We perform a study on quantum entropy production, different kinds of correlations, and their interplay in the driven Caldeira-Leggett model of quantum Brownian motion. The model, taken with a large but finite number of bath modes, is exactly solvable, and the assumption of a Gaussian initial state leads to an efficient numerical simulation of all desired observables in a wide range of model parameters. Our paper is composed of three main parts. We first compare two popular definitions of entropy production, namely, the standard weak-coupling formulation originally proposed by Spohn [J. Math. Phys. 19, 1227 (1978)] and later on extended to the driven case by Deffner and Lutz [Phys. Rev. Lett. 107, 140404 (2011)], and the always-positive expression introduced by Esposito, Lindenberg and van den Broeck [New J. Phys. 12, 013013 (2010)], which relies on the knowledge of the evolution of the bath. As a second study, we explore the decomposition of the Esposito et al. entropy production into system-environment and intraenvironment correlations for different ranges of couplings and temperatures. Lastly, we examine the evolution of quantum correlations between the system and the environment, measuring entanglement through logarithmic negativity.

Recently, the resource theory of asymmetric distinguishability for quantum strategies was introduced by Wang et al. [Phys. Rev. Research 1, 033169 (2019).]. The fundamental objects in the resource theory are pairs of quantum strategies, which are generalizations of quantum channels that provide a framework to describe an arbitrary quantum interaction. In the present paper we provide semidefinite program characterizations of the one-shot operational quantities in this resource theory. We then apply these semidefinite programs to study the advantage conferred by adaptive strategies in discrimination and distinguishability distillation of generalized amplitude damping channels. We find that there are significant gaps between what can be accomplished with an adaptive strategy versus a nonadaptive strategy.

We present an ultrafast quantum state holography scheme based on the interference of photoelectron wave packets from multiphoton ionization (MPI) using shaper-generated trichromatic pulse sequences. By superimposing a probe and a reference electron wave packet generated by MPI with a pump-probe-reference triple-pulse sequence, the phases from the MPI dynamics are imprinted into the resulting photoelectron hologram. In the experiment, we combine white-light supercontinuum pulse shaping with differential photoelectron detection using a velocity-map-imaging spectrometer. To illustrate the holographic scheme, we study the interference of a probe wave packet from $(2+1)$ resonance-enhanced MPI of potassium atoms via the $3d$ state with a reference wave packet from nonresonant three-photon MPI of its $4s$ ground state. Phase control of the hologram is exerted by variation of the relative phases of the individual pulses in the sequence. We analyze the hologram to determine the time- and energy-dependent quantum phases resulting from either the free time evolution of the probe wave packet or the detuning of the pump pulse with respect to the resonance and the ensuing time evolution in the excited state of the atom. Our results show that the trichromatic shaper-based photoelectron holography scheme is a powerful tool for time-resolved and phase-sensitive background-free observation of ultrafast quantum dynamics.

Spontaneous and photoinduced delayed electron detachment from pentacene anions was measured in the electrostatic ion storage ring TMU E-ring. The dissipation mechanisms of the internal energy of the stored hot ions were inferred both from the temporal profile of the spontaneous detachment and from the laser-firing time dependence of the photoinduced temporal profile. Simulations based on statistical emission processes reproduced the observed behaviors, providing the value for the radiative cooling rates in the time range of a few tens of milliseconds. Using the obtained information on the competition between electron detachment and radiative cooling, we discuss the anion survival probabilities as a function of the initial internal energies and the nature of the observed quasiexponential decrease in the temporal profile of the spontaneous detachment at long times.

Resolved sideband cooling is a standard technique for cooling trapped ions below the Doppler limit to near their motional ground state. Yet the most common methods for sideband cooling implicitly rely on low Doppler-cooled temperatures and tightly confined ions and they cannot be optimized for different experimental conditions. Here we introduce a framework which calculates the fastest possible pulsed sideband cooling sequence for a given number of pulses and set of experimental parameters and we verify its improvement compared to traditional methods using a trapped ${}^{171}{\mathrm{Yb}}^{+}$ ion. After extensive cooling, we find that the ion motional distribution is distinctly nonthermal and thus not amenable to standard thermometry techniques. We therefore develop and experimentally validate an improved method to measure ion temperatures after sideband cooling. These techniques will enable more efficient cooling and thermometry within trapped-ion systems, especially those with high initial temperatures or spatially extended ion wave packets.

The SU(2) symmetric Fermi-Hubbard model (FHM) plays an essential role in strongly correlated fermionic many-body systems. In the one particle per site and strongly interacting limit $U/t\gg 1$, it is effectively described by the Heisenberg Hamiltonian. In this limit, enlarging the spin and extending the typical SU(2) symmetry to $\mathrm{SU}\left(N\right)$ has been predicted to give exotic phases of matter in the ground state, with a complicated dependence on $N$. This raises the question of what—if any—are the finite-temperature signatures of these phases, especially in the currently experimentally relevant regime near or above the superexchange energy. We explore this question for thermodynamic observables by numerically calculating the thermodynamics of the $\mathrm{SU}\left(N\right)$ FHM in the two-dimensional square lattice near densities of one particle per site, using determinant quantum Monte Carlo and numerical linked cluster expansion. Interestingly, we find that for temperatures above the superexchange energy, where the correlation length is short, the energy, number of on-site pairs, and kinetic energy are universal functions of $N$. Although the physics in the regime studied is well beyond what can be captured by low-order high-temperature series, we show that an analytic description of the scaling is possible in terms of only one- and two-site calculations.

Entanglement is a defining feature of quantum mechanics that can be a resource in engineered and natural systems, but measuring entanglement in experiment remains elusive especially for large chemical systems. Most practical approaches require determining and measuring a suitable entanglement witness which provides some level of information about the entanglement structure of the probed state. A fundamental quantity of quantum metrology is the quantum Fisher information (QFI), which is a rigorous witness of multipartite entanglement that can be evaluated from linear response functions for certain states. In this paper, we explore measuring the QFI of molecular exciton states of the first-excitation subspace from spectroscopy. In particular, we utilize the fact that the linear response of a pure state subject to a weak electric field over all possible driving frequencies encodes the variance of the collective dipole moment in the probed state, which is a valid measure for QFI. The systems that are investigated include the molecular dimer, $N$-site linear aggregate with nearest-neighbor coupling, and $N$-site circular aggregate, all modeled as a collection of interacting qubits. Our theoretical analysis shows that the variance of the collective dipole moment in the brightest dipole-allowed eigenstate is the maximum QFI. The optical response of a thermally equilibrated state in the first-excitation subspace is also a valid QFI. Theoretical predictions of the measured QFI for realistic linear dye aggregates as a function of temperature and energetic disorder due to static variations of the host matrix show that two- to three-partite entanglement is realizable. This paper lays some groundwork and inspires measurement of multipartite entanglement of molecular excitons with ultrafast pump-probe experiments.

We report a spectroscopic investigation of the $3d{\delta }_{\mathrm{\Omega }}$ ($\mathrm{\Omega }=\frac{3}{2},\frac{5}{2}$) Rydberg states of the ${\mathrm{MgAr}}^{+}$ molecular cation. Vibrational levels of the $3d{\delta }_{3/2}$ spin-orbit component with $v\ge 15$ were observed to directly predissociate into charge-transfer continua correlating to the $\mathrm{Mg}\left(3s^2{}^{1}S\right)$ $+$ ${\mathrm{Ar}}^{+}\left(3p^5{}^{2}P\right)$ dissociation limit, making it possible to record their spectra by isolated-core multiphoton Rydberg dissociation spectroscopy. Vibrational states below $v=15$, which do not predissociate, were characterized with partial rotational resolution using a multiphoton excitation and dissociation quantum-control scheme that radiatively couples the $3d\delta$ vibronic states to predissociative levels of the $3d\pi$ state. An effective Hamiltonian approach was used to theoretically investigate this scheme, including the origin of the control over the spectral lineshapes provided by the laser detuning. This approach provided results in quantitative agreement with the measured spectra. Potential-energy functions were derived for the $3d{\delta }_{\mathrm{\Omega }}$ states from experimental data. They are very similar to the one of the $X^{2+}{}^1{\mathrm{\Sigma }}^{+}$ ground state of the ${\mathrm{MgAr}}^{2+}$ doubly charged ion, highlighting the Rydberg character of the $3d\delta$ state. Whereas homogeneous charge-transfer interactions play a major role in the dynamics of the Rydberg states of molecular ions, the present study shows that heterogeneous interactions leave these states essentially unaffected.

We apply input-output theory with quantum pulses [A. H. Kiilerich, K. M.ølmer, Phys. Rev. Lett. 123, 123604 (2019)] to a model of a type of two-photon detector consisting of one molecule that can detect two photons arriving sequentially in time. The underlying process is distinct from the usual two-photon absorption process where two photons arriving simultaneously and with frequencies adding up to the resonance frequency are absorbed by a single molecule in one quantum jump. Our detector model includes a Hamiltonian description of the amplification process necessary to convert the microscopic change in the single molecule to a macroscopic signal.

Analog quantum simulation offers a hardware-specific approach to studying quantum dynamics, but mapping a model Hamiltonian onto the available device parameters requires matching the hardware dynamics. We introduce a paradigm for quantum Hamiltonian simulation that leverages digital decomposition techniques and optimal control to perform analog simulation. We validate this approach by constructing the optimal analog controls for a superconducting transmon device to emulate the dynamics of an extended Bose-Hubbard model. We demonstrate the role of control time, digital error, and pulse complexity, and we explore the accuracy and robustness of these controls. We conclude by discussing the opportunity for implementing this paradigm in near-term quantum devices.

In quantum error correction using imperfect primitives, errors of high weight arising from a few faults are major concerns since they might not be correctable by the quantum error correcting code. Fortunately, some errors of different weights are logically equivalent, and the same correction procedure is applicable to all equivalent errors; thus correcting high-weight errors is sometimes possible. In this work, we introduce a technique called weight parity error correction (WPEC) which can correct Pauli error of any weight in some stabilizer codes provided that the parity of the weight of the error is known. We show that the technique is applicable to concatenated codes constructed from the $\left[\right[7,1,3\left]\right]$ Steane code or the $\left[\right[23,1,7\left]\right]$ Golay code. We also provide a fault-tolerant error correction protocol using WPEC for the $\left[\right[49,1,9\left]\right]$ concatenated Steane code which can correct up to three faults and requires only two ancillas.

To characterize entanglement of tripartite ${\mathbb{C}}^d\otimes {\mathbb{C}}^d\otimes {\mathbb{C}}^d$ systems, we employ algebraic-geometric tools that are invariants under stochastic local operation and classical communication, namely $k$-secant varieties and one-multilinear ranks. Indeed, by means of them, we present a classification of tripartite pure states in terms of a finite number of families and subfamilies. At the core of it stands out a fine-structure grouping of three-qutrit entanglement.

This study presents a theoretical approach to model the dissociative recombination (DR) of molecular ions in which direct and indirect mechanisms are competing with each other. This is often the case for the ions having low-energy electronic resonances, such as open-shell molecular ions. The approach combines the UK $R$-matrix method for fixed-nuclei electron-ion scattering, the vibronic frame transformation with outgoing-wave dissociative functions obtained using a complex absorbing potential, and molecular quantum-defect theory. The present first-principles approach is applied to the ${\mathrm{CH}}^{+}$ ion. The contribution of the Rydberg series converging to the two lowest excited electronic states of the ion, ${\mathrm{a}}^3\mathrm{\Pi }$ and $A^1\mathrm{\Pi }$, plays a significant role in the DR cross section. The obtained DR cross section is in good agreement with experimental measurements at energies between 0.3 and 3 eV, but is much higher for energies below 0.3 eV. The disagreement is probably due to the rotational structure neglected in the present theory. The nature of prominent resonances in the computed results is analyzed by considering DR probabilities for different partial waves of the incident electron. It was found that the $d$-type partial waves $(d\sigma \mathrm{and} d\pi )$ contribute considerably to the DR of ${\mathrm{CH}}^{+}$ in its ground vibronic state.

A comprehensive theoretical treatment of laser-assisted electron-atom radiative recombination in the presence of short laser pulses is presented. Our formulation lacks various unphysical effects observed in previous works, such as oscillations and high-energy tails in the spectrum of emitted radiation; however, it accounts for a contribution from the field-free process. As a result, the energy distribution of emitted radiation consists of a point spectrum embedded in a continuum. We demonstrate that the features of the latter are determined by the laser field. For instance, in the case of a train of pulses, comb structures appear in the radiation spectrum. We attribute them to constructive interference between probability amplitudes of recombination assisted by each pulse from the train. Finally, we show that the vector potential describing the laser field is encoded in the spectrogram of emitted radiation. This suggests the use of the spectrogram for a complete temporal reconstruction of the laser field, irrespective of whether it is an isolated pulse or a pulse train.

A diagrammatic theory is developed for the classical description of nonlinear response functions of weakly anharmonic oscillators. The expansion employs a harmonic basis of contributions to the phase-space probability density that exactly describes the response properties of harmonic systems and is directly analogous to the expansion of the quantum-mechanical density matrix into coherence and population terms. Introducing weak anharmonicity into the system potential leads to nonvanishing response functions that have a simple diagrammatic description in terms of transitions between these classical coherences and populations. The theory is used to explicitly calculate two-dimensional (2D) infrared (IR) spectra both with and without static disorder. Significantly, it is demonstrated that, to first order in the anharmonicity, the classical and quantum third-order response functions are identical, supporting the utility of classical methods for describing 2D IR spectra in weakly anharmonic systems.

The quantum Rabi model (QRM) describes the interaction between a two-level system (qubit) and a quantum harmonic oscillator. In the limit where the qubit frequency is smaller than the harmonic frequency, the QRM can be well approximated by the adiabatic approximation (AA). The AA is widely used due to its simplicity and explicit physical interpretation. However, the level crossings in the spectrum of the QRM predicted by the AA are determined by the zeros of Laguerre polynomials, which deviate from the exact points. We propose an approximation to the QRM that predicts the level crossings correctly. This is done by exploiting a surprising connection between isolated exact solutions to the QRM and the Laguerre polynomials in the AA. We thus refer to this approach as the generalized adiabatic approximation (GAA). By construction, the GAA always predicts the exact exceptional spectrum and approximates the regular spectrum remarkably well in a much larger parameter regime than the AA. This generalized approach offers a framework to deal with the family of Rabi-type light-matter interaction models in a simple but accurate manner.

By the Fourier transform of the yields of ${\mathrm{Ar}}^{2+}$ and ${\mathrm{Kr}}^{2+}$ obtained by the pump-probe measurements using intense near-infrared few-cycle laser pulses, we determined the spin-orbit splitting energies in the electronic ground states of ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Kr}}^{2+}$ with high precision. For ${\mathrm{Ar}}^{+}$ and ${}^{84}\mathrm{Kr}^{+}$, the spin-orbit splitting energies for ${}^{2}P_{1/2}-{}^{2}P_{3/2}$ were determined to be 1431.583 33(12) and $5370.29642\left(50\right)\mathrm{c}{\mathrm{m}}^{\text{–}1}$ and the isotope effect was examined for ${}^{83}\mathrm{Kr}^{+}$, ${}^{84}\mathrm{Kr}^{+}$, and ${}^{86}\mathrm{Kr}^{+}$. For ${}^{84}\mathrm{Kr}^{2+}$, the spin-orbit splitting energies for ${}^{3}P_1-{}^{3}P_2$ and ${}^{3}P_0-{}^{3}P_2$ were determined to be 4548.2144(40) and $5312.8059\left(40\right)\mathrm{c}{\mathrm{m}}^{\text{–}1}$, respectively.

We investigate the effect of losses on an interacting quantum gas. We show that, for gases in dimensions higher than one, assuming together a vanishing correlation time of the reservoir where dissipation occurs, and contact interactions lead to a divergence of the energy increase rate. This divergence is a combined effect of the contact interactions, which impart arbitrary large momenta to the atoms, and the infinite energy width of the reservoir associated with its vanishing correlation time. We show how the divergence is regularized when taking into account the finite energy width of the reservoir, and, for a large energy width, we give an expression for the energy increase rate that involves the contact parameter. We then consider the specific case of a weakly interacting Bose-Einstein condensate, that we describe using the Bogoliubov theory. Assuming slow losses so that the gas is at any time described by a thermal equilibrium, we compute the time evolution of the temperature of the gas. Using a Bogoliubov analysis, we also consider the case where the regularization of the divergence is due to the finite range of the interaction between atoms.

Scattershot photon sources are known to have useful properties for optical quantum computing and boson-sampling purposes, in particular for scaling to large numbers of photons. This paper investigates the application of these scattershot sources towards the metrological task of estimating an unknown phase shift. In this regard, we introduce three different scalable multimode interferometers and quantify their quantum Fisher information performance using scattershot sources with arbitrary system sizes. We show that two of the interferometers need the probing photons to be in certain input configurations to beat the classical shot-noise precision limit, while the remaining interferometer has the necessary symmetry which allows it to always beat the classical limit no matter the input configuration. However, we can prove that all three interferometers give the same amount of information on average, which can be shown to beat the classical precision limit. We also perform Monte Carlo simulations to compare the interferometers in different experimentally relevant regimes, as a function of the number of samples.

We report on measurements of the anisotropic dynamical polarizability of Dy near the 626-nm intercombination line, employing modulation spectroscopy in a one-dimensional optical lattice. To eliminate large systematic uncertainties resulting from the limited knowledge of the spatial intensity distribution, we use K as a reference species with accurately known polarizability. This method can be applied independently of the sign of the polarizability, i.e., for both attractive and repulsive optical fields on both sides of a resonance. By variation of the laser polarization we extract the scalar and the tensorial part. To characterize the strength of the transition, we also derive the natural linewidth. We find our result to be in excellent agreement with literature values, which provide a sensitive benchmark for the accuracy of our method. In addition we demonstrate optical dipole trapping on the intercombination line, confirming the expected long lifetimes and low heating rates. This provides an additional tool to tailor optical potentials for Dy atoms and for the species-specific manipulation of atoms in the Dy-K mixture.