We propose feasible schemes to realize topological phases with cold atoms in momentum-space lattices through carefully engineering artificial gauge potentials for neutral atoms. We show a Floquet approach to design momentum-space lattices with long-range hoppings, which can be applicable to realize the momentum-space Haldane model. We further demonstrate the realization of the two-dimensioanl spin-orbit coupling for a spinful momentum-space lattices, arising from a general formulation of non-Abelian gauge potential. Then we give a minimal realization describing quantum anomalous Hall effect in a spin-orbit-coupling induced Chern insulator. We also provide three different dynamical approaches to extract topological properties of the proposed models. The numerical results show that our approaches are still valid even for a lattice with small size and in a short evolution time, thus making it feasible for realistic experiments.

We report on the impact of the extended geometry of a ring-dimple trap on particle-loss heating of a degenerate Fermi gas. When the Fermi level is slightly greater than the depth of the dimple and a broad, low-density “halo” is present, the overall heating rate is reduced relative to the case of a bare ring. We find that the experimentally measured heating rates for the overfilled dimple are in good agreement with a model of the hole-induced heating caused by background-gas collisions. This suppression of the heating rate can be helpful for experimental studies of fermionic superfluids in the weak pairing limit, where achieving and maintaining low temperatures over long timescales are essential.

We propose a single resonance coupled-mode approach to light scattering by dielectric solids of revolution. By using a biorthogonal decomposition of the $S$ matrix found with the extended boundary condition method we derived all parameters required for application of the temporal coupled-mode theory in a closed form. The proposed approach allows for constructing a frequency-dependent Fano response due to a single resonance after the full-wave solution has been found at a single incident frequency.

Zero-photon subtraction (ZPS) is a conditional measurement process that can reduce the mean photon number of quantum optical states without physically removing any photons. Here we show that ZPS can also be used to transform certain super-Poissonian states into sub-Poissonian states and vice versa. Combined with a well-known “no-go” theorem on conditional measurements, this effect leads to a set of nonclassicality criteria that can be experimentally tested through ZPS measurements.

Decoherence represents a major obstacle towards realizing reliable quantum technologies. Identifying states that can be upheld against decoherence by purely coherent means, i.e., stabilizable states, for which the dissipation-induced decay can be completely compensated by suitable control Hamiltonians, can help to optimize the exploitation of fragile quantum resources and to understand the ultimate limits of coherent control for this purpose. In this work, we develop conditions for stabilizability based on the target state's eigendecomposition, both for general density operators and for the covariance matrix parametrization of Gaussian states. Unlike previous conditions for stabilizability, these spectral conditions are both necessary and sufficient and are typically easier to use, extending their scope of applicability. To demonstrate its viability, we use the spectral approach to derive upper bounds on stabilizability for a number of exemplary open system scenarios, including stabilization of generalized Greenberger-Horne-Zeilinger (GHZ) and W states in the presence of local dissipation and stabilization of squeezed thermal states under collective damping.

Exceptional points (EPs) are degeneracies of non-Hermitian systems, where both eigenvalues and eigenvectors coalesce. Classical and quantum systems exhibiting high-order EPs have recently been identified as fundamental building blocks for the development of novel, ultrasensitive optoelectronic devices. However, arguably one of their major drawbacks is that they rely on nonlinear amplification processes that could limit their potential applications, particularly in the quantum realm. In this work, we show that high-order EPs can be designed by means of linear, time-modulated, chain of inductively coupled RLC (where R stands for resistance, L for inductance, and C for capacitance) electronic circuits. With a general theory, we show that $N$ coupled circuits with $2N$ dynamical variables and time-dependent parameters can be mapped onto an $N$-site, time-dependent, non-Hermitian Hamiltonian, and obtain constraints for $\mathcal{PT}$ symmetry in such models. With numerical calculations, we obtain the Floquet exceptional contours of order $N$ by studying the energy dynamics in the circuit. Our results pave the way toward realizing robust, arbitrary-order EPs by means of synthetic gauge fields, with important implications for sensing, energy transfer, and topology.

We study the effect of the Dzyaloshinskii-Moriya (DM) interaction on the quantum speed limit (QSL) and orthogonality catastrophe (OC) in XY spin chains. With an initial sudden quench, the general expressions of fidelity and QSL time are derived in terms of the antisymmetric quasiparticle excitation spectra. The numerical and analytical studies show that depending on the system parameters, the DM interaction has various influences on QSL and fidelity. In general, the OC is witnessed and QSL time vanishes in the thermodynamic limit as the variance of the quenched Hamiltonian in ground states scales with the system size, and the conclusion is independent of the DM interaction strength. However, it is interesting to note that the QSL can uniquely detect the critical points and lines induced by the DM interaction. We further analyze the actual evolution speed, a measure of the criticality, and find its direct correspondence with the OC. We also study an interesting phenomenon arising from the magnetic field and DM interaction in the presence of classical noise. Notably, we find that the system dynamic is fault tolerant when the system is subject to a uniform classical noise in the DM interaction strength.

Superconducting circuit quantum electrodynamics is the main architecture for implementation of quantum processors. This work presents a proposal to miniaturize quantum memory devices by combining the quantum memory qubits encoded directly in the first two Fock states $\left\{\right|0\rangle ,|1\rangle \}$ and the bus channel in the same specially designed multimode cavity. One key feature of the resonant modes used for quantum memory is that the microwave current is localized, which facilitates the storage and retrieval of information in superconducting qubits while preventing crosstalk from other resonant modes. Frequency design and distinct coupling strengths in this kind of device are analyzed with electromagnetic simulations and numerical calculations. The quantum memory properties, e.g., storage speed, fidelity, and stability, are examined in four cases: (1) memory operation of single-qubit state, (2) disturbing effects, (3) coupling operation, and (4) entangled states storage. Decoherence time of the quantum processor can be extended to more than one order of magnitude longer than that without the memory. By integrating transmons with optimally designed coplanar waveguide multimode quantum memories, the present study could provide an optimistic perspective for the development of large-scale superconducting quantum processors.

Recent progress in quantum signal processing (QSP) and its generalization, quantum singular value transformation, has led to a grand unification of quantum algorithms. However, inherent experimental noise in quantum devices severely limits the length of realizable QSP sequences. We consider a model of QSP with generic perturbative noise in the signal processing basis and present a diagrammatic notation useful for analyzing such errors. To demonstrate our technique, we study a specific coherent error, that of under- or overrotation of the signal processing operator parametrized by $\epsilon \ll 1$. For this coherent error model, it is shown that while Pauli $Z$ errors are not recoverable without additional resources, Pauli $X$ and $Y$ errors can be arbitrarily suppressed by coherently appending a noisy recovery QSP without the use of additional resources or ancillas. Furthermore, through a careful accounting of errors using our diagrammatic tools, we provide an upper and lower bound on the length of this recovery QSP operator. We anticipate that the perturbative technique and the diagrammatic notation proposed here will facilitate future study of generic noise in QSP and quantum algorithms.

We generalize the theory of catalytic quantum randomness to distributed and dynamical settings. First, we expand the theory of catalytic quantum randomness by calculating the amount of (Rényi) entropy catalytically extractable from a distributed or dynamical randomness source. We show that no entropy can be catalytically extracted when one cannot implement local projective measurement on randomness source without altering its state. As an application, we prove that quantum operation cannot be hidden in correlation between two parties without using randomness, which is the dynamical generalization of the no-hiding theorem. Moreover, the formalism of distributed catalysis is applied to develop a formal definition of semantic quantum information and it follows that utilizing semantic information is equivalent to catalysis using a catalyst already correlated with the transforming system. By doing so, we unify the utilization of semantic and nonsemantic quantum information and conclude that one can always extract more information from an incompletely depleted classical randomness source, but it is not possible for quantum randomness sources.

To implement a scalable and fault-tolerant programmable quantum computing architecture, the realization of high-fidelity, robust, and efficient quantum entangling gates in a multiqubit system is an essential requirement. In this paper, we consider the optimization of the segmented amplitude-modulated entangling gates realized by applying spin-dependent forces to a chain of ions trapped in a hybrid quadratic and quartic potential. We propose a geometrical hypersurface-tangent method to optimize the laser power and the pulse gradient, carefully considering the experimental feasibility in realizing the effective Rabi frequency and its change between adjacent pulse segments. In addition, our method allows us to optimize the solution for the case of a few segments by constructing an approximated null space through trading a negligible amount of gate fidelity. Finally, we show that the present scheme can provide a unified framework to improve the robustness to an arbitrary order against random static drifts of the motional frequencies, the gate duration errors, the laser detuning drifts, and their simultaneous drifts.

Two types of long-range interactions, dipolar interaction and cavity-mediated interaction, lead to exotic quantum phases. Both interactions were realized and observed in optical lattice setups. Here, we study quantum phases of dipolar bosons trapped in optical lattices and coupled to a high-finesse cavity where both dipolar interaction and cavity-mediated interaction coexist. We perform quantum Monte Carlo simulations and find that the checkerboard solid is enhanced and the checkerboard supersolid phase can exist in a wide range of densities (e.g., $0.27\lesssim n\lesssim 0.73$). Our unbiased numerical results suggest that both solid and supersolid phases can be achieved experimentally with magnetic atoms coupled to a cavity.

We investigate the classicality of linear dipolar droplet arrays through a normal mode analysis of the dynamical properties in comparison to the supersolid regime. The vibrational patterns of isolated-droplet crystals that time-evolve after a small initial kick closely follow the properties of a linear droplet chain. For larger kick velocities, however, droplets may coalesce and separate again, showing distinct deviations from classicality. In the supersolid regime the normal modes are eliminated by a counterflow of mass between the droplets, signaled by a reduction of the center-of-mass motion. Our study effectively captures the vibrational patterns of isolated droplets in the presence of three-body loss and can be generalized to systems with a larger number of droplet arrays.

The model of continuous spontaneous localization (CSL) is the most prominent consistent modification of quantum mechanics predicting an objective quantum-to-classical transition. Here we show that precision interferometry with Bose-Einstein-condensed atoms can serve to lower the current empirical bound on the localization rate parameter by several orders of magnitude. This works by focusing on the atom count distributions rather than just mean population imbalances in the interferometric signal of squeezed Bose-Einstein condendates, without the need for highly entangled Greenberger-Horne-Zeilinger-like states. In fact, the interplay between CSL-induced diffusion and dispersive atom-atom interactions results in an amplified sensitivity of the condensate to CSL. We discuss experimentally realistic measurement schemes utilizing state-of-the-art experimental techniques to test new regions of parameter space and, pushed to the limit, to probe and potentially rule out large relevant parameter regimes of CSL.

It is shown that the applicability conditions for the inverse Bernoulli transform method to solve the inverse problem of photocount statistics are determined by the fulfillment of the associativity condition for multiplying the matrices included in this transformation. A general criterion for evaluating the photocount distributions $Q_m$ in the case of few-photon light, which makes it possible to establish whether the solution to the inverse problem of photocount statistics by the inverse Bernoulli transform method is applicable for $\eta <0.5$, is found. As an example of the application of the obtained criterion, the critical quantum efficiency ${\eta }_{\text{cr}}$ is found for compound Poisson distribution, below which the solution of the inverse problem of photocount statistics becomes incorrect. Additionally it is shown that the normalization of $Q_m$ is not sufficient to obtain a correct solution using the inverse Bernoulli transform.

Penning traps have been used for performing quantum simulations and sensing with hundreds of ions and provide a promising route toward scaling up trapped ion quantum platforms because of the ability to trap and control hundreds or thousands of ions in two- and three-dimensional crystals. In both Penning traps and the more common radiofrequency Paul traps, lasers are often used to drive multiqubit entangling operations. A leading source of decoherence in these operations is off-resonant spontaneous emission. While many trapped ion quantum computers or simulators utilize clock qubits, other systems, especially those with high magnetic fields such as Penning traps, rely on Zeeman qubits, which require a more complex calculation of this decoherence. We therefore examine theoretically the impacts of spontaneous emission on quantum gates performed with trapped-ion ground-state Zeeman qubits in a high magnetic field. In particular, we consider two types of gates—light-shift $\left({\widehat{\sigma }}_i^z{\widehat{\sigma }}_j^z\right)$ gates and Mølmer-Sørensen $\left({\widehat{\sigma }}_i^x{\widehat{\sigma }}_j^x\right)$ gates—obtained with laser beams directed approximately perpendicular to the magnetic field (the quantization axis) and compare the decoherence errors in each. Within each gate type, we also compare different operating points with regard to the detunings, polarizations, and required intensity of the laser beams used to drive the gates. We show that both gates can have similar performance at their optimal operating conditions at high magnetic fields and examine the experimental feasibility of various operating points. By examining the magnetic field dependence of each gate, we demonstrate that, when the $P$ state fine-structure splitting is large compared to the Zeeman splittings, the theoretical performance of the Mølmer-Sørensen gate is significantly better than that of the light-shift gate. Additionally, for the light-shift gate, we make an approximate comparison between the fidelities that can be achieved at high fields with the fidelities of state-of-the-art two-qubit trapped ion quantum gates. We show that, with regard to spontaneous emission, the achievable infidelity with our current configuration is about an order of magnitude larger than that of the best low-field gates, but we also discuss several alternative configurations with potential error rates that are comparable with those for state-of-the-art trapped ion gates.

We present a theoretical study of the radiative recombination (RR) of heavy highly charged ions with arbitrary (longitudinally and transversely) polarized electrons. In order to investigate how the spin state of incident electrons affects the linear polarization of emitted photons, we apply the density matrix theory and solutions of the relativistic Dirac equation. Explicit analytical expressions are obtained for the dependencies of the differential cross section and the polarization Stokes parameters of RR photons on the components of the incident electrons polarization vector. Detailed calculations of the degree of linear polarization and the tilt angle that characterizes the orientation of the polarization axis are carried out for radiative recombination with bare ${\mathrm{U}}^{92+}$ and ${\mathrm{Xe}}^{54+}$ ions for different polarization states of the incoming electrons. Based on the results of these calculations, we argue that the linear polarization of RR photons is very sensitive to the spin state of an electron target, and this sensitivity can be easily observed with the help of modern Compton polarization detectors.