We introduce a type of shape-invariant rotating beam in linear media with harmonic potentials. This type of beam is constructed by superposing a series of properly selected Laguerre-Gaussian beams of different orders. During propagation, the beam is shape invariant, but the pattern rotates and its size varies periodically. The transverse position of the beam at the exit plane of a medium can be steered by tuning the input parameters, even if the transverse position at the entrance plane is fixed.

We investigate the collective radiance characteristics of qubits in the ultrastrong-coupling regime, where the radiance witness is defined based on the resonator-qubit dressed basis. An ultrastrong hyperradiance effect is demonstrated when the dressed state of the system is resonantly driven. Interestingly, we show that, besides the resonator-qubit coupling strength, the parity-symmetry-breaking-induced cascade transition can significantly enhance the collective radiance of the qubits, which allows us to manipulate the transitions between subradiance, superradiance, and hyperradiance via adjusting the parity symmetry of the system with an external magnetic field. This work extends the collective radiance theory to the ultrastrong-coupling regime, and offers potential applications in the engineering of laser devices.

In this work, taking $N$ noninteracting two-level atoms coupled to an anisotropic three-dimensional photonic crystal into account, we study exact non-Markovian dynamics of the atoms with the photonic crystal as an environment. We find that there exists a threshold number $N_{\mathrm{CR}}$ for the identical atoms, beyond which the system environment has a bound state regardless of how weak the system-environment coupling is. This means all identical atoms do not dissipate to the ground state. Further, when the whole system has no bound state, we find that the amplitude of collective excitation approaches a limit value $2/3$ as the number of atoms approaches to infinity. For nonidentical atoms with different transition frequencies, there exist $N$ bound states at most that lead to $N$-wave mixing in the system dynamics. The present prediction is possible to observe in the recent circuit QED experiment, and the results suggest that coherence in a many-body system can be maintained in bound states by manipulating the environment.

We construct seven kinds of three-qubit biseparable states to show that every class of biseparable states in the partial separability classification proposed by Szalay and Kökényesi [S. Szalay and Z. Kökényesi, Phys. Rev. A 86, 032341 (2012)] is nonempty.

A fundamental task in any physical theory is to quantify a certain physical quantity in a meaningful way. In this paper, we show that both fidelity distance and affinity distance satisfy the strong contractibility, and the corresponding resource quantifiers can be used to characterize a large class of resource theories. Under two assumptions, namely, convexity of “free states” and closure of free states under “selective free operations,” our general framework of resource theory includes quantum resource theories of entanglement, coherence, partial coherence, and superposition. In partial coherence theory, we show that fidelity of partial coherence of a bipartite state is equal to the minimal error probability of a mixed quantum state discrimination task and vice versa, which complements the main result in Xiong and Wu [J. Phys. A: Math. Theor. 51, 414005 (2018)]. We also compute the analytic expression of fidelity of partial coherence for $(2,n)$ bipartite $X$ states. At last, we study the correlated coherence in the framework of partial coherence theory. We show that partial coherence of a bipartite state, with respect to the eigenbasis of a subsystem, is actually a measure of discord-type quantum correlation.

Quantum field theory (QFT) simulations are a potentially important application for noisy intermediate scale quantum (NISQ) computers. The ability of a quantum computer to emulate a QFT therefore constitutes a natural application-centric benchmark. Foundational quantum algorithms to simulate QFT processes rely on fault-tolerant computational resources, but to be useful on NISQ machines, error-resilient algorithms are required. Here we outline and implement a hybrid algorithm to calculate the lowest energy levels of the paradigmatic $1+1$–dimensional ${\varphi }^4$ interacting scalar QFT. We calculate energy splittings and compare results with experimental values obtained on currently available quantum hardware. We show that the accuracy of mass-renormalization calculations represents a useful metric with which near-term hardware may be benchmarked. We also discuss the prospects of scaling the algorithm to full simulation of interacting QFTs on future hardware.

The entanglement survival time is defined as the maximum time a system which is evolving under the action of local Markovian, homogeneous-in-time noise is capable of preserving the entanglement it had at the beginning of the temporal evolution. In this paper we study how this quantity is affected by the interplay between the coherent preserving and dissipative contributions of the corresponding dynamical generator. We report the presence of a counterintuitive, nonmonotonic behavior in such a functional, capable of inducing sudden death of entanglement in models which, in the absence of unitary driving, are capable of sustaining entanglement for arbitrarily long times.

We calculate excitation energies for low states of nobelium, including states with an open $5f$ subshell. An efficient version of the many-electron configuration-interaction method for treating the atom as a 16-external-electron system is developed and used. The method is tested on calculations for ytterbium, which has an external electron structure similar to that of nobelium. The results for nobelium are important for prediction of its spectrum and for interpretation of recent measurements. Ytterbium is mostly used to study the features of the method.

In order to realize more sensitive measurement of the electron's electric dipole moment ($e\mathrm{EDM}$), it would be worthwhile to find new laser-cooled molecules with large internal effective electric field ($E_{\mathrm{eff}}$), high electric polarizability, and long lifetime of the $e\mathrm{EDM}$ measurement state. Here we demonstrate the theoretical feasibility of laser cooling and trapping the mercuric monofluoride (${}^{202}\mathrm{Hg}{}^{19}\mathrm{F}$, $X^2{\mathrm{\Sigma }}_{1/2}$) radicals, as well as their application in the $e\mathrm{EDM}$ measurement. We investigated the electronic, rovibrational, and hyperfine structures and verified the highly diagonal Franck-Condon factors of the main transitions by the Rydberg-Klein-Rees inversion method and the Morse approximation. Hyperfine manifolds of the $X^2{\mathrm{\Sigma }}_{1/2}\left(\upsilon =0\right)$ rotational states were examined with the effective Hamiltonian approach and a feasible sideband modulation scheme was proposed. In order to enhance optical cycling, the microwave remixing method was employed to address all the necessary levels. The Zeeman effect and the hyperfine structure magnetic $g$ factors of the $X^2{\mathrm{\Sigma }}_{1/2}\left(\upsilon =0,N=1\right)$ state were studied subsequently. Finally, its statistical sensitivity for the $e\mathrm{EDM}$ measurement was estimated to be about $6\times {10}^{-32}e\mathrm{cm}$ in the trap, indicating that ${}^{202}\mathrm{Hg}{}^{19}\mathrm{F}$ might be a promising laser-cooled $e\mathrm{EDM}$ candidate when compared with the most recent ThO result of $d_e=\left(4.3\pm {3.1}_{\mathrm{stat}}\pm {2.6}_{\mathrm{syst}}\right)\times {10}^{-30}e\mathrm{cm}$ [V. Andreev et al. (The ACME Collaboration), Nature (London) 562, 355 (2018)].

We present numerical simulations on the correlated two-electron momentum distributions for the recollisional ($e, 2e$) process in nonsequential double ionization (NSDI) of helium by an 800-nm laser field at an intensity around $4.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, based on the improved quantitative rescattering model, in which the lowering of the kinetic energy required for the electrons to escape from the parent ion due to the presence of electric field at the time of recollision has been taken into account. According to the QRS model, the correlated two-electron momentum distributions for laser-induced ($e, 2e$) collision in NSDI can be factorized as a product of the returning-electron wave packet (RWP) and the field-free differential cross section (DCS) for ionization of the parent ion by the impact of the laser-induced returning electron. The RWPs which describe the momentum distribution of the returning electrons are obtained within the strong-field approximation for high-order above threshold ionization. In the calculations of the DCSs for electron impact ionization of ${\mathrm{He}}^{+}$, the precollision Coulomb interaction between the incident electron and the parent ion and the postcollision Coulomb interaction between the two outgoing electrons are considered. In addition, the dynamic screening (DS) of the three-body Coulomb interactions in the final state, due to the fact that the strength of the interaction of any two particles is affected by the presence of the third one, has also been taken into account. It has been found that while the postcollision Coulomb interaction is responsible for the observed fingerlike structure, the precollision Coulomb interaction changes the orientation of the two fingers from $\mathsf{V}$-type to parallel, the final state DS reduces the separation of the two fingers, and the lowering of threshold energy shifts the two fingers towards smaller momenta. All these effects improve the agreement of the simulated results with the experimental results.

Strong-field tunneling ionization is the initial step for various ultrafast dynamics in intense laser-atom and/or molecule interactions. Time-resolving tunneling ionization is of crucial importance for accurately understanding these ultrafast processes. In our previous work [Phys. Rev. Lett. 121, 253203 (2018).], we proposed an attosecond photoelectron interferometer based on strong-field photoelectron holography to resolve the tunneling ionization. Here, we show more details about how the complex ionization time of tunneling is retrieved with our scheme. We demonstrate that the Coulomb interaction, which is a tedious and complex problem in dealing with photoelectron interference in strong-field ionization, can be safely canceled in our scheme. This is further confirmed by performing our scheme for both the model atoms with short-range and Coulomb potentials. Additionally, the validity and accuracy of our scheme are confirmed by its application in different targets and different laser parameters. Our scheme provides a reliable and versatile method to resolve the time information of strong-field tunneling ionization.

We propose an algorithm for retrieving the spectral phase of an isolated attosecond pulse based on the photoelectron spectra generated in atoms by the attosecond pulse in the presence of a time-delayed infrared laser. Instead of employing the whole set of the streaking spectra as in the previous phase retrieval algorithms, we calculate the autocorrelation (AC) of the streaking trace, and use it to extract the spectral phase. We illustrate that this method can be used to extract narrow- as well as broadband attosecond pulses. The method converges much faster and is more accurate. We also define two parameters, $V$ and $A$. By comparing the AC pattern and the $V$ and $A$ parameters from the experiment and from the retrieved pulse, this method provides a metric to evaluate the accuracy of the retrieved pulse.

The archetypal quantum interferometry experiment yields an interference pattern that results from the indistinguishability of two spatiotemporal paths available to a photon or to a pair of entangled photons. A fundamental challenge in quantum interferometry is to perform such experiments with a higher number of paths and over large distances. We demonstrate that using indistinguishable frequency paths instead of spatiotemporal ones allows for robust, high-dimensional quantum interferometry in optical fibers. In our system, twin photons from an Einstein-Podolsky-Rosen pair are offered up to nine frequency paths after propagation in long-haul optical fibers and we show that the multipath quantum interference patterns can be faithfully restored after the photons travel a total distance of up to $60\mathrm{km}$.

We study theoretically and experimentally high-order harmonic generation (HHG) using two noncollinear driving fields focused in gases. We show that these two fields form a nonstationary blazed active grating in the generation medium. The intensity and phase structure of this grating rule the far-field properties of the emission, such as the relative amplitude of the diffraction orders. Full macroscopic calculations and experiments support this wave-based picture of the process, complementing and extending its standard “photon” picture. This insight into the HHG process allows us to envision structuration schemes to convert femtosecond lasers to attosecond pulses with increased efficiency.

Most matter-wave interferometry (MWI) schemes for quantum sensing have so far been evaluated in ideal situations without noise. In this work, we provide assessments of generic multiqubit MWI schemes under Markovian dephasing noise. We find that, for certain classes of the MWI schemes with scale factors that are nonlinearly dependent on the interrogation time, the optimal precision of maximally entangled probes decreases with increasing particle number $N$, for both independent and collective dephasing situations. This result challenges the conventional wisdom found in dephasing Ramsey-type interferometers. We initiate the analyses by investigating the optimal precision of multiqubit Sagnac atom interferometry for rotation sensing. And we show that, due to the competition between the unconventional interrogation-time quadratic phase accumulation and the exponential dephasing processes, the Greenberger–Horne–Zeilinger (GHZ) state, which is the optimal input state in noiseless scenarios, leads to vanishing quantum Fisher information in the large-$N$ regime. Then our assessments are further extended to generic MWI schemes for quantum sensing with entangled states and under decoherence. Finally, a quantum error-correction logical GHZ state is tentatively analyzed, which could have the potential to recover the Heisenberg scaling and improve the sensitivity.

Nonlocal quantum correlation is at the heart of the bizarre nature of quantum physics. While there are various classes of nonlocal quantum correlation, steerability of a quantum state by local measurements provides unique operational features. Here, we theoretically and experimentally investigate the “hidden” property of quantum steerability. In particular, we find that there are initially unsteerable states which can reveal the steerability by using local filters on individual quantum systems. It is remarkable that a certain set of local filters is more effective in revealing steerability than Bell nonlocality whereas there exists another set of filters that is more effective in revealing Bell nonlocality than steerability. This result suggests the structural difference between entanglement and steerability. Finally, we present a counterintuitive result that mixed states originating from nonmaximally pure entangled states can have hidden steerability while the mixed state from a maximally pure entangled state fails to show steerability.

The coined quantum walk is a discretization of the Dirac equation of relativistic quantum mechanics, and it is the basis of many quantum algorithms. We investigate how it searches the complete bipartite graph of $N$ vertices for one of $k$ marked vertices with different initial states. We prove an intriguing dependence on the number of marked and unmarked vertices in each partite set. For example, when the graph is irregular and the initial state is the typical uniform superposition over the vertices, then the success probability can vary greatly from one time step to the next, even alternating between 0 and 1, so the precise time at which measurement occurs is crucial. When the initial state is a uniform superposition over the edges, however, the success probability evolves smoothly. As another example, if the complete bipartite graph is regular, then the two initial states are equivalent. Then if two marked vertices are in the same partite set, the success probability reaches $1/2$, but if they are in different partite sets, it instead reaches 1. This differs from the complete graph, which is the quantum walk formulation of Grover's algorithm, where the success probability with two marked vertices is $8/9$. This reveals a contrast to the continuous-time quantum walk, whose evolution is governed by Schrödinger's equation, which asymptotically searches the regular complete bipartite graph with any arrangement of marked vertices in the same manner as the complete graph.

Linear-optical realizations of Bell state measurement (BSM) on two single-photon qubits succeed with probability $p_s$ no higher than 0.5. However, predetection quadrature squeezing, i.e., quantum noise limited phase sensitive amplification, in the usual linear-optical BSM circuit, can yield $p_s\approx 0.643$. The ability to achieve $p_s>0.5$ has been found to be critical in resource-efficient realizations of linear-optical quantum computing and all-photonic quantum repeaters. Yet, the aforesaid value of $p_s>0.5$ is not known to be the maximum achievable using squeezing, thereby leaving it open whether close-to-$100\%$ efficient BSM might be achievable using squeezing as a resource. In this paper, we report insights on why squeezing-enhanced BSM achieves $p_s>0.5$. Using this, we show that the previously reported $p_s\approx 0.643$ at single-mode squeezing strength $r=0.6585$—for unambiguous state discrimination (USD) of all four Bell states—is an experimentally unachievable point result, which drops to $p_s\approx 0.59$ with the slightest change in $r$. We, however, show that squeezing-induced boosting of $p_s$ with USD operation is still possible over a continuous range of $r$, with an experimentally achievable maximum occurring at $r=0.5774$, achieving $p_s\approx 0.596$. Finally, deviating from USD operation, we explore a trade space between $p_s$, the probability with which the BSM circuit declares a “success,” versus the probability of error $p_e$, the probability of an input Bell state being erroneously identified given the circuit declares a success. Since quantum error correction could correct for some $p_e>0$, this tradeoff may enable better quantum repeater designs by potentially increasing the entanglement generation rates with $p_s$ exceeding what is possible with traditionally studied USD operation of BSMs.

We present a framework for the realization of dissipative evolutions of spin-boson models, including multiphoton exchange dynamics, as well as nonlinear transition rates. Our approach is based on the implementation of a generalized version of a dissipative linear quantum Rabi model. The latter comprises a linearly coupled spin-boson term, spin rotations, and standard dissipators. We provide numerical simulations of illustrative cases supporting the good performance of our method. Our work allows for the simulation of a large class of fundamentally different quantum models where the effect of distinct dissipative processes can be easily investigated.

We present a simplified approach to the dissociative electron attachment of polyatomic molecules. A reduced nuclear coordinate driving the dissociative process immediately following the resonance capture is introduced and allows an estimation of the cross section. The model is applied to the ${\mathrm{H}}_2\mathrm{CN}$ molecule, which is considered as a precursor in the formation of the ${\mathrm{CN}}^{-}$ anion observed in the IRC +10216 carbon star. The computed rate coefficient suggests that the dissociative electron attachment of ${\mathrm{H}}_2\mathrm{CN}$ may not be an efficient reaction to form ${\mathrm{CN}}^{-}$ in the circumstellar envelope of IRC +10216.