The preparation of low-entropy starting conditions is a key requirement for many experiments involving neutral atoms. Here we propose a method to autonomously assemble arbitrary spatial configurations of atoms within arrays of optical tweezers or lattice sites, enabled by a combination of tunneling and ground-state laser cooling. In contrast to previous methods, our protocol does not rely on either imaging or evaporative cooling. This circumvents limitations associated with imaging fidelity and loss, especially in systems with small spatial scales, while providing a substantial improvement in speed relative to evaporative approaches. These features may make it well suited for preparing arbitrary initial conditions for Bose-Hubbard or Rydberg interacting systems.

Topological phases with broken time-reversal symmetry and Chern number $\left|\mathcal{C}\right|\ge 2$ are of fundamental interest, but it remains unclear how to engineer the desired topological Hamiltonian within the paradigm of spin-orbit-coupled particles hopping only between nearest neighbours of a static lattice. We show that phases with higher Chern number arise when the spin-orbit coupling satisfies a combination of spin and spatial rotation symmetries. We leverage this result both to construct minimal two-band tight-binding Hamiltonians that exhibit $\left|\mathcal{C}\right|=2,3$ phases and to show that the Chern number of one of the energy bands can be inferred from the particle spin polarization at the high-symmetry crystal momenta in the Brillouin zone. Using these insights, we provide a detailed experimental scheme for the specific realization of a time-reversal-breaking topological phase with $\left|\mathcal{C}\right|=2$ for ultracold atomic gases on a triangular lattice subject to spin-orbit coupling. The Chern number can be directly measured using Zeeman spectroscopy; for fermions the spin amplitudes can be measured directly via time of flight, while for bosons this is preceded by a short Bloch oscillation. Our results provide a pathway to the realization and detection of novel topological phases with higher Chern numbers in ultracold atomic gases.

For many-electron systems, the second-order reduced density matrix (2-RDM) provides sufficient information for characterizing their properties of interests in physics and chemistry. In this paper, we present a set of nontrivial constraints on 2-RDM based on the basic geometric property of Hilbert space and the commutation relations of operators. Numerical examples are provided to demonstrate the pronounced violation of these constraints by the variational 2-RDMs. It is shown that, for a strongly correlated model system, the constraint violation may be responsible for a considerable portion of the variational error in ground-state energy. Our findings provide new insights into the structural subtlety of many-electron 2-RDMs.

We propose a fast and dephasing-tolerant scheme for the preparation of steady Knill-Laflamme-Milburn (KLM) state between a pair of Rydberg atoms by dissipation. In this scheme, arbitrary initial state can be engineered to the KLM state with high fidelity (99.24%). Compared to most schemes based on Rydberg dissipation dynamics, the time to prepare steady state can be significantly reduced in our scheme $(80 \mu \mathrm{s})$ due to the use of strong resonant dipole-dipole interactions. In addition, the scheme can effectively suppress the dephasing error of the Rydberg atoms caused by interatomic distance fluctuations, which may severely disrupt the desired systematic dynamics. Thus the present scheme is of interest and deserves further experimental investigation to enrich the dissipation dynamics based on the Rydberg atoms.

Developments of quantum techniques have drastically improved the control ability of current quantum processors. Characterizing quantum simulations performed on these processors has become a vital topic in quantum information processing. Traditional characterizing techniques such as quantum process and state tomography fully characterize quantum simulations, but always with the exponentially increased resource cost, unfortunately. It may place their applications only in small-scale quantum systems. Meanwhile, many potential characterizing methods have been put forward in the past decades. In this work we experimentally test and compare different methods for characterizing quantum simulations of Ising Hamiltonians on the nuclear magnetic resonance (NMR) platform, including the recent quantum Hamiltonian tomography with polynomial complexity to estimate the generator of quantum simulations and nonrepetitive direct fidelity estimation with the Monte Carlo method to estimate the fidelity of dynamical states. We also experimentally estimate the parameters of Hamiltonian via full quantum process tomography and quantum state tomography for the comparison. These methods have merits and demerits in accuracy, the required resource, and prior knowledge. Our experiments test the performance of different methods while providing a beneficial reference for characterizing quantum simulations in the future.

We propose a scheme for a high-dimensional nonlocal controlled quantum computation network. A client with limited quantum technology can control a distributed computation between two quantum servers in a secure manner, such that the client's computational information remains private. We define a measure to quantify the client's control power when performing nonlocal quantum gates. We find that, given the same channel resources, the client's control power over nonlocal quantum gate operations between quantum servers is much greater than that of quantum state transfer between quantum servers. Our scheme prevents quantum servers in the network from stealing each other's information or jointly stealing the client's information. Our protocol provides new avenues for building quantum computing networks and methods to develop the resource theory of quantum channels.

When an ion beam is injected into a tapered insulating capillary, the induced self-organized radial Coulomb potential in the capillary is able to focus the beam like an electrostatic lens. However, because of the continued accumulation of charge in the capillary, an equilibrium is rarely attained and the injected beam is eventually Coulomb blocked by the capillary's potential. We propose an original add-on to the capillary setup, which can be tested experimentally and which is expected to hinder the Coulomb blocking. We investigate numerically the benefits and limits of the modified capillary setup. We show how the intensity and emittance of the injected beam control the transmission rate through conically tapered capillaries. Our results indicate that the add-on succeeds to stabilize the asymptotic transmission rate for a larger range of beam intensities and emittances, while reaching a near optimal transmitted fraction up to 90%.

We have applied relativistic coupled-cluster (RCC) theory to determine the isotope-shift (IS) constants of the first eight low-lying states of the Li, ${\mathrm{Be}}^{+}$, and ${\mathrm{Ar}}^{15+}$ isoelectronic systems. Though the RCC theory with singles, doubles, and triples approximation (RCCSDT method) is an exact method for these systems for a given set of basis functions, we notice large differences in the results from this method when various procedures in the RCC theory framework are adopted to estimate the IS constants. This has been demonstrated by presenting the IS constants of the aforementioned states from the finite-field, expectation value, and analytical response (AR) approaches of the RCCSDT method. Contributions from valence triple excitations, Breit interaction, and lower-order QED effects to the evaluation of these IS constants are also highlighted. Our results are compared with high-precision calculations reported using few-body methods wherever possible. We find that results from the AR procedure are more reliable than the other two approaches. This analysis is crucial for understanding the roles of electron correlation effects in the accurate determination of IS constants in the heavier atomic systems, where few-body methods cannot be applied.

We apply the improved molecular strong-field approximation to investigate high-order above-threshold ionization (HATI) of heteronuclear diatomic molecules by an orthogonally polarized two-color (OTC) laser field. The OTC field consists of two linearly polarized components with frequencies $r\omega$ and $s\omega$, where $r$ and $s$ are integers, and $\omega$ is the fundamental frequency. The molecule is aligned in the OTC laser field polarization plane. We show that the photoelectron momentum distribution obeys one reflection symmetry which is valid for arbitrary values of the relative phase between the OTC field components in the case when $r+s$ is odd. For molecules oriented along the polarization axis $z_L$ of the $r\omega$ component (${\theta }_L=0^{\circ }$) and $r$ even and $s$ odd, the HATI spectrum exhibits the reflection symmetry with respect to the $z_L$ axis. When the molecular orientation is along the $x_L$ axis, which is perpendicular to the polarization axis of the $r\omega$ component (${\theta }_L={90}^{\circ }$), the spectrum exhibits the reflection symmetry with respect to the $x_L$ axis for $r$ odd and $s$ even. In addition, we analyze the asymmetry in the photoelectron spectra of heteronuclear molecules by comparing them with the photoelectron spectra obtained ionizing a homonuclear diatomic molecule. We also explore the influence of the shift of the relative phase by ${180}^{\circ }$ on the HATI spectra. We explain some characteristics of the obtained HATI spectra using a generalization of the classical two-dimensional simple man's model which includes ionization probabilities calculated using the imaginary-time method. Finally, we analyze the interference minima for different heteronuclear diatomic molecules and for particular values of the emission angle, laser-field parameters, and internuclear distance. These minima are well fitted with the curve obtained using the derived condition for the two-center destructive interference minima.

We calculate the correlated two-electron momentum distributions for nonsequential double ionization of helium in a 400-nm laser pulse with a peak intensity of $6.5\times {10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^2$, which is below the recollision threshold, by using the quantitative rescattering model in which the lowering of the threshold due to the presence of the electric field at the instant of recollision is taken into account. While distinct correlated back-to-back emission of the electrons along the polarization direction is predicted in accord with other existing theoretical simulations, we suggest an alternative mechanism that is responsible for the anticorrelation. At intensities below the recollision threshold, recollision excitation can only take place when the barrier of the Coulomb potential is sufficiently suppressed by the electric field. The excited electron begins to ionize at a “delayed” recollision time after a crossing of the field and hence the probability of being tunnel-ionized after the subsequent peak of the field is increased. It is demonstrated that the “delayed” recollision time predominantly determines the parallel momentum distribution of the tunneling electron and plays a decisive role in forming anticorrelation.

Using an optical potential with subwavelength resolution in the form of sharp $\delta$-like peaks, potential landscapes are created with increased anharmonicity in placement of lattice band energies and more favorable energy scales. In particular, this makes the ultracold atom $p$-band gas more stable. The article outlines the details of the construction and discusses the $p$-band stability in a canonical cosine optical lattice potential, double-well potential, and a combination of a classical cosine potential with dark state peaked potential.

In this work, we study the stationary structures and the breathing mode behavior of a two-dimensional self-bound binary Bose droplet. We employ an analytical approach using a variational ansatz with a super-Gaussian trial order parameter and compare it with the numerical solutions of the extended Gross-Pitaevskii equation. We find that the super-Gaussian is superior to the often used Gaussian ansatz in describing the stationary and dynamical properties of the system. For sufficiently large nonrotating droplets the breathing mode is energetically favorable compared to the self-evaporating process. However, for small self-bound systems our results differ based on the ansatz. Inducing angular momentum by imprinting multiply quantized vortices at the droplet center, this preference for the breathing mode persists independent of the norm.

We report the experimental observation of strong two-color optical nonlinearity in an ultracold gas of ${}^{85}\mathrm{Rb}\text{−}{}^{87}\mathrm{Rb}$ atom mixture. By simultaneously coupling two probe transitions of ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ atoms to Rydberg states in electromagnetically induced transparency (EIT) configurations, we observe significant suppression of the transparency resonance for one probe field when the second probe field is detuned at $\sim 1\mathrm{GHz}$ and hitting the EIT resonance of the other isotope. Such a cross-absorption modulation to the beam propagation dynamics can be described by two coupled nonlinear wave equations we develop. We further demonstrate that the two-color optical nonlinearity can be tuned by varying the density ratio of different atomic isotopes, which highlights its potential for exploring strongly interacting multicomponent fluids of light.

We investigate the statistical properties of partially coherent polarization singular beams embedded with a V-point polarization singularity. An analytical formula for the cross-spectral density matrix is derived for the family of partially coherent polarization singular vector beams (PSVBs) propagating through a paraxial ABCD optical system. It is observed that the far-field intensity profiles and the coherence-induced depolarization effect in partially coherent PSVBs depend on both the input spatial coherence length and the Poincaré-Hopf index (PHI) of the beam. Interestingly, it is found that in this process of coherence degradation, the polarization (Stokes, $S_{12}$) vortices are preserved. The depolarization is due to an enhanced unpolarized light field that in turn modulates the beam profile, the transverse distribution of the degree of polarization (DOP) and the degree of coherence (DOC). Furthermore, the Gaussian distribution of the DOC evolves into a non-Gaussian profile in the far-field with the number of ring dislocations equal to the magnitude of PHI of the beam. The degeneracy associated with the intensity profile, the Stokes intensity distribution, the DOP, and DOC profiles of these partially coherent PSVBs carrying opposite polarity of PHI are also discussed to complete this study. Subsequently, all of these findings are experimentally verified by generating a family of partially coherent PSVBs with controllable spatial coherence. The modulation of the spatial coherence length in the source plane leads to efficient control of its intensity, the DOC and DOP profiles on propagation, which are of importance in particle trapping, material thermal processing, free-space optical communications, and detection of a phase object.

We study at the single-photon level the nonreciprocal excitation transfer between emitters coupled with a common waveguide. Non-Markovian retarded effects are taken into account due to the large separation distance between different emitter-waveguide coupling ports. It is shown that the excitation transfer between the emitters of a small-atom dimer can be obviously nonreciprocal by introducing between them a coherent coupling channel with nontrivial coupling phase. We prove that for dimer models the nonreciprocity cannot coexist with the decoherence-free giant-atom structure although the latter markedly lengthens the lifetime of the emitters. In view of this, we further propose a giant-atom trimer which supports both nonreciprocal transfer (directional circulation) of the excitation and greatly lengthened lifetime. Such a trimer model also exhibits incommensurate emitter-waveguide entanglement for different initial states in which case the excitation transfer is, however. reciprocal. We believe that the proposals in this paper are of potential applications in large-scale quantum networks and quantum information processing.

We introduce a protocol to transfer excitations between two noninteracting qubits via purely dissipative processes (i.e., in the Lindblad master equation there is no coherent interaction between the qubits). The fundamental ingredients are the presence of collective (i.e., nonlocal) dissipation and unbalanced local dissipation rates (the qubits dissipate at different rates). The resulting quantum trajectories show that the measurement back-action changes the system wave function and induces a passage of the excitation from one qubit to the other. While similar phenomena have been witnessed for a non-Markovian environment, here the dissipative quantum state transfer is induced by an update of the observer knowledge of the wave function in the presence of a Markovian (memoryless) environment—this is a single quantum trajectory effect. That is, a postselection of a jumpless trajectory allows a transfer even for a non-Markovian environment where no quantum jumps have taken place. Beyond single quantum trajectories and postselection, such an effect can be observed by histogramming the ratio of quantum jumps at different times along several realizations. By investigating the effect of the temperature in the presence of unbalanced local dissipation, we demonstrate that, if appropriately switched on and off, the collective dissipator can act as a Maxwell's demon. These effects are a generalized measure equivalent to the standard projective measure description of quantum teleportation and Maxwell's demon. They can be witnessed in state-of-the-art setups given the extreme experimental control in, e.g., superconducting qubits, Rydberg atoms, and nitrogen-vacancy (NV) centers.

Unlike quantum correlations, the shareability of classical correlations (CCs) between two parties of a multipartite state is assumed to be free since there exist states for which CCs for each of the reduced states can simultaneously reach their algebraic maximum value. However, when one randomly picks out states from the state space, we find that the probability of obtaining those states possessing the algebraic maximum value is vanishingly small. Therefore, the possibility of a nontrivial upper bound on the distribution of CCs that is less than the algebraic maxima emerges. We explore this possibility by Haar uniformly generating random multipartite states and computing the frequency distribution for various CC measures, conventional classical correlators, and two axiomatic measures of classical correlations, namely, the classical part of quantum discord and local work of work-deficit. We find that the distributions are typically Gaussian-like and their standard deviations decrease with the increase in number of parties. It also reveals that, among the multiqubit random states, most of the reduced density matrices possess a low amount of CCs which can also be confirmed by the mean of the distributions, thereby showing a kind of restrictions on the shareability of classical correlations for random states. Furthermore, we also notice that the maximal value for random states is much lower than the algebraic maxima obtained for a set of states, and the gap between the two increases further for states with a higher number of parties. We report that, for a higher number of parties, the classical part of quantum discord and local work can follow a monogamy-based upper bound on shareability while classical correlators have a different upper bound. The trends of shareability for classical correlation measures in random states clearly demarcate between the axiomatic definition of classical correlations and the conventional ones.

In this work we revisit the discussion on the process of measurements by a detector in a uniformly accelerated rectilinear motion, interacting linearly with a massive scalar field. We employ the Glauber theory of photodetection. In this formalism, the detector, interacting with the field prepared in an arbitrary state of the Rindler Fock space, is excited only by absorption processes. Here we intend to point out the main differences between the Glauber model and the more commonly used Unruh-DeWitt model. To understand more plainly measurement processes within the Glauber framework, we investigate the transition probability rate associated with a uniformly accelerated broadband detector prepared in the ground state. We show how the Unruh-Fulling-Davies effect emerges within the rotating-wave approximation. We also consider the behavior of this broadband detector in a frame which is inertial in the remote past but in the far future becomes uniformly accelerated. The usage of the Glauber model helps to clarify the accelerated or inertial observer interpretations of the measurements performed by the accelerated detector.