We have measured the tune-out wavelength of lithium isotope ${}^7\mathrm{Li}$ at 671 nm. We have used our atom interferometer to measure the phase shift due to the dynamical Stark effect as a function of the laser frequency when a laser beam was focused on a single interferometer arm. The tune-out wavelength is a function of the hyperfine $F,m_F$ sublevel, and we have prepared the atoms in the sublevel $F=2,m_F=+2$ or $-2$. We find that the tune-out frequency of the $F=2,m_F=+2$ sublevel is at $3388\left(8\right)\mathrm{MHz}$ to the blue of the ${}^{2}S_{1/2},F=1\to {}^{2}P_{1/2},F^{\prime }=2$ transition, the accuracy of this measurement being limited by imperfect optical pumping. This measurement is in excellent agreement with its theoretical value, and, once corrected for different tensorial contributions, our measurement agrees with the more precise measurement of Copenhaver et al. [Phys. Rev. A 100, 063603 (2019)], who also used atom interferometry but different experimental conditions.

We develop a coherent-squeezed-state expansion for the ground state of the quantum phase transition in the quantum Rabi model. The wave function undergoes a transition from a localized state in the normal phase to a two-peaks delocalized state in the super-radiant phase. Interestingly, the two displaced wave packets are squeezed in the momentum space, and overlap strongly to form a correlated state. We develop a superposition of coherent-squeezed states to describe the phase transition correctly, which includes the higher-order excitations at the critical value. The corresponding ground-state energy and the average photon number are in agreement with numerical ones, exhibiting an improvement over previous coherent-state expansions. Moreover, our method successfully captures the reduced quantum fluctuations in momentum variable in the phase transition.

We employ a recently developed optical pulse propagation formulation suitable for slow and stopped light regimes to demonstrate light stopping by a (transient) grating coupling in a toy system, which is, however, applicable to a range of wave systems. We study the complete spatiotemporal dynamics, the characteristic timescales, the coupling efficiencies, and the spectral contents of the stopped pulse. Finally, we demonstrate numerically a delay line which surpasses the delay-bandwidth limit.

We address a particular instance where open quantum systems may be used as quantum probes for an emergent property of a complex system, as the temperature of a thermal bath. The inherent fragility of the quantum probes against decoherence is the key feature making the overall scheme very sensitive. The specific setting examined here is that of quantum thermometry, which aims to exploit decoherence as a resource to estimate the temperature of a sample. We focus on temperature estimation for a bosonic bath at equilibrium in the Ohmic regime (ranging from sub-Ohmic to super-Ohmic), by using pairs of qubits in different initial states and interacting with different environments, consisting either of a single thermal bath or of two independent ones at the same temperature. Our scheme involves pure dephasing of the probes, thus avoiding energy exchange with the sample and the consequent perturbation of temperature itself. We discuss the role of correlations among the probes and the presence of a local versus a global bath. We show that entanglement improves thermometry at short times if the two qubits are embedded in a common bath, whereas if the interaction time is not constrained, then coherence rather than entanglement is the key resource in quantum thermometry.

Quantum walks are accepted as a generic model for quantum transport. The character of the transport crucially depends on the properties of the walk like its geometry and the driving coin. We demonstrate that increasing transport distance between source and target or adding redundant branches to the actual graph may surprisingly result in a significant enhancement of transport efficiency. We explain analytically the observed nonclassical effects using the concept of trapped states for several intriguing geometries, including the ladder graph, the Cayley tree, and its modifications.

Highly charged ion systems play an important role in simulation and diagnostics of astrophysical and fusion plasmas. Due to dominant strong Coulomb interactions from the nuclear charge, electron correlations are generally thought to be less prominent than relativistic effects and other effects in HCIs. In this paper, we demonstrate the importance of strong intrashell electron correlations from the quasidegeneracy of $2s, 2p$ orbitals in highly charged B-like ${\mathrm{Ar}}^{13+}$ ions. Such strong intrashell electron interactions manifest as an atomic system with many channel interactions, which will ultimately form a much more complex atomic spectrum than the neutral B atom in the isoelectronic sequence. As a typical example, the relativistic eigenchannel $R$-matrix method (R-R-eigen code) was successfully applied to investigate the complex spectrum of ${\mathrm{Ar}}^{13+}$ ions with $J^{\pi }=1.5^{+}$ symmetry in discrete and autoionization regions. Calculated results obviously show interesting behaviors of strong intrashell electron correlations in HCIs. Additionally, it is also beneficial to understand discrepancies in the low-energy electron-ion recombination spectra of Be-like ${\mathrm{Ar}}^{14+}$ ions between experimental observations at the main cooler-storage ring of the Heavy Ion Research Facility in Lanzhou and distorted wave calculation.

Atomic properties of S-like W are evaluated through a state-of-the-art method, namely, the multiconfiguration Dirac-Hartree-Fock method combined with the relativistic configuration-interaction approach. The level energies, wavelengths, and transition parameters involving the 88 lowest levels of ${\mathrm{W}}^{58+}$ (W lix) are calculated. We discuss in detail the relative importance of the valence- and core-valence electron correlation effects, the Breit interaction, the higher-order retardation correction beyond the Breit interaction through the transverse photon interaction, and the quantum electrodynamical corrections. The present level energies are highly accurate, with uncertainties close to what can be achieved from spectroscopy. As such, they provide benchmark tests for other theoretical calculations of S-like W and should assist the spectroscopists in their assignment and identification of observed lines in complex spectra.

We study the ground state of a two-component dipolar Fermi gas in a spherically symmetric harmonic trap at zero temperature. We obtain the ground state of the system with equal but opposite values of the magnetic moment on the basis of the Thomas–Fermi–von Weizsäcker approximation. It is found that two types of phase-separated states which break the spherical symmetry emerge across the phase transition between a spin-symmetric phase and a phase-separation phase. One is an axially symmetric phase-separated state and the other is a one-dimensional alternating-spin layered state. We also study a Fermi-Fermi mixture of polarized Dy and Er atoms and show that a structural transition of phase separation can be observed in current experiments by varying the relative number of particles.

We investigate theoretically the polarization dynamics of vector cavity solitons in a birefringent fiber resonator exhibiting net anomalous dispersion. The well-known Lugiato-Lefever equation is adopted to capture the intracavity field dynamics when two orthogonal modes are coupled via nonlinear cross-phase modulation. Numerical simulations reveal the coexistence of different nonlinear states, such as cavity solitons (CSs) with a modulation instability pattern, two identical CSs, or a nonidentical pair of CSs depending on the external parameters. We distinguish three different types of coexisting CSs based on their polarization properties. Stokes parameters and Jones vector analysis are employed to extract the spatial energy flux distribution and the complex electric field associated with each polarization mode, respectively. We observe that the group-velocity mismatch affects the individual CS dynamics which has direct consequences of their polarization states. Our detailed findings with the polarization properties of CSs will enrich the understanding of dissipative vector CS dynamics in a passive birefringent fiber resonator.

The decoherence interpretation of quantum measurements is applied to Wigner's-friend experiments. A framework in which all the experimental outcomes arise from unitary evolutions is proposed. Within it, a measurement is not completed until an uncontrolled environment monitorizes the state composed of the system, the apparatus, and the observer. The (apparent) wave-function collapse and the corresponding randomness result from tracing out this environment; it is thus the ultimate responsible party for the emergence of definite outcomes. Two main effects arise from this fact. First, external interference measurements, trademark of Wigner's-friend experiments, modify the memory records of the internal observers; this framework provides a univocal protocol to calculate all these changes. Second, it can be used to build a consistent scenario for the recently proposed extended versions of the Wigner's-friend experiment. In regard to the work of Frauchiger and Renner [Nat. Commun. 9, 3711 (2018)], this framework shows that the agents' claims become consistent if the changes in their memories are properly taken into account. Furthermore, the particular setup discussed by Brukner [Entropy 20, 350 (2018)] cannot be tested against the decoherence framework, because it does not give rise to well-defined outcomes according to this formalism. A variation of this setup, devised to fill this gap, makes it possible to assign joint truth values to the observations made by all the agents. This framework also narrows the requisites for such experiments, making them virtually impossible to apply to conscious (human) beings. Notwithstanding, it also opens the door to future realizations on quantum machines.

A configuration to identify the stochastic effect of the quantum radiation reaction is proposed, where electrons are put in a standing wave of two colliding ultraintense lasers. By comparing the simulation results of the semiclassical and quantum models, we find that if the semiclassical model is used, a particular trapping of electrons between two electric-field nodes is induced, which, however, does not exist once the correct quantum model is applied. This results in different features of the electron density, energy, and angular distributions, which can be measured in experiments.

In general quantum operations, or quantum channels cannot be inverted by physical operations, i.e., by completely positive trace-preserving maps. An arbitrary state passing through a quantum channel loses its fidelity with the input. Given a quantum channel $\mathcal{E}$, we discuss the concept of its quasi-inverse as a completely positive trace-preserving map ${\mathcal{E}}^{qi}$ which when composed with $\mathcal{E}$ increases its average input-output fidelity in an optimal way. The channel ${\mathcal{E}}^{qi}$ comes as close as possible to the inverse of a quantum channel. We give a complete classification of such maps for qubit channels and provide quite a few illustrative examples.

A way to construct Boltzmann entropy, i.e., the entropy as a function of a microscopic pure state, for quantum field systems is proposed. Operators that shift the field in wave-vector space are used in the construction. By employing an assumption that some terms emerging due to the shift are negligible in the thermodynamic limit, it is shown that, for almost all states in the ensemble of pure states corresponding to a thermodynamic state, the value of the proposed Boltzmann entropy coincides with that of the thermodynamic entropy for the thermodynamic state. For general self-interacting fields, the Boltzmann entropy evolves with time under Hamiltonian dynamics, so that it is capable of characterizing the thermalization of isolated quantum field systems.

The competition between staggered imaginary potentials and lattice dimerization results in a unidirectional propagating coalescing state of a non-Hermitian Su-Schrieffer-Heeger (SSH) ring at an exceptional point. A segment of the SSH ring inherits the unidirectional feature and exhibits some intriguing dynamic behaviors when it is embedded in a Hermitian chain as a $\mathcal{PT}$-symmetric scattering center. Based on the Bethe ansatz scattering solution for the interface between Hermitian and non-Hermitian regions, we show that the SSH scattering center supports the following wave-packet dynamics. (i) A left incident wave packet is allowed perfect transmission, while a right incident wave packet stimulates an amplified reflecting wave train with the length proportional to the size of the SSH segment. Accordingly, a unidirectional invisibility of Bragg scatterers is observed when a multi-SSH segment is considered. (ii) A left incident wave packet can be well confined in a SSH segment if the right lead is removed. (iii) In addition, a wave packet is perfectly absorbed when its size closes to the length of the SSH segment. The underlying mechanism stems from a subtle property of the SSH segment, which supports two quasicoalescing wave packets but with opposite group velocities. Our findings are applicable to the schemes of realizing quantum state storage, quantum diode, and lasing device.

We quantify the resources required for entangling two uncoupled spin qubits through an intermediate mesoscopic spin system (MSS) by indirect joint measurement. Indirect joint measurement benefits from coherent magnification of the target qubits' state in the collective magnetization of the MSS, such that a low-resolution collective measurement on the MSS suffices to prepare postselected entanglement on the target qubits. A MSS consisting of two noninteracting halves, each coupled to one of the target qubits, is identified as a geometry that allows implementing the magnification process with experimentally available control tools. It is proved that the requirements on the amplified state of the target qubits and the MSS perfectly map to the specifications of micro-macro entanglement between each target qubit and its nearby half of the MSS. In the light of this equivalence, the effects of experimental imperfections are explored; in particular, bipartite entanglement between the target qubits is shown to be robust to imperfect preparation of the MSS. Our study provides an approach for using an intermediate spin system for connecting separated qubits. It also opens another path in exploring entanglement between microscopic and mesoscopic spin systems.

We propose a quantum tomography scheme for pure qudit systems which adopts a certain version of random basis measurements and a generative learning method, along with a built-in fidelity estimation approach to assess the reliability of the tomographic states. We prove the validity of the scheme theoretically, and we perform numerically simulated experiments on several target states that have compact matrix product state representation, demonstrating its efficiency and robustness. We find the number of replicas required by a fixed fidelity criterion grows only linearly as the system size scales up, which saturates a lower bound from information theory. Thus the scheme achieves the highest possible scalability that is crucial for practical quantum state tomography.

In this paper, a flexible and effective scheme is proposed to realize nonadiabatic geometric quantum gates with the invariant based reverse engineering and the nonadiabatic holonomic quantum computation ($\mathrm{NHQC}+$) presented in recent work [B.-J. Liu et al., Phys. Rev. Lett. 123, 100501 (2019)] for extensible geometric quantum computation. The scheme provides variabilities for most of control parameters, and can build up multiple evolution paths with different geometric phases acquired in a cycling evolution. As the computational basis can be covered by the evolution paths, the realization of the nonadiabatic geometric quantum computation is possible without auxiliary levels. Moreover, multiple types of nonadiabatic geometric quantum gates can also be easily constructed by only adjusting boundary conditions of control parameters with the method. To show the applications of the scheme, we discuss the implementations of nonadiabatic geometric quantum gates of spin qubits in a double-quantum-dot system with numerical simulations. The results indicate that the scheme possesses robustness against the errors and noise. Therefore, the scheme may offer some interesting perspectives for the realization of the nonadiabatic geometric quantum computation.

In this work, we present a quantum neighborhood preserving embedding and a quantum local discriminant embedding for dimensionality reduction and classification. We demonstrate that these two algorithms have an exponential speedup over their respectively classical counterparts. Along the way, we propose a variational quantum generalized eigenvalue solver that finds the generalized eigenvalues and eigenstates of a matrix pencil $(\mathcal{G},\mathcal{S})$. As a proof of principle, we implement our algorithm to solve $2^5\times 2^5$ generalized eigenvalue problems. Finally, our results offer two optional outputs with quantum or classical form, which can be directly applied in another quantum or classical machine learning process.

The long-range interactions among two- or three-atom systems are of considerable importance in the cold and ultracold research areas for many-body systems. For an ion and an atom, the long-range interaction potential is dominated by the induction (or polarization) potential resulting from the (classical) effect of the ion's electric field on the atom and the leading term of the induction potential is much stronger than the (quantum mechanical) dispersion (or van der Waals) interaction. The present paper focuses on the long-range interaction of the $\mathrm{Li}\left(2{}^{2}S\right)-\mathrm{Li}\left(2{}^{2}S\right)-{\mathrm{Li}}^{+}\left(1{}^{1}S\right)$ system, to see what changes this induction effect (originating in the electric field of the ${\mathrm{Li}}^{+}$ ion) yields in the long-range additive and nonadditive interactions of this three-body system. Using perturbation theory for energies, we evaluate the coefficients $C_n$ in the potential energy for the three well-separated constituents, where $n$ refers to the corresponding order in inverse powers of distance, obtaining the additive interaction coefficients $C_4, C_6, C_7, C_8, C_9$ and the nonadditive interaction coefficients $C_7, C_9$. The obtained coefficients $C_n$ are calculated with highly accurate variationally generated nonrelativistic wave functions in Hylleraas coordinates. Our calculations may be of interest for the study of three-body recombination and for constructing precise potential energy surfaces. We also provide precise evaluations of the long-range potentials for the two-body $\mathrm{Li}\left(2{}^{2}S\right)-{\mathrm{Li}}^{+}\left(1{}^{1}S\right)$ system. For both the two-body and three-body cases, we provide results for the like-nuclei cases of ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$.