In this article we discuss the accuracy of effective one-dimensional (1D) theories used to describe the behavior of ultracold atomic ensembles confined in quantum wires by a harmonic trap. We derive, within a fully many-body approach, the effective Hamiltonian describing this class of systems and we calculate the beyond-mean-field corrections to the energy of the ground state arising from virtual transitions towards the excited state of the confining potential. We find that, due to the Pauli principle, effective finite-range corrections are one order of magnitude larger than effective three-body interactions. By comparing to exact solutions of the purely 1D problem, we conclude that a 1D effective theory provides a good description of the ground state of the system for a rather large range of interaction parameters.

A nonperturbative approach is developed to analyze superconducting circuits coupled to quantized electromagnetic continuum within the framework of the functional renormalization group. The formalism allows us to determine complete physical pictures of equilibrium properties in the circuit quantum electrodynamics (cQED) architectures with high-impedance waveguides, which have recently become accessible in experiments. We point out that nonperturbative effects can trigger a breakdown of the supposedly effective descriptions, such as the spin-boson and boundary sine-Gordon models, and lead to qualitatively unique phase diagrams. The origin of the failure of conventional understandings is traced to strong renormalizations of circuit parameters at low-energy scales. Our results indicate that a nonperturbative analysis is essential for a comprehensive understanding of cQED platforms consisting of superconducting circuits and long high-impedance transmission lines.

The propagation and scattering of electromagnetic waves in dielectric media is of theoretical and experimental interest in a wide variety of fields. An understanding of observational results generally requires a numerical solution of Maxwell equations—usually implemented on conventional computers using sophisticated numerical algorithms. In recent years, advances in quantum information science and in the development of quantum computers have piqued curiosity about taking advantage of these resources for an alternate numerical approach to Maxwell equations. This requires a reformulation of the classical Maxwell equations into a form suitable for quantum computers which, unlike conventional computers, are limited to unitary operations. In this paper, a unitary framework is developed for the propagation of electromagnetic waves in a spatially inhomogeneous, passive, nondispersive, and anisotropic dielectric medium. For such a medium, generally, the evolution operator in the combined Faraday-Ampere equations is not unitary. There are two steps needed to convert this equation into a unitary evolution equation. In the first step, a weighted Hilbert space is formulated in which the generator of dynamics is a pseudo-Hermitian operator. In the second step, a Dyson map is constructed which maps the weighted-physical-Hilbert space to the original Hilbert space. The resulting evolution equation for the electromagnetic wave fields is unitary. Utilizing the framework developed in these steps, a unitary evolution equation is derived for electromagnetic wave propagation in a uniaxial dielectric medium. The resulting form is suitable for quantum computing.

We numerically study two methods of measuring tunneling times using a quantum clock. In the conventional method using the Larmor clock, we show that the Larmor tunneling time can be shorter for higher tunneling barriers. In the second method, we study the probability of a spin-flip of a particle when it is transmitted through a potential barrier including a spatially rotating field interacting with its spin. According to the adiabatic theorem, the probability depends on the velocity of the particle inside the barrier. It is numerically observed that the probability increases for higher barriers, which is consistent with the result obtained by the Larmor clock. By comparing outcomes for different initial spin states, we suggest that one of the main causes of the apparent decrease in the tunneling time can be the filtering effect occurring at the end of the barrier.

We show that quantum nondemolition (QND) measurements can be used to realize measurement-based imaginary-time evolution. In our proposed scheme, repeated weak QND measurements are used to estimate the energy of a given Hamiltonian. Based on this estimated energy, adaptive unitary operations are applied such that only the targeted energy eigenstates are fixed points of the evolution. In this way, the system is deterministically driven towards the desired state. The nonlinear nature of the QND measurement, which allows for producing interactions between systems, is explicitly derived in terms of measurement operators. We show that for suitable interaction times, single-qubit QND Hamiltonians can be converted to effective multiqubit imaginary time operations. We illustrate our techniques with the example of preparing a four-qubit cluster state, which is prepared using only collective single-qubit QND measurements and single-qubit adaptive operations.

The energy loss rate from energetic proton and helium ions to electrons of a transition metal titanium (Ti) is studied using real-time time-dependent density functional theory. Nonequilibrium simulations with and without semicore electrons explicitly included in describing the electronic structure of target atoms are performed to understand their involvement in the dissipation mechanism. It is found that the low-lying $3s$ and $3p$ semicore excitations play significant roles in determining the profile of the stopping curve around and above the stopping maximum. Additionally, we investigate the effect of impact geometry on electronic stopping. An important conclusion is that although off-channeling geometry, which makes possible the strong interaction with tightly bound electrons, indeed improves the amplitude of the stopping curve, especially for the regime around the stopping maximum, it does not shift the position of the stopping maximum. Our results about the relation between effective charge and electronic stopping are in qualitative agreement with the linear response theory.

Non-Hermitian skin effect (NHSE) is a unique feature studied extensively in noninteracting non-Hermitian systems. In this paper, we extend the NHSE originally discovered in noninteracting systems to interacting many-body systems by investigating an exactly solvable non-Hermitian model, i.e., the prototypical Lieb-Liniger Bose gas with imaginary vector potential. We show that this non-Hermitian many-body model can also be exactly solved through the Bethe ansatz. By solving the Bethe ansatz equations accurately, the explicit eigenfunction is obtained, and the model's density profiles and momentum distributions are calculated to characterize the NHSE quantitatively. We find that the NHSE is gradually suppressed on the repulsive side but does not vanish as the repulsive interaction strength increases. On the attractive side, the NHSE for bound-state solutions is enhanced as interaction strength grows. In contrast, for the scattering state the NHSE shows a nonmonotonic behavior in the attractive side. Our paper provides an example of the NHSE in exactly solvable many-body systems, and we envision that it can be extended to other non-Hermitian many-body systems, especially to integrable models.

We analyze the transverse modes in flat-flat mirror microchip-laser resonators, which occur due to the gain guiding and the thermal lensing. We rigorously calculate the transverse-mode functions, their eigenfrequencies, and their generation-threshold conditions. We apply a plausible assumption that the mode amplitudes in multi-transverse-mode emission are proportional to the individual gain factors of each mode, and we estimate the beam quality factor for the multi-transverse-mode emission. This simple and intuitive approach leads to the beam quality estimations corresponding well to the experimental measurements performed here.

We derive the area theorem for light pulses interacting with an inhomogeneously broadened ensemble of two-level atoms in a single-mode optical waveguide and present its analytical solution for Gaussian-type modes, which demonstrates the significant difference from the formation of $2\pi$ pulses by plane waves. We generalize this theorem to the description of photon echo and apply it to the two-pulse (primary) echo and the revival of silenced echo (ROSE) protocol of photon echo quantum memory. We implemented ROSE protocol in a single-mode laser-written waveguide made of an optically thin crystal ${\mathrm{Tm}}^{3+}$:${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ . The experimental data obtained are satisfactorily explained by the developed theory. Finally, we discuss the obtained experimental results and possible applications of the derived pulse-area approach.

In this work, we solved the classical equations of motion and Poisson equation self-consistently, equivalent to the nonlinear Vlasov-Poisson equation, for a projectile moving in a static free-electron gas to calculate the full noncentral self-consistent electron-ion potential, and thus the ion stopping power. We investigated the origin of the Barkas effect, namely, the first nonlinear effect for projectiles at high velocities responsible for the difference between the energy-loss results for positively and negatively charged ions traversing the same target. This effect is strongly enhanced by the multipolar part of the electron-ion potential as first suggested by Lindhard [J. Lindhard, Nucl. Instr. and Meth. 132, 1438 (1976)]. Moreover, this effect is partially related to the nonconservation of the angular momentum in electron-ion collisions. These nonlinear calculations are applied to understanding the stopping of protons and antiprotons in Al at high projectile energies.

The role of a large dipole moment in rotating neutral molecules interacting with low-energy electrons is studied using an accurate ab initio approach accounting for electronic and rotational degrees of freedom. It is found that theory can reproduce weakly bound (dipole-bound) states observed in a recent photodetachment experiment with ${\mathrm{C}}_3{\mathrm{N}}^{-}$ [Phys. Rev. Lett. 127, 043001 (2021)]. Using a similar level of theory, the cross section for radiative electron attachment to the ${\mathrm{C}}_3\mathrm{N}$ molecule, forming the dipole-bound states, was determined. The obtained cross section is too small to explain the formation of ${\mathrm{C}}_3{\mathrm{N}}^{-}$ in the interstellar medium, suggesting that it is likely formed by a different process.

We propose an efficient approach for actively controlling the Rabi oscillations in nanophotonic emitter-cavity analogs based on the presence of an element with optical gain. Inspired by recent developments in parity-time ($\mathcal{PT}$)-symmetry photonics, we show that nano- or microcavities where intrinsic losses are partially or fully compensated by an externally controllable amount of gain offer unique capabilities for manipulating the dynamics of extended (collective) excitonic emitter systems. In particular, we discuss how one can drastically modify the dynamics of the system, increase the overall occupation numbers, enhance the longevity of the Rabi oscillations, and even decelerate them to the point where their experimental observation becomes less challenging. Furthermore, we show that there is a specific gain value that leads to an exceptional point, where both the emitter and cavity occupation oscillate practically in phase, with occupation numbers that can significantly exceed unity. By revisiting a recently introduced Rabi-visibility measure, we provide robust guidelines for quantifying the coupling strength and achieving strong-coupling with adaptable Rabi frequency via loss compensation.

In multipartite systems, great progress has been made recently on the study of strong quantum nonlocality without entanglement. However, the existence of orthogonal product sets with strong quantum nonlocality in even party systems remains unknown. Here the even number is greater than 4. In this paper, we successfully construct strongly nonlocal orthogonal product sets in $n$-partite systems for all even $n$, which answers the open questions given by Halder et al. [Phys. Rev. Lett. 122, 040403 (2019)] and Yuan et al. [Phys. Rev. A 102, 042228 (2020)] for any possible even party systems. Thus, we find general construction of strongly nonlocal orthogonal product sets in space ${\otimes }_{i=1}^n{\mathcal{C}}^{d_i}$ $(n,d_i\ge 3)$ and show that there do exist incomplete orthogonal product bases that can be strongly nonlocal in any possible $n$-partite systems for all even $n$. Our newly constructed orthogonal product sets are asymmetric. We analyze the differences and connections between these sets and the known orthogonal product sets in odd party systems. In addition, we present a local state discrimination protocol for our sets by using additional entangled resources. When at least two subsystems have dimensions greater than three, the protocol consumes less entanglement than a teleportation-based protocol. A strongly nonlocal set implies that the information cannot be completely accessed as long as it does not happen that all parties are together. As an application, we connect our sets with local information hiding in multipartite systems.

This paper focuses on exact synthesis of two-qubit unitaries using Clifford and $T$ gates. We propose an ancilla-free synthesis algorithm (i) which uses $T$ gates no more than ten times the minimum possible number of $T$ gates, also known as the $T$ count, and (ii) whose time complexity is linear with the $T$ count and thus instance optimal. Our synthesis algorithm relies on a characterization of the $T$ count of two-qubit unitaries based on Lie group homomorphism, which may be interest of its own. Precisely, we show that for any two-qubit unitary generated by Clifford and $T$ gates, its $T$ count is equivalent to the least denominator exponent of its SO(6) representation up to a factor of at most 10.

Using the signed Laplacian matrix, and weighted and hybrid graphs, we present additional ways to interpret graphs as grid states. Hybrid graphs offer the most general interpretation. Existing graphical methods that characterize entanglement properties of grid states are adapted to these interpretations. These additional classes of grid states are shown to exhibit rich entanglement properties, including bound entanglement. Further, we introduce graphical techniques to construct bound-entangled states in a modular fashion. We also extend the grid states model to hypergraphs. Our work, on one hand, opens up possibilities for constructing additional families of mixed quantum states in the grid state model. On the other hand, it can serve as an instrument for investigating entanglement problems from a graph theory perspective.

Spin squeezing plays a crucial role in quantum metrology and quantum information science. Its generation is the prerequisite for further applications but still faces an enormous challenge since the existing physical systems rarely contain the required squeezing interactions. Here we propose a universal scheme to generate spin squeezing in coupled spin models with collective spin-spin interactions, which commonly exist in various systems. Our scheme can transform the coupled spin interactions into squeezing interactions and reach the extreme squeezing with Heisenberg-limited measurement precision scaling as $1/N$ for $N$ particles. Only constant and continuous driving fields are required, which is a requirement that is accessible to a series of current realistic experiments. This work greatly enriches the variety of systems that can generate the Heisenberg-limited spin squeezing, with broad applications in quantum precision measurement.

We study quantum-impurity models as a platform for quantum thermometry. A single quantum spin-$\frac{1}{2}$ impurity is coupled to an explicit, structured, fermionic thermal sample system, which we refer to as the environment or bath. We critically assess the thermometric capabilities of the impurity as a probe, when its coupling to the environment is of Ising or Kondo exchange type. In the Ising case, we find sensitivity equivalent to that of an idealized two-level system, with peak thermometric performance obtained at a temperature that scales linearly in the applied control field, independent of the coupling strength and environment spectral features. By contrast, a richer thermometric response can be realized for Kondo impurities, since strong probe-environment entanglement can then develop. At low temperatures, we uncover a regime with a universal thermometric response that is independent of microscopic details, controlled only by the low-energy spectral features of the environment. The many-body entanglement that develops in this regime means that low-temperature thermometry with a weakly applied control field is inherently less sensitive, while optimal sensitivity is recovered by suppressing the entanglement with stronger fields.