In many states of the quantum electromagnetic field, including the vacuum state, entanglement exists between different space-time regions—even spacelike separated ones. These correlations can be harvested and, thereby, detected by quantum systems which locally interact with the field. Here, we propose an experimental implementation of such an entanglement-harvesting scheme which is based on electro-optic sampling (EOS). We demonstrate that state-of-the-art EOS experiments enable one to harvest entanglement from the vacuum field and to study quantum correlations within general THz fields. We further show how Bell nonlocality present in the vacuum field can be probed. Finally, we introduce an approach to mitigate shot noise in single-beam EOS configurations. These findings pave the way for experimental inquiries into foundational properties of relativistic quantum field theory, and empower EOS as a diagnostic tool in THz quantum optics.

The quantum Chernoff bound is a famous result about discriminating two different states in a setting where large numbers of copies are available, which gives the analytic asymptotic rate at which the minimal error probability decays to zero exponentially. It is however a challenging task to calculate the quantum Chernoff bound exactly in practical scenarios. In this paper, from the viewpoint of differential geometry, we demonstrate a remarkable link between the quantum Chernoff bound and Wigner-Yanase skew information. As a result, the quantum Chernoff bound can be estimated efficiently by virtue of the skew information. We present several examples to illustrate the efficiency of estimation about the quantum Chernoff bound via Wigner-Yanase skew information.

Long-range interactions can fundamentally alter properties in gapped topological phases such as emergent massive edge modes. However, recent research has shifted attention to topological nontrivial critical points or phases, and it is natural to explore how long-range interactions influence them. In this work, we investigate the topological behavior at the quantum critical point of extended Kitaev chains with long-range interactions, which can be derived from the critical Ising model via the Jordan-Wigner transformation in the short-range limit. Specifically, we analytically find that the critical edge modes at the critical point remain stable against long-range interactions. More importantly, we observe that these critical edge modes remain massless even when long- range interactions become very strong. As a by-product, we numerically find that the critical behavior of the long-range model belongs to the free-Majorana-fermion universality class, which is entirely different from the long-range universality class in the usual long-range spin models. Our work could shed new light on the interplay between long-range interactions (frustrated) and the gapless topological phases of matter.

Quantum embedding is a fundamental prerequisite for applying quantum machine learning techniques to classical data and has substantial impacts on performance outcomes. In this study, we present neural quantum embedding (NQE), a method that efficiently optimizes quantum embedding beyond the limitations of positive and trace-preserving maps by leveraging classical deep-learning techniques. NQE enhances the lower bound of the empirical risk, leading to substantial improvements in classification performance. Moreover, NQE improves robustness against noise. To validate the effectiveness of NQE, we conduct experiments on IBM quantum devices for image data classification, resulting in a remarkable accuracy enhancement from 0.52 to 0.96. In addition, numerical analyses highlight that NQE simultaneously improves the trainability and generalization performance of quantum neural networks, as well as of the quantum kernel method.

Matter-wave interferometers with microparticles will enable the next generation of quantum sensors to probe minute quantum phase information. Therefore, estimating the loss of coherence and the degree of entanglement degradation for such interferometers is essential. In this paper, we provide a noise analysis in frequency-space focusing on electromagnetic sources of dephasing. We assume that our matter-wave interferometer has a residual charge or dipole which can interact with a neighboring particle in the ambience. We investigate the dephasing due to the Coulomb, charge-induced dipole, charge-permanent dipole, and dipole-dipole interactions. All these interactions constitute electromagnetically driven dephasing channels that can affect single or multiple interferometers. As an example, we apply the obtained formulas to situations with two adjacent microparticles, which can provide insight for the noise analysis in the quantum gravity-induced entanglement of masses (QGEM) protocol and the c-not gate: we compute the dephasing due to a gas of environmental particles interacting via dipole-dipole and charge-charge couplings, respectively. To obtain simple analytical dephasing formulas, we employ uniform probability distributions for the impact parameter and for the angles characterizing the relative orientation with respect to the interferometer and a Gaussian distribution for the velocities of the environmental particles. In both cases, we show that the dephasing rate grows with the number density of particles present in the vacuum chamber, as expected.

We consider a class of games between two competing players that take turns acting on the same many-body quantum register. Each player can perform unitary operations on the register, and after each one of them acts on the register the energy is measured. Player A aims to maximize the energy while player B to minimize it. This class of zero-sum games has a clear second mover advantage if both players can entangle the same portion of the register. We show, however, that if the first player can entangle a larger number of qubits than the second player (which we refer to as having quantum edge), then the second mover advantage can be significantly reduced. We study the game for different types of quantum edge of player A versus player B and for different sizes of the register, in particular, scenarios in which absolutely maximally entangled states cannot be achieved. In this case, we also study the effectiveness of using random unitaries. Last, we consider mixed initial preparations of the register, in which case the player with a quantum edge can rely on strategies stemming from the theory of ergotropy of quantum batteries.

The surface code is a quantum error-correcting code for one logical qubit, protected by spatially localized parity checks in two dimensions. Due to fundamental constraints from spatial locality, storing more logical qubits requires either sacrificing the robustness of the surface code against errors or increasing the number of physical qubits. We bound the minimal number of spatially nonlocal parity checks necessary to add logical qubits to a surface code while maintaining, or improving, robustness to errors. We saturate the lower limit of this bound, when the number of added logical qubits is a constant, using a family of hypergraph product codes, interpolating between the surface code and constant-rate low-density parity-check codes. Fault-tolerant protocols for logical gates in the quantum code can be inherited from its classical parent codes. We provide near-term practical implementations of this code for hardware based on trapped ions or neutral atoms in mobile optical tweezers. Long-range-enhanced surface codes outperform conventional surface codes using hundreds of physical qubits, and they represent a practical strategy to enhance the robustness of logical qubits to errors in near-term devices.

Due to their significant application in reducing algorithm depth, fSim gates have attracted a lot of attention. However, during the implementation of quantum gates, fluctuations in control parameters and decoherence caused by the environment may lead to a decrease in the fidelity of the gate. Implementing an fSim gate that is robust to these factors in one step remains an unresolved issue. In this paper, we propose a one-step implementation of a nonadiabatic geometric fSim gate composed of a nonadiabatic holonomic controlled-phase (cp) gate and a nonadiabatic noncyclic geometric iswap gate with parallel paths in a tunable superconducting circuit. Compared to the composite nonadiabatic geometric fSim gate composed of a nonadiabatic holonomic cp gate and a nonadiabatic geometric iswap gate, our scheme takes only half the time and demonstrates robustness to parameter fluctuations, as well as to environmental impacts. Moreover, the scheme does not require complex controls, making it very easy to implement in experiments, and can be achieved in various circuit structures. Our scheme may provide a promising path toward quantum computation and simulation.

We employed our recent version of the Schwinger multichannel method, implemented with a model potential [F. F. Frighetto, A. S. Barbosa, and S. d'A. Sanchez, Phys. Rev. A 108, 012818 (2023)] to calculate cross sections for elastic positron scattering by a set of hydrocarbons. Initially, by comparing the model potential calculations for methane, ethylene, and acetylene with our best possible ab initio calculations to date, we obtained the cutoff parameter for the carbon atom. Our results show that this parameter depends on the hybridization of the carbon atom for each molecule. We then applied the same cutoff parameter for the model potential to calculate the cross sections for other saturated and unsaturated hydrocarbons with encouraging results. We also discuss the positron binding to these hydrocarbons and compare the present results with available data from the literature.

Under intense laser fields, atoms undergo core polarization, i.e., redistribution of the electron density around the nucleus. We systematically investigate the effect of core polarization on high-order harmonic generation (HHG) from solids using time-dependent density-functional theory in a laser-driven linear chain of atoms. It is shown that the presence of core polarization induces substantial variations in HHG compared with the results with a frozen core. Our study further provides insight into these phenomena by combining an approach that distinguishes the effects of core polarization from different shells and compares the contributions of inner and outer shells. Ultimately, we propose and demonstrate a two-color pump-probe scheme to effectively control the core-polarization effects, specifically by activating inner-shell electrons. This work highlights the significance of multielectron effects for solids and demonstrates the feasibility of manipulating the physical properties of materials through core polarization.

We propose a laser resonance chromatography (LRC) experiment on ${}^{229}\mathrm{Th}^{3+}$, with the goal of detecting the ion's electronic ground $5f^2{F}_{5/2}$ state and metastable $7s^2{S}_{1/2}$ state by means of their ion mobilities. To this end, we first model the ion-neutral interaction potentials for the two electronic states with a relativistic Fock space coupled cluster method and complete basis-set extrapolation scheme. The interaction potentials are used to simulate the state-specific reduced ion mobilities in terms of the operating temperature and the external electric field. The ion mobilities differ by more than 7% at 300 K and moderate field strengths; thus, separation of the ${}^{229}\mathrm{Th}^{3+}$ metastable state lies within the reach of LRC experiments targeting optical probing and monitoring of the nuclear clock transition in this isotope.

The confinement-induced resonances for trapped bosons in the cigar-shaped and pancake geometries are studied within the generalized Gross-Pitaevskii equations, which are a simplified version of the Hartree-Fock-Bogoliubov approximation. Although the Hartree-Fock-Bogoliubov method is considered applicable only for small interparticle interactions, the resonance denominators for the chemical potential are obtained in both quasi-one and quasi-two dimensions. A useful integral representation of the one-particle Green's function are found for the cylindrical confinement. We find the position of a smoothed resonance for the chemical potential in the pancake geometry at positive scattering length.

We consider the problem of unitary Bose polarons, i.e., impurities interacting via a potential with infinite scattering length with a bath of weakly interacting bosons. We provide an analytic expression for the energy of a heavy impurity whose interaction potential has a range larger than the healing length of the bath. Furthermore, we perform numerically exact diffusion Monte Carlo calculations and we demonstrate that the simple Gross-Pitaevskii theory provides a remarkably accurate description of heavy unitary Bose polarons throughout the whole experimentally relevant range of gas densities.

We report on the development of an advanced ultracold dysprosium apparatus, which incorporates a cold atom microscope (CAM) with a design resolution of a quarter micrometer. The CAM and the cooling and trapping regions are within the same vacuum glass vessel ensuring simple atom transport between them. We demonstrate the essential experimental steps of laser and evaporative cooling, lattice loading, transporting, and precise positioning of a cloud of the bosonic isotope ${}^{164}\mathrm{Dy}$ at the CAM focal plane. Basic characterization of the CAM and future plans in enabling its full capacity are outlined. We also present a feasible platform for simulating complex spin models of quantum magnetism, such as the XYZ model, by exploiting a set of closely spaced opposite parity levels in Dy with a large magnetic and electric dipole moment. We isolate a degenerate isospin-1/2 system, which possesses both magnetic and electric dipole-dipole coupling, containing Ising, exchange, and spin-orbit terms. The last gives rise to a spin model with asymmetric tunable rates that depend on the lattice geometry.

Inverse Compton scattering of ultra-relativistic electron beams in the field of a high-intensity laser produces photon beams with angular and spectral distributions that are strongly dependent on the laser intensity. Here we show that the laser intensity at the interaction point can be accurately inferred from the measurement of the angular number-density distribution of Compton-scattered photon beams. The theoretical expressions, supported by numerical simulations, are accurate to within 10%–15% in a wide range of laser intensities (dimensionless intensity $5\le a_0\le 50$) and electron energies ($250\mathrm{MeV}\le E\le 15\mathrm{GeV}$), and accounts for experimental features such as the finite transverse size of the electron beam, low-energy cutoffs in the photon detector, and the possibility of a transverse misalignment between the electron beam and the laser focus.

The quantum dynamics of a dense and dipole-dipole-coupled ensemble of two-level emitters interacting via their environmental thermostat is investigated. The static dipole-dipole interaction strengths are being considered strong enough but smaller than the transition frequency. Therefore, the established thermal equilibrium of ensemble's quantum dynamics is described with respect to the dipole-dipole-coupling strengths. We have demonstrated the quantum nature of the spontaneously scattered light field in this process for weaker thermal baths as well as non-negligible dipole-dipole couplings compared to the emitter's transition frequency. Furthermore, the collectively emitted photon intensity suppresses or enhances depending on the environmental thermal baths' intensities.

Quantum Wasserstein divergences are modified versions of quantum Wasserstein distances defined by channels and they have been conjectured to be genuine metrics on quantum state spaces by De Palma and Trevisan. We prove triangle inequality for quantum Wasserstein divergences for every quantum system described by a separable Hilbert space and any quadratic cost operator under the assumption that a particular state involved is pure and all the states have finite energy. We also provide strong numerical evidence suggesting that the triangle inequality holds in general for an arbitrary choice of states.

How can we analyze quantum correlations in large and noisy systems without quantum state tomography? An established method is to measure total angular momenta and employ the so-called spin-squeezing inequalities based on their expectations and variances. This allows detection of metrologically useful entanglement, but efficient strategies for estimating such nonlinear quantities have yet to be determined. In this paper we show that spin-squeezing inequalities can not only be evaluated by measurements of the total angular momentum but also by two-qubit correlations, either involving all pair correlations or randomly chosen pair correlations. Then we analyze the estimation errors of our approaches in terms of a hypothesis test. For this purpose, we discuss how error bounds can be derived for nonlinear estimators with the help of their variances, characterizing the probability of falsely detecting a separable state as entangled. We focus on the spin-squeezing inequalities in multiqubit systems. Our methods, however, can also be applied to spin-squeezing inequalities for qudits or for the statistical treatment of other nonlinear parameters of quantum states.

The Pauli strings appearing in the decomposition of an operator can be can be grouped into commuting families, reducing the number of quantum circuits needed to measure the expectation value of the operator. We detail an algorithm to completely partition the full set of Pauli strings acting on any number of qubits into the minimal number of sets of commuting families, and we provide python code to perform the partitioning. The partitioning method scales linearly with the size of the set of Pauli strings and it naturally provides a fast method of diagonalizing the commuting families with quantum gates. We provide a package that integrates the partitioning into qiskit, and use this to benchmark the algorithm with dense Hamiltonians, such as those that arise in matrix quantum mechanics models, on IBM hardware. We demonstrate computational speedups close to the theoretical limit of ${(3/2)}^m$ relative to qubit-wise commuting groupings, for $m=2,...,6$ qubits.