We present a detailed theory of the Rydberg blockade, including contributions from multiple intermediate-state excitations. Two fields drive transitions between ground and Rydberg levels via an off-resonance intermediate state. Assuming a perfect blockade, we calculate the probability to excite fully symmetric collective states having either zero or one Rydberg excitation, but an arbitrary number of intermediate-state excitations. Both “bare” state and “dressed” state approaches are used for (1) constant amplitude driving fields and (2) adiabatic pulse driving fields. It is shown that a dressed state approach offers distinct advantages when multiple intermediate-state excitations occur. In the case of fixed amplitude fields, the multiple intermediate excitations can result in comblike modulated populations of individual states having one Rydberg excitation and $n\ll N$ intermediate-state excitations. However, when summed over all such state populations, most of the modulation disappears and the system is described to a good approximation by an effective two-level model. In the case of adiabatic, pulsed fields, there is no such modulation and an effective two-level model (in the dressed basis), corrected for light shifts, can be used to model the system. In addition to solving this problem using conventional methods, we show that similar results could be obtained using a form of the Holstein-Primakoff transformation.

The freedom to define branch cuts of the complex function is used to derive an integral representation of the quantum carpet, thus producing a generalization of the Poincaré recurrence theorem in the case of the continuous spectrum. This approach provides a different way to renormalize resonant states to be both space and time convergent. The coherence of quantum carpets was related to the properties of the Wigner function in the canonical time-frequency phase space. It has been shown that the distortion of the Wigner function shape is directly responsible for the lack of the ability of the dynamics to produce revivals equally as sharp as the initial wave packet.

We achieve the controllable multipartite Einstein-Podolsky-Rosen (EPR) steering and its monogamy relations by means of the dynamical Casimir effect in the frame of superconducting quantum network. We employ the external driving flux of the superconducting quantum interference device as a switch. It is interesting that the system exhibits different types of the EPR steering and monogamy relations by tuning parameters. In the system, the one-way or two-way EPR steerings for different parties have been realized, and the direction of the one-way EPR steering is reversed by tuning coupling parameters. Remarkably, we can also obtain the EPR steering orderly sudden death for different modes. In addition, the system not only shows the type-I and type-II monogamy relations, but also converts steering parties to get monogamy relations' swapping and cycling. Finally, we discuss the influence of temperature. In the system, the higher the temperature is, the earlier sudden death for the EPR steering is. These results are important for quantum information and lay a foundation for us to better understand the important role of the multipartite EPR steering in secure quantum communication.

Quantum network correlations have attracted strong interest because they may reveal new types of locality violations. Here we investigated network nonlocality sharing in the extended bilocal scenario via weak measurements, which adds one more observer to either end of the well-known entanglement-swapping scenario. Intriguingly, network nonlocality sharing can be revealed from the multiple violation of the Branciard-Rosset-Gisin-Pironio inequality by considering two contrasting motivations of intermediate observers, which has no counterpart in standard Bell scenarios. In addition, the noise resistance of network nonlocality sharing was also discussed.

Due to the unconditional security of quantum communication and flexible and economical implementation of wireless communication, quantum wireless networks (QWNs) have attracted wide attention. Different from fiber-based or satellite-based quantum communication, quantum wireless networks have the characteristics of high dynamics, limited node energy, and quantum resource competition. We have found that the existing quantum wireless network routing protocols suffer from path collision, high energy consumption, and low utilization of quantum resources. In this paper, a more practical and efficient entanglement routing protocol, which can find contention-free paths for concurrent source destination pairs, is proposed. Simulation results show that the proposed protocol can not only greatly improve the throughput and success rate of the network, but also significantly prolong the network lifetime. The research developed by this work may stimulate more investigations of entanglement routing problems in QWNs.

We put forward a rich variety of optical vortex structures nested in a localized beam envelope supported by cubic-quintic media confined in a rotating harmonic trap. The globally linked vortex cluster comprises an even number of vortices with topological charges equaling 1 and $-1$ alternately. In the nonrotating frame, single-charged vortices reside evenly on a ring. Yet, the system rotation induces the Coriolis force, which in turn leads to a strong asymmetry of the vortex cluster. With the increase of rotation frequency, vortex clusters with a different number of vortices transform into rotating nonlinear states with different symmetries. Meanwhile, the beam envelope is deformed obviously. Nonrotating vortex clusters are stable provided that their power exceeds a certain critical value. Unstable rotating states are very robust and survive over thousands of diffraction lengths.

We demonstrate a generalized invariance property: that the ratio between the intrinsic dynamics of complex media and the perceived timescale of scattered intensity fluctuations depends only on macroscopic properties and not on the detailed circumstances of geometry-dependent light-matter interaction. We show that the necessary conditions can be established in a diffusive Cauchy cavity where the relation between these two timescales can be controlled at will. This approach provides the flexible means to study the intrinsic dynamics of systems with arbitrary geometry and in various conditions of interaction.

The dynamics of fundamental solitons is examined for periodic and defective lattices when the lattice period is varied. The existence domain and stability intervals of solitons are determined and it is shown that the solitons can exist and stay stable for a wide range of parameters. It is observed that the domain of existence is extended by increased lattice period for the periodic lattice and the square lattice with an edge dislocation. It is also demonstrated that stability of solitons around a vacancy defect and near edge dislocation can be improved by decreased lattice period, whereas a higher lattice period supports the stability of periodic lattice solitons. Further, it is shown that there are a lower limit for the period of a square lattice and an upper limit for the period of a lattice with a vacancy defect for no collapse of the solitons in their entire existence domains. Thus modification of the lattice period provides great controllability of the soliton dynamics. It is also observed that the deeper (or strong) vacancy defect in the lattice extends the stability domain of solitons for larger lattice periods.

Computational ghost imaging generally requires a large number of patterns to obtain a high-quality result. It has been shown that both premodulated orthogonal patterns and postprocessing orthonormalization improve imaging quality and reduce the required pattern number. In this work, we propose and experimentally demonstrate a sub-Nyquist computational ghost imaging technique using the orthonormal spectrum-encoded speckle patterns. Our method enables the reconstruction of grayscale images at very low sampling ratios. Additionally, we show that this technique can be combined with compressive sensing to enhance image quality further. Reconstructed images are analyzed using quality indicators such as mean-square error, signal-to-noise ratio, correlation coefficient, and mean-square error of the detected edge. With our method, high-quality images can be obtained at a sampling ratio significantly lower than conventional methods.

The understanding of the emergence of equilibrium statistical mechanics has progressed significantly thanks to developments from typicality, canonical and dynamical, and from the eigenstate thermalization hypothesis. Here we focus on a nonequilibrium scenario in which two nonintegrable systems prepared in different states are locally and nonextensively coupled to each other. Using both perturbative analysis and numerical exact simulations of up to 28 spin systems, we demonstrate the typical emergence of nonequilibrium quasi-steady current for weak coupling between the subsystems. We also identify that these currents originate from a prethermalization mechanism, which is the weak and local breaking of the conservation of the energy for each subsystem.

We report that the position-inhomogeneous quantum walk (IQW) can be utilized to produce the maximal high-dimensional entanglement while maintaining the quadratic speedup spread of the wave function. Our calculations show that the maximal coin-walker entanglement can be generated in any odd steps or asymptotically in even steps, and the nearly maximal entanglement can be obtained in even steps after 2. We implement the IQW by a stable resource-saving time-bin optical network, in which a polarization Sagnac loop is employed to realize the precisely tunable phase shift. Our approach opens up an efficient way for high-dimensional entanglement engineering as well as promotes investigations on the role of coin-walker interactions in QW-based applications.

In this paper, we investigate coherence protection of a quantum system coupled to a hierarchical environment by utilizing noise. As an example, we solve the Jaynes-Cummings model in the presence of both a classical and a quantized noise. The master equation is derived beyond the Markov approximation, where the influence of memory effects from both noises is taken into account. More importantly, we find that the performance of the coherence protection sensitively depends on the non-Markovian properties of both noises. By analyzing the mathematical mechanism of the coherence protection, we show the decoherence caused by a non-Markovian noise with longer memory time can be suppressed by another Markovian noise with shorter memory time. Last but not least, as an outlook, we try to analyze the connection between the atom-cavity entanglement and the atomic coherence, then discuss the possible clue to search the required noise. The results presented in this paper show the possibility of protecting coherence by utilizing noise and may open an alternative path to design noise-assisted coherence protection schemes.

We investigate the generation of nonclassical states of spins coupled to a common cavity by means of a collective driving of the spins. We propose a control strategy using specifically designed series of short coherent and squeezing pulses, which have the key advantage of being experimentally implementable with the state-of-the art techniques. The parameters of the control sequence are found by means of optimization algorithms. We consider the cases of two and four spins, the goal being either to reach a well-defined target state or a state maximizing a measure of nonclassicality. We discuss the influence of cavity damping and spin offset on the generation of nonclassical states. We also explore to what extent squeezing fields help to enhance the efficiency of the control process.

The ground-state hyperfine splitting of light muon-electron ions of lithium, beryllium, boron, and helium is calculated on the basis of analytical perturbation theory in terms of small parameters of the fine-structure constant and electron-muon mass ratio. The corrections of vacuum polarization, nuclear structure, and recoil effects and electron vertex corrections are taken into account. The dependence of the corrections on the nucleus charge Z is studied. The obtained total values of hyperfine splitting intervals can be used for comparison with future experimental data.

The temporal evolution of the dissociative nuclear wave packet (NWP) is intuitive for studying and understanding the dissociation mechanisms of molecules driven by laser fields. Yet, the time-domain information is encoded in the phase term of the wave function and is vanishing when the wave packet is being detected. Here, we demonstrate that the dynamic information for the dissociative NWP in the repulsive state of ${\mathrm{H}}_2^{+}$ can be reconstructed using the wave-packet interference approach. In this approach, a reference pulse following the driving pulse is applied and creates a secondary dissociative NWP for coherent interference with the target NWP. By analyzing the modulation of the observed kinetic energy release distribution, we obtain the time-dependent yield of the target NWP and find a delay of its peak with respect to the field maximum. The delay is reduced when shorter or weaker driving pulses are applied. We attribute the delay to the competition between the three-photon and net-two-photon dissociation channels of ${\mathrm{H}}_2^{+}$.

Transporting cold atoms between distant sections of a vacuum system is a central ingredient in many quantum simulation experiments, in particular in setups, where large optical access and precise control over magnetic fields is needed. In this work, we demonstrate optical transport of cold cesium atoms over a total transfer distance of about 43 cm in less than $30\mathrm{ms}$. The high speed is facilitated by a moving lattice, which is generated via the interference of a Bessel and a Gaussian laser beam. We transport about $3\times {10}^6$ atoms at a temperature of a few microkelvins with a transport efficiency of about $75\%$. We provide a detailed study of the transport efficiency for different accelerations and lattice depths and find that the transport efficiency is mainly limited by a fast initial loss most likely due to the sudden onset of the acceleration and the potential depth along the direction of gravity. To highlight the suitability of the optical-transport setup for quantum simulation experiments, we demonstrate the generation of a pure Bose-Einstein condensate with about $2\times {10}^4$ atoms. We find a robust final atom number within $2\%$ over a duration of 2.5 h with a standard deviation of $<5\%$ between individual experimental realizations.

We investigate the system of a heavy impurity embedded in a paired two-component Fermi gas at the crossover from a Bose-Einstein condensate (BEC) to a Bardeen-Cooper-Schrieffer (BCS) superfluid via an extension of the functional determinant approach (FDA). FDA is an exact numerical approach applied to study manifestations of Anderson's orthogonality catastrophe (OC) in the system of a static impurity immersed in an ideal Fermi gas. Here, we extend the FDA to a strongly correlated superfluid background described by a BCS mean-field wave function. In contrast to the ideal Fermi gas case, the pairing gap in the BCS superfluid prevents the OC and leads to genuine polaron signals in the spectrum. Thus our exactly solvable model can provide a deeper understanding of polaron physics. In addition, we find that the polaron spectrum can be used to measure the superfluid pairing gap, and in the case of a magnetic impurity, the energy of the subgap Yu-Shiba-Rusinov (YSR) bound state. Our theoretical predictions can be examined with state-of-art cold-atom experiments.

Conventional techniques for laser cooling, by coherent scattering off of internal states or through an optical cavity mode, have so far proved inefficient on mechanical oscillators heavier than a few nanograms. That is because larger oscillators vibrate at frequencies much too small compared to the scattering rates achievable by their coupling to auxiliary modes. Decoherence mechanisms typically observed in heavy low-frequency elastically suspended oscillators also differ markedly from what is assumed in conventional treatments of laser cooling. We show that for a low-frequency anelastic oscillator forming the mechanically compliant end mirror of a cavity, detuned optical readout, together with measurement-based feedback to stiffen and dampen it, can harness ponderomotively generated quantum correlations to realize efficient cooling to the motional ground state. This will pave the way for experiments that call for milligram-scale mechanical oscillators prepared in pure motional states, for example, for tests of gravity's effect on massive quantum systems.

Extreme events (EEs) are rare and unexpected events that emerge in many natural systems and are the object of intensive studies. Here, we experimentally investigate the evolution of deterministic optical EEs in a slave laser (SL) subject to chaotic optical injection from a master laser (ML) with optical feedback. Although the ML operates in the long-cavity regime, we observe occasional pulses that are high enough to be considered EEs, and the appearance probability of EEs can be significantly enhanced in the SL for certain injection parameters outside the injection-locking regions of the master-slave configuration. Additionally, the evolution of EEs can be readily identified from the two-dimensional maps. The numerical results show good qualitative agreement with the experiment. These results reveal the importance of injection parameters for the appearance of EEs in chaotic lasers and can be easily extended to other coupled systems.

Classical machine learning has succeeded in the prediction of both classical and quantum phases of matter. Notably, kernel methods stand out for their ability to provide interpretable results, relating the learning process with the physical order parameter explicitly. Here we exploit quantum kernels instead. They are naturally related to the fidelity, and thus it is possible to interpret the learning process with the help of quantum information tools. In particular, we use a support vector machine (with a quantum kernel) to predict and characterize second-order quantum phase transitions. We explain and understand the process of learning when the fidelity per site (rather than the fidelity) is used. The general theory is tested in the Ising chain in transverse field. We show that for small-sized systems, the algorithm gives accurate results, even when trained away from criticality. Besides, for larger sizes we confirm the success of the technique by extracting the correct critical exponent $\nu$. Finally, we present two algorithms, one based on fidelity and one based on the fidelity per site, to classify the phases of matter in a quantum processor.