Motivated by recent experimental advances in bubble traps which confine atoms on a thin shell, we investigate the stabilization of droplet states of a Bose-Bose mixture on a spherical surface due to quantum fluctuations. We analytically calculate the energy of a weakly interacting Bose-Bose mixture on a spherical surface with attractive interspecies and repulsive intraspecies interactions using the Bogoliubov approximation. Our findings show that the energy per particle has a minimum at a finite density in the presence of the Lee-Huang-Yang (LHY) term, which indicates the presence of liquidlike droplet state stabilized by quantum fluctuations. We also numerically solve the extended Gross-Pitaevskii equation, incorporating both bubble traps and an additional repulsive LHY correction derived from the Bogoliubov spectrum of a three-dimensional homogeneous mixture. Our calculations show that, due to the size limitations of the sphere, droplet states can only exist within specific parameter ranges. We also verify the transition between droplet states and uniform condensates in spherical confinement potentials, as well as the coexistence of the two. Our results pave the way for future exploration of quantum droplets in curved geometries with nontrivial real-space topologies.

We present an innovative optical imaging system for measuring parameters of a small particle such as a macromolecule or nanoparticle at the quantum limit of sensitivity. In comparison to the conventional confocal interferometric scattering (iSCAT) approach, our setup adds a second arm to form a Michelson interferometer that allows us to tune a relative phase. We evaluate the quantum Cramér-Rao bound (QCRB) for different quantum states, including single-mode coherent states, multifrequency coherent states, and phase-averaged coherent states. Our results show that the proposed setup can achieve the QCRB of sensitivity and outperform iSCAT for all considered quantum states for the mass and phase estimation of a particle.

Quantum entanglement can be used to extend the baseline of telescope arrays in order to increase the spatial resolution. In one proposal by Marchese and Kok [Phys. Rev. Lett. 130, 160801 (2023)], identical single photons are shared between receivers and interfere with a star photon. In this paper we consider two outstanding questions: (i) what is the precise effect of the low photon occupancy of the mode associated with the starlight? and (ii) what is the effect on the achievable resolution of imperfect indistinguishability (or partial distinguishability) between the ground and star photons? We find that the effect of distinguishability is relatively mild, but low photon occupancy of the optical mode of the starlight quickly deteriorates the sensitivity of the telescope for higher auxiliary photon numbers.

Many-body quantum batteries composed of $N$ two-level systems benefit from collective effects, demonstrating excellent charging performance. In this paper, we consider a charging scheme where an atomic ring array serves as the quantum battery system, coupled to a central charger atom that is pumped by an incoherent thermal source. Assuming no interactions among the atoms on the ring, the system can be described using the Dicke model, where collective coupling results in a significant initial charging power. When dipole-dipole interactions among the atoms are taken into account, the charging power of the battery decreases due to the introduction of intricate collective dissipation. However, the symmetric coherent interactions facilitate the rapid relaxation of the system's dynamics to the eigenstates, thereby increasing the proportion of useful work within the battery. Moreover, we investigate the impact of the ring size and the detuning of the charger atom on the steady-state ergotropy of the quantum battery. Our findings indicate that, at zero detuning, the system exhibits a larger effective charging ring size. As the magnitude of the detuning increases, the effective charging ring size decreases. By analyzing the radiation properties of the system in its steady state, we also explain why the steady-state ergotropy is higher when the charger atom is positively detuned.

Quantum error correction (QEC) plays a critical role in preventing information loss in quantum systems and provides a framework for reliable quantum computation. Identifying quantum codes with nice code parameters for physically motivated noise models remains an interesting challenge. While past work has primarily focused on qubit codes, here we identify a [4,1] qudit error correcting code tailored to protect against amplitude-damping noise. We show that this four-qudit code satisfies the error correction conditions for all single-qudit and a few two-qudit damping errors up to the leading order in the damping parameter $\gamma$. We devise a protocol to extract syndromes that unambiguously identify this set of errors, leading to a noise-adapted recovery scheme that achieves a fidelity loss of $O\left({\gamma }^2\right)$. For the $d=2$ case, our QEC scheme is identical to the known example of the four-qubit code and the associated syndrome-based recovery. We also assess the performance of this code using the Petz recovery map and note some interesting deviations from the qubit case. Finally, we generalize this construction to a family of $[2M+2,M]$ qudit codes that can approximately correct all the single-qudit and a few two-qudit amplitude-damping errors.

Routing classical and quantum information is a fundamental task for quantum information technologies and processes. Here, we consider information encoded in the position of a quantum walker on a graph, and we design an optimal structure to achieve perfect quantum routing, exploiting chirality and weighting of the edges. The topology, termed the lily graph, enables perfect (i.e., with fidelity one) and robust routing of classical (localized) or quantum (superposition) states of the walker to $n$ different, orthogonal, spatial regions of the graph, corresponding to the $n$ possible outputs of the device. The routing time is independent of the input signal and the number of outputs, making our scheme a robust and scalable solution for all quantum networks.

Quantum simulation in its current state faces experimental overhead in terms of physical space and cooling. We propose boson sampling as an alternative compact synthetic platform performing at room temperature. Identifying the capability of estimating matrix permanents, we explore the applicability of boson sampling for tackling the dynamics of quantum systems without having access to information about the full state vector. By mapping the time-evolution unitary of a Hamiltonian onto an interferometer via continuous-variable gate decompositions, we present proof-of-principle results of localization characteristics of a single particle. We study the dynamics of one-dimensional tight-binding systems in the clean and quasiperiodic-disordered limits to observe Bloch oscillations and dynamical localization, and the delocalization-to-localization phase transition in the Aubry-André-Harper model, respectively. Our computational results obtained using boson sampling are in complete agreement with the dynamical and static results of noninteracting tight-binding systems obtained using conventional numerical calculations. Additionally, our study highlights the role of the number of sampling measurements or “shots” for simulation accuracy.

The coherent superposition of two current-carrying electronic states can form a chiral state in an atom. We study strong-field ionization of such a chiral electronic state in a few-cycle carrier-envelope-phase-stabilized circularly polarized laser field with an attoclock configuration by numerically solving the three-dimensional time-dependent Schrödinger equation. We find that the photoelectron momentum along the laser propagation direction reveals a high sensitivity to the photoelectron emission angle in the laser polarization plane. The time-to-angle mapping relation of the attoclock allows us to obtain a time-resolved photoelectron circular dichroism spectrum on the attosecond timescale. We reconstruct the temporal evolution of the chiral bound wave packet from the time-resolved photoelectron circular dichroism spectrum, which shows the phenomenon of the electronic chirality flips occurring on the attosecond timescale.

We theoretically explore the precise control of a molecular polariton by strongly coupling the lowest three rotational states of a single molecule with a single-mode cavity. We examine two distinct cavity resonance configurations: a fundamental frequency cavity $({\omega }_c=2B$ with the rotational constant $B)$ resonating with the lowest two rotational states, and a second harmonic cavity $({\omega }_c=4B)$ coupling with the first and second excited rotational states. We propose two control schemes based on the two polariton configurations and derive the corresponding pulse-area theorems to achieve a theoretical maximum orientation of 0.7746, identical to the molecule without the cavity. The control schemes are analyzed in carbonyl sulfide (OCS) molecules in their ground rotational state. Our numerical simulation results demonstrate the theoretical control schemes and analyze the sensitivity of the molecular polariton orientation degree to the control field bandwidth and phases. This work provides a valuable reference for achieving maximum field-free orientation of ultracold three-state molecules in a cavity using analytically designed pulses.

We introduce an application of the Hartmann sensor, traditionally designed for wavefront sensing, to measure the coherence properties of optical signals. By drawing an analogy between the coherence matrix and the density matrix of a quantum system, we recast the sensor operation as a quantum estimation problem. We experimentally demonstrate its effectiveness in the regime where signals from different apertures significantly overlap, enabling information extraction beyond the reach of standard wavefront sensing.

Resonances in the form of quasinormal modes (QNMs) for open scattering systems can be generally identified in the far field through peaks of scattering spectra (e.g. cross sections of scattering, extinction, and absorption). Nevertheless, when the resonant frequencies of different QNMs are spectrally overlapped or sufficiently close, the scattering peaks merge and it then becomes extremely challenging to reveal the mode constituents underlying the scattering spectra solely in the far field. Here in this paper, we study open systems scattering electromagnetic plane waves, and reveal that spectrally close or even overlapped QNMs can be selectively excited through tuning incident directions and polarizations. Such a far-field technique can be further applied to categorize the nature of degenerate QNMs sharing identical complex eigenfrequencies (coalescent eigenvalues): at effectively Hermitian degeneracies (conical or Dirac points), the eigenvectors are not coalescent and thus the QNMs can still be selectively excited, producing distinct scattering patterns, while for non-Hermitian degeneracies (exceptional points) eigenvectors also coalesce and thus selective QNM excitation does not exist, leading to invariant scattering patterns. Our technique sheds light on the borderlands of Mie theory, QNMs, non-Hermitian photonics, and singular optics, which can empower new explorations and applications and cross-fertilize all those disciplines.

We propose schemes to design and control a time-dependent decoherence-free subspace (DFS) in a dissipative atom-cavity system. These schemes use atoms with three internal energy levels, which allows for the DFS to be multidimensional—a condition important for quantum sensing, simulation, and computation. We consider the use of tunable external driving lasers to transfer the system from a coherent spin state to a highly degenerate DFS. We find that the typical state in the DFS is highly entangled. Throughout evolution the state is kept in an instantaneous DFS, thereby allowing for pure states to be prepared. We develop adiabatic shortcuts to carry out this evolution with higher purity and fidelity than standard adiabatic and dissipative methods.

We investigate the emergence and properties of long-distance genuine multipartite entanglement, induced via three localized magnetic defects, in a one-dimensional transverse-field $XX$ spin-$1/2$ chain. Using both analytical and numerical techniques, we determine the conditions for the existence of bound states localized at the defects. We find that the reduced density matrix (RDM) of the defects exhibits long-distance genuine multipartite entanglement (GME) across the whole range of the Hamiltonian parameter space, including regions where the two-qubit concurrence is zero. We quantify the entanglement by using numerical lower bounds for the GME concurrence, as well as by analytically deriving the GME concurrence in regions where the RDM is of rank 2. Our work provides insights into generating multipartite entanglement in many-body quantum systems via local control techniques.

We develop a numerical methodology for the computation of entanglement measures for mixed quantum states. Using the well-known Schrödinger-Hughston-Jozsa-Wootters theorem, the computation of convex-roof entanglement measures is reframed as a search for unitary matrices: a nonconvex optimization problem. To address this nonconvexity, we modify a genetic algorithm, known in the literature as differential evolution, constraining the search space to unitary matrices by using a QR factorization. We then refine results using a quasi-Newton method. We benchmark our method on simple test problems and, as an application, compute entanglement between a system and its environment over time for pure dephasing evolutions. We also study the temperature dependence of Gibbs state entanglement for a class of block-diagonal Hamiltonians to provide a complementary test scenario with a set of entangled states that are qualitatively different. We find that the method works well enough to reliably reproduce entanglement curves, even for comparatively large systems. The modified genetic algorithm is a derivative-free, nonconvex computational method that broadly applies to the computation of convex-roof entanglement measures.

We propose an efficient quantum state tomography method inspired by compressed sensing and threshold quantum state tomography that can drastically reduce the number of measurement settings to reconstruct the density matrix of an $N$-qudit system. We validate our algorithm with demonstrations on IBMQ and show the efficient and accurate reconstruction of $N\le 7$ qubit systems, reproducing GHZ, $\mathrm{W}$, and random states with $O\left(1\right)$, $O\left(N^2\right)$, and $O\left(N\right)$ settings.

Constructing Majorana zero modes (MZMs) is one of the most promising schemes for achieving topological quantum computing. However, recent research primarily focuses on preparing stable materials that carry MZMs and detecting their existence. Further combining structures that carry MZMs for quantum computing would facilitate the synergistic development of material fabrication and quantum computing. Here, we take the T-junction structure carrying MZMs as the basic unit to construct a two-dimensional (2D) grid structure. Based on this structure, the protocols for performing braiding operations for single-qubit and two-qubit gates are defined. Furthermore, the Deutsch-Jozsa (D-J) algorithm is derived to demonstrate the vast application potential of the 2D grid structure in more complex quantum algorithms.

We propose a method for the estimation of the initial polarization of spinful nuclei of the ${}^{13}\mathrm{C}$ isotope in diamond via a measurement of the evolution of the coherence of a nitrogen-vacancy-center spin qubit. Existing polarization measurement methods are difficult to implement experimentally because they require direct interference in the environment of the qubit. Here, in order to obtain the information, it is necessary to measure the qubit coherence at certain points of time, which are unambiguously determined by the applied magnetic field. For sufficiently high magnetic fields, the minimum value of the measured coherence constitutes an upper bound on the product of the initial polarizations of each environmental spin. The most significant advantage of the method, which allows to infer initial values of nuclear polarizations without any direct access to the environment, lies in its simplicity and the small amount of experimental resources that it requires. We exemplify the operation of the scheme on a realistic, randomly generated environment of eight nuclear spins, obtaining a reasonably accurate estimation of the initial polarization.

Dysprosium atoms have proven to be a promising platform for quantum simulation due to their strong magnetic moment and high tunability of interactions. In this work, we propose dysprosium atoms for simulating the one-dimensional spin-1 XXZ Heisenberg model, which is known to have a rich phase diagram including the famous Haldane phase. To realize the model, we make use of the strong dipolar exchange interactions that naturally occur in the ground state of dysprosium due to its large electron angular momentum of $J$ = 8. To implement spin-1 particles, we encode the spin degree of freedom into three Zeeman sublevels, which are energetically isolated by applying a magnetic field. Using the density-matrix renormalization group, we analyze the ground-state properties of the resulting effective model. We find that a chain of fermionic dysprosium atoms in a suitable magnetic field can form a Haldane state with the characteristic spin-1/2 edge modes. Furthermore, we discuss the use of AC Stark shifts and Raman-type schemes for bosonic dysprosium to isolate effective spin-1 systems and to increase the tunability of model parameters.

We demonstrate a way to calculate photoelectron spectra on a numerical domain much smaller than the actual extension of the wave function. Specifically, a hydrogen atom exposed to a laser pulse in the moderately ultraviolet region is studied numerically within the dipole approximation. Outgoing waves are absorbed by means of a complex absorbing potential. The corresponding photoelectron spectra are calculated by aggregating the absorbed part of the outgoing particle. Via a density operator formulation and projections onto time-independent scattering states, we obtain convergent spectra using numerical domains for which a significant part of the unbound wave is absorbed during interaction with the time-dependent field. We compare this numerical approach with methods based on approximating time-dependent scattering states by Volkov states, which do not encompass the long-range nature of the Coulomb potential. This comparison demonstrates how the present method, which was introduced by Selstø [Phys. Rev. A 106, 042213 (2022)], is able to account for Coulomb corrections to the photoelectron spectra.

The existence of subpeak structures between the Autler-Townes (AT) doublet of the photoelectron spectrum during resonant ionization of an atom has been predicted in a large number of theoretical calculations, but a convincing physical explanation is still lacking. In this study we establish a temporal interference picture to elucidate the physical origin of these subpeaks. We derive an analytical estimation for the photoelectron spectra based on a three-level model, which agrees well with the numerical solution of the time-dependent Schrödinger equation at moderately strong pulse intensities. Our interpretation suggests that the interference of the photoelectron wave packet emitted during each Rabi cycle results in the subpeak structure between the AT doublet. By analyzing the phase difference between two temporally adjacent photoelectron wave packets, we successfully predict the interference minima in the photoelectron energy spectra.