We simulate the deceleration and transmission of antiprotons with keV-scale kinetic energies through polymer foils using a molecular dynamics approach, which includes a model of nuclear stopping based on the attractive interaction potentials between antiprotons and target atoms calculated by quantum chemical methods. Antiprotons scatter into larger angles with higher cross sections than protons. This causes a significant fraction of antiprotons to annihilate in the foil instead of emerging with energies of a few keV, especially when coatings of materials with high atomic number are applied to the surfaces. The simulation results are in good agreement with data from two experiments that involved pulsed antiproton beams with incident energies between 63 keV and 122 keV that traverse polymer foils with thicknesses of $\approx 1.3$ $\mu \mathrm{m}$ and 1.8 $\mu \mathrm{m}$. The 25-nm-thick layers of Ag on the latter foil reduced the transmission of antiprotons. The results will be utilized to design the degrader foils in laser spectroscopy experiments of antiprotonic helium atoms and experiments involving Penning traps that are carried out at the ELENA facility of CERN.

We study theoretically the collective excitations of a spin-orbit-coupled spin-1 Bose-Einstein condensate with antiferromagnetic spin-exchange interactions in a cigar-shaped trapping potential at zero and finite temperatures using the Hartree-Fock-Bogoliubov theory with Popov approximation. The collective modes at zero temperature are corroborated by real-time evolution of the ground state subjected to a perturbation suitable to excite a density or a spin mode. We also calculate a few low-lying modes analytically and find very good agreement with the numerical results. We confirm the presence of excitations belonging to two broad categories, namely, density and spin excitations, based on the calculation of dispersion. The degeneracy between a pair of spin modes is broken by the spin-orbit coupling. At finite temperature, spin and density excitations show qualitatively different behavior as a function of temperature.

We study the fluctuation-mediated interactions between two atoms in the presence of an aplanatic lens, demonstrating an enhancement in their resonant dipole-dipole interaction. We derive the field propagation of the linear optical system in terms of the electromagnetic Green's tensor for an aplanatic lens. The collective internal atomic dynamics is analyzed via a Lindblad master equation, which allows one to characterize the dispersive and dissipative interactions between atoms. We thus demonstrate that the resonant dipole-dipole coupling between the atoms can be enhanced in the focal plane of the lens, and the lens-modified energy exchange between the atoms can create a mutual trapping potential. Our work opens avenues for expanding dipole-dipole interactions to macroscopic scales and the experimental platforms to study them.

We consider statistical methods based on finite samples of locally randomized measurements in order to certify different degrees of multiparticle entanglement in intermediate-scale quantum systems. We first introduce hierarchies of multiqubit criteria, satisfied by states which are separable with respect to partitions of different size, involving only second moments of the underlying probability distribution. Then, we analyze in detail the statistical error of the estimation in experiments and present several approaches for estimating the statistical significance based on large deviation bounds. The latter allows us to characterize the measurement resources required for the certification of multiparticle correlations, as well as to analyze given experimental data in detail.

We introduce a spacetime discretization of the Dirac equation that has the form of a quantum automaton and that is invariant upon changing the representation of the Clifford algebra, like the Dirac equation itself. Our derivation follows Dirac's original one: We required that the square of the discrete Dirac scheme be what we define as an acceptable discretization of the Klein-Gordon equation. In contrast to standard lattice gauge theory in discrete time, in which unitarity needs to be proven, we show that the quantum automaton delivers naturally unitary Wilson fermions for any choice of Wilson's parameter.

Protocols for nonadiabatic quantum control often require the use of classical time-varying fields. Assessing the thermodynamic cost of such protocols, however, is far from trivial. Here, we study the irreversible entropy produced by the classical apparatus generating the control fields, thus providing a direct link between the cost of a control protocol and dissipation. We focus, in particular, on the case of time-dependent magnetic fields and shortcuts to adiabaticity. Our results are showcased with two experimentally realizable case studies: the Landau-Zener model of a spin-$\frac{1}{2}$ particle in a magnetic field and an ion confined in a Penning trap.

Advanced quantum information protocols rely on the operation of multinodal quantum networks where entanglement is distributed across the nodes. Existing protocols of entanglement generation are probabilistic, with the efficiency dropping exponentially with the size of the system. We formulate an approach for the deterministic generation of entangled states of a multinodal quantum network of arbitrary size by coupling a single photon standing wave with the nodes of the network. We show experimentally how this can be implemented in a simple binodal system. Since this approach relies on collective excitation of the network—not on local interaction with individual nodes—it allows generation of entanglement with unitary efficiency, independent of the size and the nature of the network.

We discuss a class of quantum speed limits (QSLs) based on unified quantum ($\alpha ,\mu$)-entropy for nonunitary physical processes. The bounds depend on both the Schatten speed and the smallest eigenvalue of the evolved state and the two-parametric unified entropy. We specialize these results to quantum channels and non-Hermitian evolutions. In the first case, the QSL depends on the Kraus operators related to the quantum channel, while in the second case the speed limit is recast in terms of the non-Hermitian Hamiltonian. To illustrate these findings, we consider a single-qubit state evolving under the (i) amplitude damping channel and (ii) the nonunitary dynamics generated by a parity-time-reversal symmetric non-Hermitian Hamiltonian. The QSL is nonzero at earlier times, while it becomes loose as the smallest eigenvalue of the evolved state approaches zero. Furthermore, we investigate the interplay between unified entropies and speed limits for the reduced dynamics of quantum many-body systems. The unified entropy is upper bounded by the quantum fluctuations of the Hamiltonian of the system, while the QSL is nonzero when entanglement is created by the nonunitary evolution. Finally, we apply these results to the XXZ model and verify that the QSL asymptotically decreases as the system size increases. Our results find applications to nonequilibrium thermodynamics, quantum metrology, and equilibration of many-body systems.

Entanglement has shown promise in enhancing information processing tasks in a sensor network via distributed quantum sensing protocols. As noise is ubiquitous in sensor networks, error correction schemes based on Gottesman-Kitaev-Preskill (GKP) states are required to enhance the performance, as shown in [Q. Zhuang, J. Preskill, and L. Jiang, New J. Phys. 22, 022001 (2020)] assuming homogeneous noise among sensors and perfect GKP states. Here, we extend the analyses of performance enhancement to finite-squeezed GKP states in a heterogeneous noise model. To begin with, we study different concatenation schemes of GKP-two-mode-squeezing codes. While traditional sequential concatenation schemes in previous works do improve the suppression of noise, we propose a balanced concatenation scheme that outperforms the sequential schemes in the presence of finite GKP squeezing. We then apply these results to two specific tasks in distributed quantum sensing (parameter estimation and hypothesis testing) to understand the trade-off between imperfect squeezing and performance. For the first task, we consider an energy-constrained scenario and provide an optimal way to distribute the energy of the finite-squeezed GKP states among the sensors. For the later task, we show that the error probability can still be drastically lowered via concatenation of realistic finite-squeezed GKP codes.

Graph states are entangled resource states for universal measurement-based quantum computation. Although matter qubits such as superconducting circuits and trapped ions are promising candidates to generate graph states, it is technologically hard to entangle a large number of them due to several types of noise. Since they must be sufficiently cooled to maintain their quantum properties, thermal noise is one of the major ones. In this paper, we show that, for any temperature $T$, the fidelity $\langle G\left|{\rho }_T\right|G\rangle$ between an ideal graph state $|G\rangle$ at zero temperature and a thermal graph state ${\rho }_T$, which is a graph state at temperature $T$, can be efficiently estimated by using only one measurement setting. A remarkable property of our protocol is that it is passive, while existing protocols are active, namely, they switch between at least two measurement settings. Since thermal noise is equivalent to an independent phase-flip error, our estimation protocol also works for that error. By generalizing our protocol to hypergraph states, we apply our protocol to the quantum-computational-supremacy demonstration with instantaneous quantum polynomial time circuits. Our results should make the characterization of entangled matter qubits extremely feasible under thermal noise.

The Gaussian process is a widely used model for regression problems in supervised machine learning. However, predicting new inputs via a Gaussian process model becomes computationally inefficient when training a large data set. This paper proposes a fast quantum algorithm for prediction based on the Gaussian process regression. The proposed quantum algorithm consists of two subalgorithms: the first one aims to efficiently prepare the squared exponential covariance matrices and covariance functions vector with annihilation and creation operators; the other is to obtain predictive mean values and covariance values for new inputs. Evidence is also shown that the proposed quantum Gaussian process regression algorithm can achieve quadratic speedup over the classical counterpart.

Photoionization branching ratios of all of the spin-orbit doublets in the noble gas atoms Ne, Ar, Kr, and Xe have been investigated theoretically at energies well above the thresholds. The results confirm in all cases that the general behavior of the branching ratios is to decrease monotonically below their statistical value with increasing photon energy. This effect gets more pronounced with increasing $Z$. In addition, the branching ratios can be strongly affected by interchannel coupling with inner shell photoionization channels. Although these effects are strongest in the neighborhood of the inner shell thresholds, they can persist over a broad range of energies,

We propose to detect phonon and roton excitations in a two-dimensional Bose-Einstein condensate with Raman-induced spin-orbit coupling by perturbing the atomic cloud with a weak barrier. The two excitation modes can be observed by moving the barrier along different directions in appropriate parameter regimes. Phonon excitations are identified by the appearance of solitary waves, while roton excitations lead to distinctive spatial density modulations. We show that this method can also be used to determine the anisotropic critical velocities of superfluid.

We report on the study of a detection scheme based on a Bloch separator in two-photon Bragg interferometers. We increase the spatial separation between the two output ports of the interferometer by selectively imparting 30 Bloch oscillations to one of them before their detection via time of flight. This method allows increasing the duration of the interferometer by reducing the time for discriminating the ports of the interferometer at detection. We study in detail the impact of this separator on the performance of a dual gravity sensor, and in particular on its measurement sensitivities to gravity acceleration and gravity gradients.

We demonstrate a comparison of different multiplexing architectures based on quantum detector tomography. Using the purity of their measurement outcomes, we gain insight into the photon-number-resolving ability of the devices. Further, we calculate the information each measurement outcome can extract from a Hilbert space with given dimension. Our work confirms that more multiplexing outcomes enable higher photon-number-resolving ability; however, the splitting between those outcomes must be optimized as well.

Superconducting circuits coupled to acoustic waveguides have extended the range of phenomena that can be experimentally studied using tools from quantum optics. In particular, giant artificial atoms permit the investigation of systems in which the electric dipole approximation breaks down and pronounced non-Markovian effects become important. While previous studies of giant atoms focused on the realm of the rotating-wave approximation, we go beyond this and perform a numerically exact analysis of giant atoms strongly coupled to their environment, in regimes where counter-rotating terms cannot be neglected. To achieve this, we use a Lanczos transformation to cast the field Hamiltonian into the form of a one-dimensional chain and employ matrix-product state simulations. This approach yields access to a wide range of system-bath observables and to relatively unexplored parameter regimes.

Quantum communication research has in recent years shifted to include multipartite networks for which questions of quantum network routing naturally emerge. To understand the potential for multipartite routing, we focus on the most promising architectures for future quantum networks—those connecting nodes close to each other. Nearest-neighbor networks, such as rings, lines, and grids, have been studied under different communication scenarios to facilitate the sharing of quantum resources especially in the presence of bottlenecks. We analyze the potential of nearest-neighbor entangling gate quantum networks and identify some serious limitations by demonstrating that rings and lines cannot overcome common bottleneck communication problems.

We introduce a three-parameter family of Bell functionals that extends those studied previously [Kaniewski, Phys. Rev. Res. 2, 033420 (2020)] by including a marginal contribution. An analysis of their largest value achievable by quantum realizations naturally splits the family into two branches, and for the first of them we show that this value is given by a simple function of the parameters defining the functionals. In this case we completely characterize the realizations attaining the optimal value and show that these functionals can be used to self-test any partially entangled state of two qubits. The optimal measurements, however, are not unique and form a one-parameter family of qubit measurements. The second branch, which includes the well-known $I_{3322}$ functional, is studied numerically. We identify the region in the parameter space where the optimal value can be attained with two-dimensional quantum systems and characterize the state and measurements attaining this value. Finally, we show that the set of realizations introduced by Pál and Vértesi [Phys. Rev. A 82, 022116 (2010)] to obtain the maximal violation of the $I_{3322}$ inequality succeeds in approaching the optimal value for a large subset of the functionals in this branch. In these cases we analyze and discuss the main features of the optimal realizations.