We propose an all-static method to realize an artificial magnetic field for charge neutral particles without introducing any time modulation. Our proposal consists of one-dimensional tubes subject to harmonic trapping potentials with shifted centers. We show that this setup realizes an artificial magnetic field in a hybrid real-momentum space. We discuss how characteristic features of particles in a magnetic field, such as chiral edge states and the quantized Hall response, can be observed in this setup. We find that the mean-field ground state of bosons in this setup in the presence of long-range interactions in physical real space can have quantized vortices in the hybrid real-momentum space; such a state with vortices exhibits a supersolid structure in the physical real space. Our method can be applied to a variety of synthetic quantum matter, including ultracold atomic gases, coupled photonic cavities, coupled waveguides, and exciton-polariton lattices.

Recently the authors of Phys. Rev. Lett. 125, 090401 (2020) considered the following scenario: Alice and Bob each have half of a pair of entangled qubit states. Bob measures his half and then passes his part to a second Bob who measures again, and so on. The goal is to maximize the number of Bobs that can have an expected violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality with the single Alice. By taking the maximally entangled pure two-qubit state $|\varphi \rangle =\frac{1}{\sqrt{2}}\left(\right|00\rangle +|11\rangle )$ as an example, it has been constructively proved that arbitrarily many independent Bobs can share the nonlocality with the single Alice. Here we demonstrate that arbitrarily many independent observers can share the nonlocality of a single arbitrary dimensional bipartite entangled but not necessary two-qubit entangled state. Furthermore, taking the generalized Greenberger-Horne-Zeilinger (GHZ) states as an example, we show that at most two Charlies can share the genuine nonlocality of a single generalized GHZ state with an Alice and a Bob.

We simulate a system of coupled kicked tops to generate random density matrices distributed according to the Bures-Hall measure, which has an important role in quantum information theory. We study the effects of stochasticity and coupling parameters of the coupled-kicked-top system on the behavior of the associated bipartite pure state, eigenvalues and eigenvectors of the reduced state, and average entropies. For strongly chaotic phase space and adequate coupling between the constituting tops (subsystems), we find that the results of simulation agree with analytical results of random matrix theory. We also examine local fluctuation properties of the Bures-Hall eigenvalues using the distribution of nearest-neighbor spacing ratios. In the limit of high-dimensional density matrices, the empirical ratio distribution is found to approach the Wigner-surmise-like result for the Gaussian unitary ensemble. Additionally, we present some closed-form results for the Bures-Hall spectral density and corresponding moments using Meijer $G$ functions.

Resource theories provide a general framework for the characterization of properties of physical systems in quantum mechanics and beyond. Here we introduce methods for the quantification of resources in general probabilistic theories (GPTs), focusing in particular on the technical issues associated with infinite-dimensional state spaces. We define a universal resource quantifier based on the robustness measure, and show it to admit a direct operational meaning: in any GPT, it quantifies the advantage that a given resource state enables in channel discrimination tasks over all resourceless states. We show that the robustness acts as a faithful and strongly monotonic measure in any resource theory described by a convex and closed set of free states, and can be computed through a convex conic optimization problem. Specializing to continuous-variable quantum mechanics, we obtain additional bounds and relations, allowing an efficient computation of the measure and comparison with other monotones. We demonstrate applications of the robustness to several resources of physical relevance: optical nonclassicality, entanglement, genuine non-Gaussianity, and coherence. In particular, we establish exact expressions for various classes of states, including Fock states and squeezed states in the resource theory of nonclassicality and general pure states in the resource theory of entanglement, as well as tight bounds applicable in general cases.

The rate at which quantum communication tasks can be performed using direct transmission is fundamentally hindered by the channel loss. Quantum repeaters allow one, in principle, to overcome these limitations, but their introduction necessarily adds an additional layer of complexity to the distribution of entanglement. This additional complexity—along with the stochastic nature of processes such as entanglement generation, Bell swaps, and entanglement distillation—makes finding good quantum repeater schemes nontrivial. We develop an algorithm that can efficiently perform a heuristic optimization over a subset of quantum repeater schemes for general repeater platforms. We find a strong improvement in the generation rate in comparison to an optimization over a simpler class of repeater schemes based on BDCZ (Briegel, Dür, Cirac, Zoller) repeater schemes. We use the algorithm to study three different experimental quantum repeater implementations on their ability to distribute entanglement, which we dub information processing implementations, multiplexed elementary pair generation implementations, and combinations of the two. We perform this heuristic optimization of repeater schemes for each of these implementations for a wide range of parameters and different experimental settings. This allows us to make estimates on what are the most critical parameters to improve for entanglement generation, how many repeaters to use, and which implementations perform best in their ability to generate entanglement.

We report the fluorescence emission which is driven by femtosecond laser pulses with a repetition rate of 100 MHz and a center wavelength of 1040 nm in a nitrogen gas jet. The experiment is performed in a femtosecond enhancement cavity coupled with high repetition rate laser. In contrast to previous observation at low repetition rate with a nitrogen gas jet, where the 391-nm radiation was observed but the 337-nm emission was missing, the 337-nm emission is three times stronger than the 391-nm emission in our experiment. By examining the dependence of the radiation intensity on the flow rate of the nitrogen gas and the polarization of the pump pulse, the formation mechanism of the ${\mathrm{N}}_2\left(C{}^3\mathrm{\Pi }_u\right)$ triplet excited state, i.e., the upper state of the 337-nm emission, is investigated. We attribute the main excitation process to the inelastic collision excitation process and exclude the possibility of the dissociative recombination as the dominate pathway. The role of the steady-state plasma that is generated under our experimental conditions is also discussed.

We report an observation of the subnatural-linewidth spectroscopy in cold ${}^6\mathrm{Li}$ atoms by combining the interference contrast method and the time-delayed coincidence technique. The measured linewidth of $D_1$ lines is as narrow as 3.6 MHz, which is well below its natural linewidth of 5.8 MHz. The hyperfine splitting of the $2P_{1/2}$ state is measured as 26.112(20) MHz with this subnatural-linewidth spectroscopy. The transitions of the $D_2$ lines are partially resolved by using the developed technique. The time-delayed coincidence technique based on the interference contrast method in cold atoms provides a route for robust fluorescence measurements and determination of unresolved transitions.

Based on the strong-field approximation (SFA), we investigate the interference between different returning orbits of the rescattering electron in nonsequential double ionization. We find that the effect of interference is negligible in the recollision-impact ionization process but is prominent in the recollision excitation with subsequent ionization process and induces fast oscillation with constant period in the laser intensity dependence of the asymmetry parameter. The dominant contribution to the fast oscillation comes from the mixed interference between different pairs of returning orbits in different excitation channels. However, the oscillation disappears after laser focus averaging is performed due to its rather small period.

We characterize interisotope interorbital interactions between atoms in the ${}^{1}S_0$ ground state and the ${}^{3}P_0$ metastable state in interacting Fermi-Fermi mixtures of ${}^{171}\mathrm{Yb}$ and ${}^{173}\mathrm{Yb}$. We perform high-precision clock spectroscopy to measure interaction-induced energy shifts in a deep three-dimensional optical lattice and determine the corresponding scattering lengths. We find the elastic interaction of the interisotope mixtures ${}^{173}\mathrm{Yb}_{\mathrm{e}}\text{−}{}^{171}\mathrm{Yb}_{\mathrm{g}}$ and ${}^{173}\mathrm{Yb}_{\mathrm{g}}\text{−}{}^{171}\mathrm{Yb}_{\mathrm{e}}$ to be weakly attractive and very similar, while the corresponding two-body decay coefficients differ by more than two orders of magnitude. By comparing different spin mixtures we experimentally demonstrate the $\mathrm{SU}\left(2\right)\otimes \mathrm{SU}\left(6\right)$ symmetry of the elastic interactions. Furthermore, we measure the interorbital spin-exchange interaction in ${}^{171}\mathrm{Yb}$ and confirm its previously observed antiferromagnetic nature.

Topological nodal rings as the simplest topological nodal lines recently have been extensively studied in optical lattices. However, the realization of complex nodal line structures like nodal chains in this system remains a crucial challenge. Here we propose an experimental scheme to realize and detect topological nodal chains in optical Raman lattices. Specifically, we construct a three-dimensional optical Raman lattice which supports next-nearest-neighbor spin-orbit couplings and hosts topological nodal chains in its energy spectra. Interestingly, the nodal chains realized are protected by mirror symmetry and could be tuned into a large variety of shapes, including the inner and outer nodal chains. We also demonstrate that the shapes of the nodal chains could be detected by measuring spin polarizations. Our study opens up the possibility of exploring topological nodal-chain semimetal phases in optical lattices.

We present a detailed study of the fields and propagation characteristics around the focus of ultrashort radially polarized laser beams (RPLBs) having low-order spatiotemporal couplings (STCs). The three STCs considered are the focusing of the different frequencies to different positions along the longitudinal coordinate, the focusing of the frequencies to different positions along one transverse coordinate, and the beam waist or Rayleigh range having a dependence on frequency. The STCs considered are deemed low-order because they are primarily linear in frequency. The combination of a low-order vector beam, ultrashort pulse duration, and the three different STCs shows promise for exotic applications in dielectric or charged particle manipulation and potentially other strong-field phenomena. The STCs presented are all developed in a standard frequency-domain model where each case involves a different chromatic term. We present the results unique to the vector nature of the RPLBs and compare them to the linearly polarized cases, opening up opportunities for control of the electric field around the focus with the additional element of polarization.

We present a topological approach to the input-output relations of photonic driven-dissipative systems acting as directional amplifiers. Our theory relies on a mapping from the optical non-Hermitian coupling matrix to an effective topological insulator Hamiltonian. This mapping is based on the singular value decomposition of non-Hermitian coupling matrices, the inverse matrix of which determines the linear input-output response of the system. In topologically nontrivial regimes, the input-output response of the lattice is dominated by singular vectors with zero singular values that are the equivalent of zero energy states in topological insulators, leading to directional amplification of a coherent input signal. In such topological amplification regime, our theoretical framework allows us to fully characterize the amplification properties of the quantum device such as gain, bandwidth, added noise, and noise-to-signal ratio. We exemplify our ideas in a one-dimensional nonreciprocal photonic lattice, for which we derive fully analytical predictions. We show that the directional amplification is near quantum limited with a gain growing exponentially with system size $N$, while the noise-to-signal ratio is suppressed as $1/\sqrt{N}$. This suggests interesting applications of our theory for quantum signal amplification and single-photon detection.

A coherent coupling among different energy photons provided by nonlinear optical interaction is regarded as a photonic version of the Rabi oscillation. Cavity enhancement of the nonlinearity reduces an energy requirement significantly and pushes the scalability of the frequency-encoded photonic circuit based on the photonic Rabi oscillation. However, the confinement of the photons in the cavity severely limits the number of interactable frequency modes. Here we demonstrate a wide-bandwidth and efficient photonic Rabi oscillation achieving full-cycle oscillation based on a cavity-enhanced nonlinear optical interaction with a monolithic integration. We also show its versatile manipulation beyond the frequency degree of freedom such as an all-optical control for polarizing photons with geometric phase. Our results will open up full control accessible to synthetic dimensional photonic systems over wide frequency modes as well as a large-scale photonic quantum information processing.

This Letter presents a nonclassical mechanism of two-atom interferometry. The two-atom interferometer introduces two different yet indistinguishable alternatives for randomly paired atoms to create a joint atom-detection event. The superposition of two-atom amplitudes yields two-atom interference with peculiar features which outshine the classic atom interferometer: (1) two-atom interference is still observable when the time delay of the interferometer is greater than the coherence time of the atom beam and (2) two-atom interference may eliminate phase noises, including background, variation, turbulence, and Raman laser induced phase noises, thereby allowing for higher sensitivity and stability sensing than is achievable by classic atom interferometers. These features are crucial in high sensitivity-accuracy acceleration and rotation measurements. The presented concept and mechanism of two-atom interference can be adapted to other matter-wave interferometers, such as two-neutron interferometer and two-electron interferometer.

The dynamics of a quantum system following a sudden, highly nonadiabatic change of its control parameter (quantum quench) is studied with quasiclassical techniques. Recent works have shown, using exact quantum mechanical approach, that equilibration after quantum quench exhibits specific features in the presence of excited-state quantum phase transitions. In this paper, we demonstrate that these features can be understood from the classical evolution of the Wigner function in phase space.

We iteratively apply a recently formulated adiabatic theorem for the strong-coupling limit in finite-dimensional closed and open quantum systems. This allows us to improve approximations to a perturbed dynamics, beyond the standard approximation based on quantum Zeno dynamics and adiabatic elimination. The effective generators describing the approximate evolutions are endowed with the same block structure as the unperturbed part of the generator, and exhibit adiabatic evolutions. This iterative adiabatic theorem reveals that adiabaticity holds eternally, that is, the system evolves within each eigenspace of the unperturbed part of the generator, with an error bounded by $O(1/\gamma )$ uniformly in time, where $\gamma$ is the strength of the unperturbed part of the generator. We prove that the iterative adiabatic theorem reproduces Bloch's perturbation theory in the unitary case, and is therefore a full generalization to open systems. We furthermore prove the equivalence of the Schrieffer-Wolff and des Cloizeaux approaches in the unitary case and generalize both to arbitrary open systems, showing that they share the eternal adiabaticity, and providing explicit error bounds. Finally we discuss the physical structure of the effective adiabatic generators and show that ideal effective generators for open systems do not exist in general.

We define a property called nondegeneracy for Bell inequalities, which describes the situation that in a Bell setting, if a Bell inequality and involved local measurements are fixed, any quantum state with a given dimension and its orthogonal quantum state cannot violate remarkably the inequality simultaneously. By choosing a proper nondegenerate Bell inequality, we prove that for an unknown bipartite quantum state of given dimension, based on the measurement statistics only, we can provide an analytic lower bound for the entanglement of formation or even the distillable entanglement, making the whole process semi device independent. We characterize the mathematical structure of nondegeneracy, and prove that quite a lot of well-known Bell inequalities are nondegenerate. We demonstrate our approach by quantifying entanglement for qutrit-qutrit states based on their violations of the Collins-Gisin-Linden-Masser-Popescu inequality.

Macrorealistic description governing the classical realm is known to get uprooted in the quantum domain. The Leggett-Garg inequality [A. J. Leggett and A. Garg, Phys. Rev. Lett. 54, 857 (1985)] proposes to probe the macrorealistic limit emerging from the quantum scenario. It places a bound on the linear combinations of temporal correlations of observables measured sequentially at different time intervals. Violation of the Leggett-Garg inequality, up to the so-called temporal Tsirelson bound (TTB), has been realized in the quantum domain. It is important to understand whether quantum theory permits larger violation beyond the TTB and up to the algebraic maximum value of the Leggett-Garg inequality. To this end, it has been shown [C. Budroni and C. Emary, Phys. Rev. Lett. 113, 050401 (2014)] that a three-term Leggett-Garg inequality can be violated beyond the TTB $3/2$ in the time evolution of $N>2$ level quantum systems by choosing suitable measurement schemes and state-update rules. Furthermore, it is revealed that the algebraic maximum value 3 can be realized in the limit $N\to \infty$. Their measurement scheme rules out violation of the Leggett-Garg inequality beyond the TTB by a qubit. Being genuinely quantum in nature, we ask whether such enhanced violation of the Leggett-Garg inequality could be witnessed in a qubit itself. Here, we show that a qubit undergoing time evolution generated by a non-Hermitian $\mathcal{P}\mathcal{T}$-symmetric Hamiltonian indeed violates the three-term Leggett-Garg inequality beyond the TTB and up to its algebraic maximum value 3.

We propose an efficient protocol to fully reconstruct a set of high-fidelity quantum gates. Usually, the efficiency of reconstructing high-fidelity quantum gates is limited by the sampling noise. Our protocol is based on a perturbative approach and has two stages. In the first stage, the unital part of noisy quantum gates is reconstructed by measuring traces of maps, and the trace can be measured by amplifying the noise in a way similar to randomized benchmarking and quantum spectral tomography. In the second stage, by amplifying the nonunital part using the unital part, we can efficiently reconstruct the nonunital part. We show that the number of measurements needed in our protocol scales logarithmically with the error rate of gates.

We present a method to approximate partition functions of quantum systems using mixed-state quantum computation. For positive-semidefinite Hamiltonians, our method has an expected running-time that is almost linear in ${[M/\left({\epsilon }_{\mathrm{rel}}\mathcal{Z}\right)]}^2$, where $M$ is the dimension of the quantum system, $\mathcal{Z}$ is the partition function, and ${\epsilon }_{\mathrm{rel}}$ is the relative precision. It is based on approximations of the exponential operator as linear combinations of certain operators related to block-encoding of Hamiltonians or Hamiltonian evolutions. The trace of each operator is estimated using a standard algorithm in the one-clean-qubit model. For large values of $\mathcal{Z}$, our method may run faster than exact classical methods, whose complexities are polynomial in $M$. We also prove that a version of the partition function estimation problem within additive error is complete for the so-called DQC1 complexity class, suggesting that our method provides a superpolynomial speedup for certain parameter values. To attain a desired relative precision, we develop a classical procedure based on a sequence of approximations within predetermined additive errors that may be of independent interest.