We propose a way to measure the qubit state of an arbitrary subensemble of atoms in an array without significantly disturbing the quantum information in the unmeasured atoms. The idea is to first site-selectively transfer atoms out of the qubit basis so that one of the two states at a time is put into an auxiliary state. Electromagnetically induced transparency (EIT) light will then protect most states while detection light is scattered from atoms in the auxiliary state, which is made immune to the EIT protection by angular momentum selection rules and carefully chosen light polarization. The two states will be measured in turn, after which it is possible to recool and return the atoms to a qubit state. These measurements can be the basis of quantum error correction.

The current stage of quantum computing technology, called noisy intermediate-scale quantum technology, is characterized by large errors that prohibit it from being used for real applications. In these devices, decoherence, one of the main sources of error, is generally modeled by Markovian master equations such as the Lindblad master equation. In this paper, the decoherence phenomena are addressed from the perspective of the steepest-entropy-ascent quantum thermodynamics framework in which the noise is in part seen as internal to the system. The framework is as well used to describe changes in the energy associated with environmental interactions. Three scenarios, an inversion recovery demonstration, a Ramsey demonstration, and a two-qubit entanglement-disentanglement demonstration, are used to demonstrate the applicability of this framework, which provides good results relative to the demonstrations and the Lindblad equation; it does so, however, from a different perspective as to the cause of the decoherence. These demonstrations are conducted on the IBM superconducting quantum device $\mathrm{ibmq}\_\mathrm{bogota}$.

Solving a quadratic nonlinear system of equations (QNSE) is a fundamental, but important, task in nonlinear science. We propose an efficient quantum algorithm for solving $n$-dimensional QNSE. Our algorithm embeds QNSE into a finite-dimensional system of linear equations using the homotopy perturbation method and a linearization technique; then we solve the linear equations with a quantum linear system solver and obtain a state which is $\epsilon$-close to the normalized exact solution of the QNSE with success probability $\mathrm{\Omega }\left(1\right)$. The complexity of our algorithm is $O(\mathrm{polylog}(n/\epsilon ))$, which provides an exponential improvement over the optimal classical algorithm in dimension $n$, and the dependence on $\epsilon$ is almost optimal. Therefore, our algorithm exponentially accelerates the solution of QNSE and has wide applications in all kinds of nonlinear problems, contributing to the research progress of nonlinear science.

Light carrying time-varying orbital angular momentum (OAM) is a recently discovered type of structured electromagnetic field compared with a typical vortex field whose OAM is static. Such so-called self-torqued light is employed for manipulating the fast magnetic, topological, and quantum excitations and increasing its intensity and having access to shorter pulse durations would be of great benefit. Here we theoretically and numerically demonstrate the generation of intense self-torqued harmonics and attosecond pulses in the relativistic regime, driven by two time-delayed relativistic vortex lasers with different OAMs $l_1$ and $l_2$. The OAM of the $n\mathrm{th}$ harmonic spans $n{l}_1$ to $n{l}_2$, and the OAM of the attosecond pulses changes from $l_1$ to $l_2$. Such intense self-torqued harmonics and attosecond pulses may offer alternative possibilities in ultrafast spectroscopy.

The Aharonov-Bohm (AB) effect is a pure quantum effect that implies a measurable phase shift in the wave function for a charged particle that encircles a magnetic flux located in a region inaccessible to the particle. Classically, such a nonlocal effect appears to be impossible since the Lorentz force depends on only the magnetic field at the location of the particle. In quantum mechanics, the Hamiltonian, and thus the Schrödinger equation, has a local coupling between the current due to the particle and the electromagnetic vector potential $\mathbf{A}$, which extends to the entire space beyond the region with finite magnetic field. This has sometimes been interpreted as meaning that in quantum mechanics $\mathbf{A}$ is in some sense more “fundamental” than $\mathbf{B}$ in spite of the former being gauge dependent and thus unobservable. Here we shall, with a general proof followed by a few examples, demonstrate that the AB effect can be fully accounted for by considering only the gauge-invariant $\mathbf{B}$ field, as long as it is included as part of the quantum action of the entire isolated system. The price for the gauge-invariant formulation is that we must give up locality—the AB phase for the particle will arise from the change in the action for the $\mathbf{B}$ field in the region inaccessible to the particle.

We consider the clock game–a task formulated in the framework of quantum information theory—that can be used to improve the existing schemes of quantum-enhanced telescopy. The problem of learning when a stellar photon reaches a telescope is translated into an abstract game, which we call the clock game. A winning strategy is provided that involves performing a quantum non-demolition measurement that verifies which stellar spatio-temporal modes are occupied by a photon without disturbing the phase information. We prove tight lower bounds on the entanglement cost needed to win the clock game, with the amount of necessary entangled bits equaling the number of time bins being distinguished. This lower bound on the entanglement cost applies to any telescopy protocol that aims to nondestructively extract the time bin information of an incident photon through local measurements, and our result implies that the protocol of Khabiboulline et al. [Phys. Rev. Lett. 123, 070504 (2019)] is optimal in terms of entanglement consumption. The full task of the phase extraction is also considered, and we show that the quantum Fisher information of the stellar phase can be achieved by local measurements and shared entanglement without the necessity of nonlinear optical operations. The optimal phase measurement is achieved asymptotically with increasing number of ancilla qubits, whereas a single qubit pair is required if nonlinear operations are allowed.

Quantum multiple parameter estimation can achieve an enhanced sensitivity beyond the classical limit. Although a theoretical ultimate sensitivity bound for multiple phase estimation is given by the quantum Cramer-Rao bound (QCRB), experimental implementations to saturate the QCRB typically require an impractical setup including entangled measurements. Since it is experimentally challenging to implement an entangled measurement, the practical sensitivity is given by the Cramer-Rao bound (CRB) relevant to the measurement probabilities. Here, we consider the problem of practical sensitivity bound for multiple phase estimation with quantum probe states and a measurement setup without entanglement, which consists of a beam splitter followed by the photon-number-resolving measurement. In this practical measurement scheme, we show that lower CRB can be achieved with a quantum probe state even with higher QCRB.

This paper investigates theoretically the effect of nonreciprocity on surface electromagnetic waves (SEWs) propagating in magneto-optical structures in mutually opposite directions, i.e., on waves with tangential wave numbers $k$ of opposite sign. Namely, of our interest is a correlation between the numbers of forward ($k>0$) and backward ($k<0$) propagating SEWs, which manifests itself as a limit on the total number of SEWs. The peculiarity of this limit is that the maximum total number of forward- and backward-propagating SEWs is less than the sum of the maximum numbers of SEWs for each of the opposite directions taken individually. We derive a relation between the electromagnetic surface impedances relevant to forward- and backward-propagating waves. Afterward, by using general properties of these impedances valid regardless of the crystallographic symmetry, we analyze the roots of dispersion equations without solving them explicitly. Various types of magneto-optical structures are considered. In particular, it is shown that, in the bicrystal formed of two half-infinite magneto-optical media, the maximum total number of forward- and backward-propagating SEWs is two at a given value of $\left|k\right|$, whereas in each of the mutually opposite directions two SEWs can exist. For example, if two forward-propagating SEWs are known to exist, then no backward-propagating SEW exists. In the case where a metal film is inserted between two different half-infinite magneto-optical media, the maximum number of SEWs in one direction is four but the maximum of the total number of forward- and backward-propagating SEWs is six rather than eight.

We present an inverse-design approach to significantly improve the figures of merit for chiral photonics with quantum emitters in topological photonic crystal slab waveguides. Beginning with a topological waveguide mode with a group index of approximately 10 and a maximum forward or backward Purcell factor at a chiral point of less than 0.5, we perform optimizations of the directional Purcell factor. We use a fully three-dimensional guided-mode expansion method to efficiently calculate waveguide band-dispersion properties and modes, while automatic differentiation is employed to calculate the gradient of objective functions. We present two example improved designs: (i) a topological mode with an accessible group index of approximately 30 and a maximum unidirectional Purcell factor at a chiral point greater than 4.5, representing a nearly 10-fold improvement to the Purcell factor, and (ii) a slow-light mode well away from the Brillouin-zone edge with a group index greater than 350 and a maximum unidirectional Purcell factor at a chiral point greater than 45.

We propose that weak continuous probing may be exploited to determine and define quantum phases of complex many-body systems based on the measurement record alone. We test the resulting phase criterion in numerical simulations of measurements on the Bose-Hubbard model and the quantum Ising chain. This yields a phase transition point in reasonable agreement with the quantum phase transition in the ground state of the closed system in the thermodynamic limit, despite the system being highly excited through the measurement dynamics. At high measurement strengths, the system's response enters a Zeno regime suppressing transitions between eigenstates of the measurement operator.

It has been found that in certain non-Hermitian Hamiltonian systems, all the eigenstates can have their amplitudes concentrating to the open boundaries of the systems and decaying exponentially into the bulk, which is totally different from their counterparts in Hermitian systems. This phenomenon has been called the “non-Hermitian skin effect.” Later, certain homogeneous open systems obeying the quadratic Lindblad equation have been shown to exhibit the non-Hermitian skin effect as well. The effect is manifested in the “chiral damping” phenomenon as the particle populations, decaying from their initial uniform unity values, show asymmetry with respect to the open boundaries. However, how such asymmetry can emerge in the homogeneous open systems awaits further understanding. Here we employ an adjoint fermion formalism to study quadratic Lindbladian systems. We find that in such homogeneous open systems, while the long time steady state and the normal modes on top of it do not show the asymmetry, it is the interference between the biexcitations of the normal modes that gives rise to the non-Hermitian skin effect. We introduce the spectral function which clearly exhibits the interference pattern. We identify that the central structure of the spectral function crucially determines the feature of the non-Hermitian skin effect.

Quantitative characterization of different entanglement detection criteria for bipartite systems is presented. We review the implication sequence of these criteria and then numerically estimate volume ratios between criteria nonviolating quantum states and all quantum states. The numerical approach is based on the hit-and-run algorithm, which is applied to the convex set of all quantum states embedded into a Euclidean vector space of the Hilbert-Schmidt inner product. We demonstrate that reduction, majorization, and the Rényi-entropy-based criteria are very ineffective compared to the positive partial transpose. In the case of the Rényi-entropy-based criterion, we show that the ratio of detectable entanglement increases with the order of the Rényi entropy.

First-principle calculations are employed to investigate the ultrafast isomerization of the acetylene cation and dication. We use the time-dependent density functional theory together with the Ehrenfest dynamics to track the coupled electron-nuclear dynamics. For both the acetylene cation and the dication, we observe nonadiabatic behaviors during the isomerization. We find that the charge transfer not only alters the electronic structure through nonadiabatic transitions, but also plays a key role in the subsequent hydrogen migration. We show that nonadiabatic transitions affect the structural modification of the excited potential energy surface, and also facilitate the ultrafast isomerization through the creation of a channel of increased negative charge that facilitates the proton movement. For the acetylene cation, we find a timescale for hydrogen isomerization of $66\pm 4$ fs, which is consistent with previous pump-probe experiments and on-the-fly calculations. For the dication, we find nonadiabatic transitions occur before the isomerization and identify a similar channel for the proton. Moreover, we find the formation of vinylidene-like structures is always accompanied by a characteristic charge separation on the carbon skeleton. These heuristics will be useful in identifying tautomers and motivating the ultrafast charge-transfer detection methods for future experiments.

We experimentally study the angular distribution of near-threshold photoelectrons of argon in the multiphoton-ionization regime. Those near-threshold photoelectrons are released from atoms by a weak 800-nm laser field and a strong 400-nm laser field with parallel polarizations. In the laser polarization direction, the yield of the near-threshold photoelectron oscillates with twice the 800-nm laser frequency, while higher-frequency components for the yield oscillations are found for the electrons emitted perpendicular to the laser polarization direction. By decomposing the photoelectron angular distribution into different partial waves, we found that those near-threshold photoelectrons are generated by the interference of different photon channels via resonant states. Moreover, we observed a large time delay between the near-threshold photoelectrons emitted along the laser polarization direction via the $5p$ and $4f$ resonant states.

Spatial light modulation is important for many scientific and industrial applications. The spatial light modulator and optical data projector both rely on precisely configurable optical elements to shape a light beam. Here we explore an image-projection approach which does not require a configurable beam-shaping element. We term this approach ghost projection on account of its conceptual relation to computational ghost imaging. Instead of a configurable beam shaping element, the method transversely displaces a single illuminated mask, such as a spatially random screen, to create specified distributions of radiant exposure. The method has potential applicability to image projection employing a variety of radiation and matter wave fields, such as hard x rays, neutrons, muons, atomic beams, and molecular beams. Building on our previous theoretical and computational studies, we here seek to understand the effects, sensitivity, and tolerance of some key experimental limitations of the method. Focusing on the case of hard x rays, we employ experimentally acquired masks to numerically study the deleterious effects of photon shot noise, inaccuracies in random-mask exposure time, and inaccuracies in mask positioning, as well as adapting to spatially nonuniform illumination. Understanding the influence of these factors will assist in optimizing experimental design and work towards achieving ghost projection in practice.

The classical and quantum theories leading to the asymptotic Bethe formula of the stopping power of matter for charged particles heavier than the electron are briefly reviewed. Models and approximations for the practical calculation of various corrections that extend the validity range of the formula are described. The asymptotic formula and the associated shell correction were determined previously from an extensive database of atomic generalized oscillator strengths, calculated for an independent-electron model with the Dirac-Hartree-Fock-Slater (DHFS) self-consistent potential, with due account for relativistic departures from the Bethe sum rule. The nonrelativistic Bloch correction is extended to the relativistic domain by means of the Lindhard-Sørensen formulation, and an accurate parametrization for point projectiles with small charges is proposed. The density-effect correction and the Barkas correction are obtained from a semiempirical model of the optical oscillator strength (OOS), built from the calculated DHFS contributions of inner electron subshells plus the OOS of outer-shell electrons represented by an analytical expression, which is determined by the composition, mass density, and empirical mean excitation energy, or $I$ value of the material. Inclusion of the shell, density-effect, Lindhard-Sørensen, and Barkas corrections into the asymptotic formula leads to the corrected Bethe formula. A general strategy is proposed to determine the stopping power in terms of only the $I$ value of the material. It is shown that, with the empirical $I$ values recommended in Report 37 of the International Commission on Radiation Units and Measurements, the stopping powers calculated numerically from the corrected formula are in close agreement with available measurements of the stopping power of elemental materials for protons and alpha particles with energies higher than 0.75 and 5 MeV, respectively.

We calculate elastic and positronium (Ps) break-up cross sections for collisions of Ps with the polar molecules CO, HCl, and LiF in the fixed-nuclei approximation. We incorporate electron exchange and correlation for these processes by using the free-electron-gas model developed earlier for Ps scattering by rare-gas atoms, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{CO}}_2$ molecules. The present target molecules provide a range of dipole moments from the weakly polar CO to the strongly polar LiF. We find that Ps scattering is similar to electron scattering when the cross sections are plotted as a function of projectile velocity for the targets with smaller dipole moments (CO, HCl). However, we do not see such a similarity for LiF which has a large dipole moment. Below the Ps break-up threshold we observe resonance structures similar to those obtained earlier for the other molecular targets that we have studied.

Imaging laser-induced rotational dynamics is an important and active field due to its applications in capturing reactions in the molecular frame and in molecular imaging. Experimental measurement of the molecular orientation distribution, as a function of the Euler angles, has been demonstrated for special cases when the detectable signal is generated along the molecular symmetry axis. Here we developed the general theory that maps the probability density distribution of the molecular orientation to the atom-pair angular distributions for nonlinear molecules. With the theory, the molecular orientation distribution can be retrieved from the measured atom-pair angular distribution, which we demonstrate experimentally using ultrafast electron diffractive imaging of impulsively aligned trifluoro-iodomethane molecules. The retrieved molecular orientation distribution is in good agreement with direct numerical simulations of the time-dependent Schrödinger equation using the experimental conditions. Unlike the existing retrieval methods, the retrieval method does not require solving the Schrödinger equation, works for any alignment method, and is in principle applicable to asymmetric top molecules.

We experimentally demonstrate an externally seeded ${\mathrm{N}}_2{}^{+}$ lasing action on the transition between the $B{}^2{\mathrm{\Sigma }}_u{}^{+}\left(v=0\right)$ and $X{}^2{\mathrm{\Sigma }}_g{}^{+}\left(v^{\prime }=0\right)$ states at 391.4 nm by irradiating a ${\mathrm{N}}_2$ gas with an intense 400-nm pump laser pulse and reveal that the populations in the rotational levels of $J^{\prime }=7–17$ in the $B{}^2{\mathrm{\Sigma }}_u{}^{+}\left(v=0\right)$ state are responsible for the lasing based on the rotational revival structure in the lasing intensity recorded by the pump-probe measurements. By introducing additionally an 800-nm control pulse, we find that the lasing intensity is suppressed when the timing of the control pulse is set between the 400-nm pump and 400-nm seed laser pulses while it can be enhanced when the control pulse overlaps temporally the seed pulse. By solving the time-dependent Schrödinger equation including the continuous ionization of ${\mathrm{N}}_2$ and multistate coupling among the $B^2{\mathrm{\Sigma }}_u{}^{+}$, $A^2{\mathrm{\Pi }}_u$, and $X{}^2{\mathrm{\Sigma }}_g{}^{+}$ states in ${\mathrm{N}}_2{}^{+}$ induced by the control pulse, we show that the population inversion in ${\mathrm{N}}_2{}^{+}$ can be achieved by the 400-nm pump laser pulse and can be further modulated by the control pulse. We show also that the ${\mathrm{N}}_2{}^{+}$ lasing intensity can be suppressed or enhanced depending on the timing and intensity of the control pulse originating from the competition between the two dynamical processes, that is, the ionization of ${\mathrm{N}}_2$ and the population transfer among the three electronic states of ${\mathrm{N}}_2{}^{+}$.