Quantum spin liquids (QSLs) define an exotic class of quantum ground states where spins are disordered down to zero temperature. We propose routes to QSLs in kagome optical lattices using applied flux. An optical flux lattice can be applied to induce a uniform flux and chiral three-spin interactions that drive the formation of a gapped chiral spin liquid. A different approach based on recent experiments using laser-assisted tunneling and lattice tilt implements a staggered flux pattern which can drive a gapless spin liquid with symmetry protected nodal lines. Our proposals, therefore, establish kagome optical lattices with effective flux as a powerful platform for exploration of QSLs.

The two-photon Hong-Ou-Mandel (HOM) interference is a pure quantum effect which indicates the degree of indistinguishability of photons. The four-photon HOM interference exhibits richer dynamics in comparison to the two-photon interference and simultaneously is more sensitive to the input photon states. We demonstrate theoretically and experimentally an explicit dependency of the four-photon interference on the number of temporal modes, created in the process of parametric down-conversion. Moreover, we exploit the splitting ratio of the beam splitter to manipulate the interference between bunching and antibunching. Our results reveal that the temporal mode structure (multimodeness) of the quantum states shapes many-particle interference.

The propagation of laser pulses in multicore fibers (MCFs) made of a central core and an even number of cores located in a ring around it is studied. Approximate quasisoliton homogeneous solutions of the wave field in the MCF considered are found. The stability of the in-phase soliton distribution is shown analytically and numerically. At low energies, its wave field is distributed over all MCF cores and has a duration that exceeds the duration of the nonlinear Schrödinger equation (NSE) soliton with the same energy by many (i.e., five to six) times. In contrast, almost all of the radiation at high energies is concentrated in the central core with a duration similar to the NSE soliton. The transition between the two types of distributions is very sharp and occurs at a critical energy, which is weakly dependent on the number of cores and on the coupling coefficient with the central core. The self-compression mechanism of laser pulses was proposed. It consists in injecting such MCFs with a wave packet being similar to the found soliton and having an energy larger than the critical value. It is shown that the compression ratio depends weakly on the energy and the number of cores and is approximately equal to six times with an energy efficiency of almost 100%. The use of longer laser pulses allows one to increase the compression ratio up to 30–40 times with an energy efficiency of more than 50%. The obtained analytical estimates of the compression ratio and its efficiency are in good agreement with the results of a numerical simulation.

We introduce thermal-difference states (TDSs), a three-parameter family of single-mode non-Gaussian bosonic states whose density operator is a weighted difference of two thermal states. We show that the states of “heralded photons” generated via parametric down-conversion (PDC) are precisely those among the TDSs that are nonclassical, meaning they have a negative $P$ function. The three parameters correspond in that context to the initial brightness of PDC and the transmittances, characterizing the linear loss in the signal and the idler channels. At low initial brightness and unit transmittances, the heralded photon state is known to be a single-photon state. We explore the influence of brightness and linear loss on the heralded state of the signal mode. In particular, we analyze the influence of the initial brightness and the loss on the state nonclassicality by computing several measures of nonclassicality, such as the negative volume of the Wigner function, the sum of quantum Fisher information for two quadratures, and the ordering sensitivity, introduced recently by us [S. De Bièvre, D. B. Horoshko, G. Patera, and M. I. Kolobov, Phys. Rev. Lett. 122, 080402 (2019)]. We finally argue that the TDSs provide benchmark states for the analysis of a variety of properties of single-mode bosonic states.

It is shown that the Fizeau drag can cause nonreciprocity. We propose the use of a nanostructured ring cavity made of ${\chi }^{\left(2\right)}$ nonlinear materials to achieve a nonreciprocal photon blockade (PB) through the Fizeau drag. Under the weak driving condition, we discuss the PB phenomena for the fundamental mode and the second-harmonic mode. We find that for the fundamental mode the PB happens when we drive the system from one side but disappears when we drive the system from the other side. However, there is no such phenomenon for the second-harmonic mode. Remarkably, the PB phenomenon occurs with a reasonably small optical nonlinearity, thus bringing the system parameters closer to the reasonably achievable realm by the current technology.

We introduce a class of partially coherent surface plasmon polariton (SPP) fields carrying optical vortices, generated through a judicious superposition of planar SPPs with a prescribed initial phase distribution and arbitrary correlations at a metal-air interface. We explore the global degree of coherence, energy densities, energy flows, orbital and spin angular momenta, and the polarization states of such partially coherent SPP vortex (SPPV) fields in terms of their coherent mode representation. The salient physical characteristics of these SPPV fields can be widely tuned by controlling the constituent SPP correlations, tailoring such SPPV fields to desired applications in statistical nanoplasmonics.

We study the trade-off relations satisfied by the genuine tripartite nonlocality in multipartite quantum systems. From the reduced three-qubit density matrices of the four-qubit generalized Greenberger-Horne-Zeilinger (GHZ) states and W states (4-qubit entangled state), we find that there exists a trade-off relation among the mean values of the Svetlichny operators associated with these reduced states. Namely, the genuine three-qubit nonlocalities are not independent. For four-qubit generalized GHZ states and W states, the summation of all their three-qubit maximal (squared) mean values of the Svetlichny operator has an upper bound. This bound is better than the one derived from the upper bounds of individual three-qubit mean values of the Svetlichny operator. Detailed examples are presented to illustrate the trade-off relation among the three-qubit nonlocalities.

We investigate the competition of coherent and dissipative dynamics in many-body systems at continuous quantum transitions. We consider dissipative mechanisms that can be effectively described by Lindblad equations for the density matrix of the system. The interplay between the critical coherent dynamics and dissipation is addressed within a dynamic finite-size scaling framework, which allows us to identify the regime where they develop a nontrivial competition. We analyze protocols that start from critical many-body ground states and put forward general dynamic scaling behaviors involving the Hamiltonian parameters and the coupling associated with the dissipation. This scaling scenario is supported by a numerical study of the dynamic behavior of a one-dimensional lattice fermion gas undergoing a quantum Ising transition in the presence of dissipative mechanisms such as local pumping, decaying, and dephasing.

We use quantum detector tomography to characterize the qubit readout in terms of measurement positive operator-valued measures (POVMs) on IBM quantum computers IBM Q 5 Tenerife and IBM Q 5 Yorktown. Our results suggest that the characterized detector model deviates from the ideal projectors, ranging from 10 to 40%. This is mostly dominated by classical errors, evident from the shrinkage of arrows from the poles in the corresponding Bloch-vector representations. There are also small deviations that are not “classical,” of order 3% or less, represented by the tilt of the arrows from the $z$ axis. Further improvement on this characterization can be made by adopting two- or more-qubit detector models instead of independent single-qubit detectors for all the qubits in one device. We also find evidence indicating correlations in the detector behavior, i.e., the detector characterization is slightly altered (to a few percent) when other qubits and their detectors are in operation. Such peculiar behavior is consistent with characterization from the more sophisticated approach of the gate set tomography. We also discuss how the characterized detectors' POVMs, despite deviation from the ideal projectors, can be used to estimate the ideal detection distribution.

The resource theory of quantum coherence originated like entanglement in quantum information theory. However, until now proper classification of quantum states is missing under coherence. In this work we have provided a classification of states under local incoherent operations. We have succeeded in deriving the necessary and sufficient condition for which two pure multipartite states are equivalent under stochastic local incoherent operations and classical communications (SLICC) and local incoherent operations and classical communications (LICC). In particular, we have succeeded in characterizing three-qubit pure states under SLICC. Our result reveals the existence of an infinite number of SLICC inequivalent classes for three-qubit systems.

Three graph invariants are introduced which may be measured from a quantum graph state and form examples of a framework under which other graph invariants can be constructed. Each invariant is based on distinguishing a different number of qubits. This is done by applying different measurements to the qubits to be distinguished. The performance of these invariants is evaluated and compared to classical invariants. We verify that the invariants can distinguish all nonisomorphic graphs with nine or fewer nodes. The invariants have also been applied to “classically hard” strongly regular graphs, successfully distinguishing all strongly regular graphs of up to 29 nodes, and preliminarily to weighted graphs. We have found that, although it is possible to prepare states with a polynomial number of operations, the average number of preparations required to distinguish nonisomorphic graph states scales exponentially with the number of nodes. We have so far been unable to find operators which reliably compare graphs and reduce the required number of preparations to feasible levels.

Energies and Auger widths of the $LL$ resonances in He-like ions from boron to argon are evaluated by means of a complex-scaled configuration-interaction approach within the framework of the Dirac-Coulomb-Breit Hamiltonian. The nuclear recoil and QED corrections are also taken into account. The results obtained are compared with other calculations based on the complex-scaling method as well as with the related results evaluated by using the stabilization and basis balancing methods.

Optical lattice systems provide exceptional platforms for quantum simulation of many-body systems. We focus on the doubly modulated Bose-Hubbard model driven by both time-dependent on-site energy and interaction, and we predict the emergence of the nearest-neighbor interaction and density assisted tunneling. By specifically designing a bichromatic driving pattern for a one-dimensional lattice, we demonstrate that the doubly modulated fields can be tuned to realize desired quantum phases, e.g., the Mott insulator phase with selective defects and the density wave phase.

We study ground-state quantum entanglement in the one-dimensional Bose-Hubbard model in the presence of a harmonic trap. We focus on two transitions that occur upon increasing the characteristic particle density: the formation of a Mott-insulating domain with site occupation one at the center of the trap (lower transition) and the emergence of a superfluid domain at the center of the Mott-insulating one (upper transition). These transitions generate discontinuities in derivatives of the total energy and have been characterized by local (nonextensive) order parameters, so we refer to them as local quantum phase transitions. We show that a second derivative of the total energy is continuous with a kink at the lower transition, and that it is discontinuous at the upper transition. We also show that bipartite entanglement entropies are order parameters for those local quantum phase transitions. We use the density-matrix renormalization group and show that the transition points extracted from entanglement measures agree with the predictions of the local density approximation in the thermodynamic limit. We discuss how to determine the transition points from results in small systems, such as the ones realized in recent optical lattice experiments that measured the second-order Renyi entanglement entropy.

We investigate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and Sarma superfluid states in alkaline-earth-like ${}^{173}\mathrm{Yb}$ atomic gases near an orbital Feshbach resonance at zero temperature with population imbalances in both the open and closed channels. We find that, in uniform space, both the Fulde-Ferrell and Sarma states are greatly enhanced by the spin-exchange interaction on the Bardeen-Cooper-Schrieffer side of the Feshbach resonance. While trapped in a harmonic potential, a cloud of long-lived ${}^{173}\mathrm{Yb}$ atomic gas with small fraction of electronically excited-state population can stabilize not only the Sarma states with both the one and two Fermi surfaces but also the Fulde-Ferrell state, and leave detectable structures in the distribution of polarizations of different bands. As the degenerate ${}^{173}\mathrm{Yb}$ cloud is readily available and the signatures predicted can be easily detected in in situ and/or time-of-flight images, our findings are helpful to realize and detect the long-sought FFLO and Sarma states in experiments.

Abraham forces are defined as electromagnetic forces on neutral objects caused by the presence of slow, time-dependent, homogeneous electromagnetic fields. Only a few experimental observations have been reported, and different formulations exist in literature. The “standard” Abraham force is usually associated with the full derivative ${\partial }_t(\mathbf{E}\times \mathbf{B})$ of homogeneous electric and magnetic fields. We show that this full derivative is wrong for a molecule, since the classical Maxwell theory contains subtle induced-dipole forces that contribute to the Abraham force, which thus breaks the dual symmetry between electric and magnetic components. We investigate other manifestations of Abraham forces and torques and estimate orders of magnitude, e.g., related to chiral, moving, or rotating matter. Some of them are also violating dual symmetry. We provide a quantum description of the Abraham force. Apart from being more elegant and less technical, this quantum treatment paves the way for future studies of Abraham forces on complex molecules involving relativistic concepts such as spin and spin-orbit coupling, and also facilitates the inclusion of the quantum vacuum.

We present a theoretical study on the generation of photon pairs via spontaneous four-wave mixing in a waveguide array supporting topologically protected guided modes. Following a statistical approach, we introduce incremental levels of disorder in the waveguide positions and analyze the effects in terms of photon path correlations and generation efficiency. For moderate level of disorder, we show that the generated two-photon state as well as the generation rate is preserved. However, we find that for higher level of disorder quantum correlations are significantly affected, despite the fact that the linear properties of the topological mode are still protected.

Photonic condensates are complex systems exhibiting phase transitions due to the interaction with their molecular environment. Given the macroscopic number of molecules that form a reservoir of excitations, numerical simulations are expensive, even for systems with few cavity modes. We present a systematic construction of molecular excitation profiles with a clear hierarchical ordering, such that only modes in the lowest order in the hierarchy directly affect the cavity photon dynamics. In addition to a substantial gain in computational efficiency for simulations of photon dynamics, the explicit spatial shape of the mode profiles offers a clear physical insight into the competition among the cavity modes for access to molecular excitations.