the subsystems, thus showing the beneficial effect of properly taking into account correlations to achieve a larger quantum capacity.

We study the simultaneous control of an ensemble of springs with different frequencies by means of an adiabatic shortcut to adiabaticity and optimal processes. The linearity of the system allows us to derive analytical expressions for the control fields and the time evolution of the dynamics. We discuss the relative advantages of the different solutions. These results are applied in two different examples. For ion cyclotron resonance, we show how to optimally control ions by means of electric field. Using a mapping between spins and springs, we derive analytical shortcut protocols to realize robust and selective excitations of two-level quantum systems.

In three dimensions, exotic new state of matter of self-bound ultradilute quantum droplets can be realized in free space, when the mean-field attraction (i.e., with mean-field energy $E_{\text{MF}}\propto -n^2$ at the density $n$) is balanced by the repulsive beyond-mean-field quantum fluctuations (i.e., $E_{\text{BMF}}\propto n^{2+\gamma }$). The parameter $\gamma >0$ typically takes the value $1/2$ if we consider the Lee-Huang-Yang (LHY) energy functional, but it can vary when the beyond-LHY effect becomes important or the three-body interaction becomes dominant. Here, we theoretically investigate how collective excitations of a three-dimensional quantum droplet are affected by the parameter $\gamma$ and a weak harmonic trapping potential, both of which could be tuned in experiments. We use both the approximate approach based on a Gaussian variational ansatz and the exact numerical solution of the Bogoliubov equations resulting from the linearized time-dependent extended Gross-Pitaevskii equation. We show that one of the key features of quantum droplets, i.e., the existence of the surface modes with dispersion relation ${\omega }_s\propto k^{3/2}$, is very robust with respect to the changes either in the parameter $\gamma$ or in the harmonic trapping potential. We predict the excitation spectrum of the droplet realized by binary ${}^{39}\mathrm{K}$ mixtures under the typical experimental conditions, which might be readily measured in current cold-atom laboratories.

In this work, we experimentally study the angle-dependent single ionization of carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$) by linearly and circularly polarized pulses. The angle dependence of the ionization probability by linearly polarized pulses extracted from time-domain measurements on an impulsively excited rotational wave packet is compared with data obtained from a direct angle-scan measurement. The results from the measurement with linear and circular polarization are consistent with the adiabatic ionization approximation. We extend the time-domain method to extract the dependence of the asymptotic momentum distribution of fragment ions on the orientation of the molecular axis, and apply it to investigate dissociative double ionization of $\mathrm{C}{\mathrm{O}}_2$. We show that such measurements can directly test the validity of the axial recoil approximation.

Nonequilibrium photon correlations of coherently excited single quantum systems can reveal their internal quantum dynamics and provide spectroscopic access. Here we propose and discuss the fundamentals of a coherent photon coincidence spectroscopy based on the application of laser pulses with variable delay and the detection of a time-averaged two-photon coincidence rate. For demonstration, two simple but important cases, i.e., an exciton-biexciton in a quantum dot and two coupled quantum emitters, are investigated based on quantum dynamics simulations demonstrating that this nonlinear spectroscopy reveals information specific to the particular single quantum system.

The physics of quantized vortex excitations in atomic Bose-Einstein condensates has been extensively studied in recent years. Although simple vortex lines are relatively easy to create, control, and measure in experiments, it is a lot more difficult to do the same for vortex ring structures. Here we suggest and explore a method for generating and controlling superfluid vortex rings, vortex ring lattices, and other three-dimensional vortex structures in toroidally trapped superfluid Bose-Einstein condensates by using the artificial magnetic field produced by an optical nanofiber.

Recently, research on nonequilibrium dynamics sparked a great amount of topics. One of particular concern is the dynamical quantum phase transition in quenched systems. In this work, we experimentally simulated the slow quenches in a quantum transverse-field Ising model crossing two quantum critical points. We studied the relationship between the appearance of the dynamical quantum phase transition and the quench rate, using the discontinuity of the dynamical topological order parameter (DTOP). In addition, an oscillation of the DTOP is also observed in our experiment. These results provide more insight into the topological aspect of the nonequilibrium dynamics.

The recently identified possibility of ground-state cooling of a mechanical oscillator in the unresolved sideband regime by combination of the dissipative and dispersive optomechanical coupling under the red sideband excitation [Phys. Rev. A 88, 023850 (2013)] is currently viewed as a remarkable finding. We present a comprehensive analysis of this protocol, which reveals its very high sensitivity to small imperfections such as an additional dissipation, the inaccuracy of the optimized experimental settings, and the inaccuracy of the theoretical framework adopted. The impact of these imperfections on the cooling limit is quantitatively assessed. A very strong effect on the cooling limit is found from the internal cavity decay rate which, even being small compared with the detection rate, may drastically push that limit up, questioning the possibility of the ground-state cooling. Specifically, the internal loss can only be neglected if the ratio of the internal decay rate to the detection rate is much smaller than the ratio of the cooling limit predicted by the protocol to the common dispersive-coupling assisted sideband cooling limit. Moreover, we establish that the condition of applicability of theory of that protocol is the requirement that the latter ratio is much smaller than one. A detailed comparison of the cooling protocol in question with the dispersive-coupling-assisted protocols which use the red sideband excitation or feedback is presented.

In the context of spontaneous wave function collapse models, we investigate the properties of the continuous spontaneous localization (CSL) collapse rate for rigid bodies in a superposition of two states located at different places. By exploiting the Euler-Maclaurin formula, we show that for standard matter the rate for a continuous mass distribution accurately reproduces the exact rate (i.e., the one for a discrete distribution). We compare the exact rate with previous estimates in the literature and we asses their validity. We find that the reduction rate displays a peculiar mass density difference effect, which we investigate and describe in detail. We show that the recently proposed layering effect is a consequence of the mass density difference effect.

We experimentally study subradiance in a dilute cloud of ultracold rubidium (Rb) atoms where $n{\lambda }_a^3\approx {10}^{-2}$ ($n$: atomic density; ${\lambda }_a$: excitation wavelength) and the on-resonance optical depth of the cloud is of order unity. We show that, in the strong excitation regime, the subradiant time scales depend on the excitation fraction of the cloud; i.e., to the intensity of the excitation pulse. In this regime, the decay dynamics are highly complicated and there is not a single decay time constant. Instead, the decay time constant varies during the dynamics. Specifically, we were able to observe signatures of superradiant-to-subradiant transition; i.e., initially the decay rate is faster than independent decay (superradiant emission), while at later times it transitions to slower (subradiant emission). We also discuss a theoretical model whose numerical results are in good agreement with the experiments.

Characterization of equilibrium topological quantum phases by nonequilibrium quench dynamics provides a novel and efficient scheme in detecting topological invariants defined in equilibrium. Nevertheless, most of the previous studies have focused on the ideal sudden quench regime. Here we provide a generic nonadiabatic protocol of slowly quenching the system Hamiltonian and investigate the nonadiabatic dynamical characterization scheme of the topological phase. The slow quench protocol is realized by introducing a Landau-Zener problem with an inversely proportional time dependence, and it can describe, in a unified way, the crossover from the sudden quench regime (deep nonadiabatic limit) to the adiabatic regime. By analytically obtaining the final state vector after nonadiabatic evolution, we can calculate the time-averaged spin polarization and the corresponding topological spin texture. We find that the topological invariants of the postquench Hamiltonian are characterized directly by the values of spin texture on the band inversion surface. Compared to the sudden quench regime, where one has to take an additional step to calculate the gradients of spin polarization, this nonadiabatic characterization provides a minimal scheme for detecting the topological invariants. By studying the slow quench dynamics of three-dimensional topological phases, we show that our findings are not restricted to one- and two-dimensional topological phases under the inversely proportional time-dependent quench protocol, but are also valid for higher-dimensional systems or different quench protocols.