In recent work, the so-called quasi-Zeno dynamics of a system has been investigated in the context of the quantum first passage problem. This dynamics considers the time evolution of a system subjected to a sequence of selective projective measurements made at small but finite intervals of time. This means that one has a sequence of steps, with each step consisting of a unitary transformation followed by a projection. The dynamics is nonunitary and, in recent work, it has been shown that it can be effectively described by two different non-Hermitian Hamiltonians. Here we explore this connection by considering the problem of detecting a free quantum particle moving on a one-dimensional lattice, where the detector is placed at the origin and the particle is initially located at some specified lattice point. We find that results for distribution times for the first detection probability, obtained from the non-Hermitian Hamiltonians, are in excellent agreement with known exact results as well as exact numerics. Interesting finite-size effects are discussed. We also study the first detection problem for the example of a particle moving in a quasiperiodic potential, an example where the unperturbed particle's motion can be ballistic, localized, or diffusive.

We consider the controlled quantum teleportation performed on a single copy of entangled three-qubit pure states and show how the fidelity of teleportation and the control power are related to the tripartite entanglement measures, namely, three-tangle and genuine concurrence. We characterize the states with extreme properties, and use them to derive tight lower and upper bounds on both the teleportation fidelity and control power for a given amount of entanglement. Furthermore, we discuss the usefulness of these two quantities for experimental detection and/or quantification of tripartite entanglement. In particular, we present the results which imply that the control power cannot be considered as a candidate for a degree of three-qubit entanglement complementary to the three-tangle as conjectured recently [K. Jeong, J. Kim, and S. Lee, Phys. Rev. A 93, 032328 (2016)]. Furthermore, we prove a mutual relation between teleportation fidelity and control power which provides attainable limits of the controller's authority for existing controlled teleportation schemes. Although those relations are derived for pure states, their extensions for mixed states are also examined.

Machine-learning representations of many-body quantum states have recently been introduced as an ansatz to describe the ground states and unitary evolutions of many-body quantum systems. We investigate one of the most important representations, the restricted Boltzmann machine (RBM), in the stabilizer formalism. A general method to construct RBM representations for stabilizer code states is given, and exact RBM representations for several types of stabilizer groups with the number of hidden neurons equal to or less than the number of visible neurons are presented. The result indicates that the representation is extremely efficient. Then we analyze a surface code with boundaries, defects, domain walls, and twists in full detail and find that almost all the models can be efficiently represented via the RBM ansatz: the RBM parameters of the perfect case, boundary case, and defect case are constructed analytically using the method we provide in the stabilizer formalism, and the domain wall and twist case is studied numerically. In addition, the case for Kitaev's $D\left({\mathbb{Z}}_d\right)$ model, which is a generalized model of the surface code, is also investigated.

We present a special quantum phenomenon named optomechanically induced entanglement in the conventional single-cavity optomechanical system driven by a strong pump input field and a relatively weak probe input field. Bipartite entanglement between the pump and probe output fields can be achieved under realistic experimental conditions when the input pump (probe) field is blue detuned by the mechanical frequency (resonant) with respect to the cavity field. The physical origin is the mechanical oscillator displacement, which plays a role similar to the atomic coherence for the well-known electromagnetically induced transparency in the traditional Λ-type atomic system. This scheme provides an alternative, convenient way to generate nondegenerate entangled bright light beams by using only coherent laser fields, and may bring great facility in realistic quantum information processing protocols.

The mixing of atomic levels with opposite parity in the electric field induced by the blackbody radiation is studied. These studies are applied to the case when the source of blackbody radiation is the Cosmic Microwave Background. Our interest is focused on the $2s$ level of hydrogen, which played an important role during the epoch of cosmological recombination. Our studies show that the broadening of the $2s$ level due to the mixing effect induced by the blackbody radiation leads to a correction to the properties of Cosmic Microwave Background. This correction is of the same order as the other important corrections found during the last decades and is at the level of accuracy of the modern experimental data obtained from space probes.

Composite pulse sequences, which produce arbitrary predefined rotations of a two-state system at an angle $\theta$ on the Bloch sphere, are presented. The composite sequences can contain arbitrarily many pulses and can compensate experimental errors in the pulse amplitude and duration to any desired order. A special attention is devoted to two classes of $\pi /2$ sequences—symmetric and asymmetric—the phases of which are given by simple formulas in terms of rational multiples of $\pi$ for any number of constituent pulses. This allows one to construct arbitrarily accurate $\pi /2$ composite rotations. These $\pi /2$ composite sequences are used to construct three classes of arbitrarily long composite $\theta$ sequences by pairing two $\pi /2$ composite sequences, one of which is shifted by a phase $\pi -\theta$ with respect to the other one.

Cryogenic buffer-gas beams are important sources of cold molecules. In this work we explore the use of a converging-diverging nozzle with a buffer-gas beam. We find that, under appropriate circumstances, the use of a nozzle can produce a beam with improved collimation, lower transverse temperatures, and higher fluxes per solid angle.

We demonstrate that all of the salient features of the Harper-Hofstadter model can be implemented with ultracold atoms trapped in a bichromatic ring-shaped lattice. Using realistic sinusoidal lattice potentials rather than assume the idealized tight-binding picture, we determine the optimal conditions necessary to realize the critical point where the spectrum becomes fractal, and we identify the nature and cause of the departures from the discrete model predictions. We also show that even with a commensurate ring with a few lattice sites, the Aubry-André localization transition can be realized. Localized states that behave like edge states with energies that reside in the band gaps can be generated by introducing a surprisingly small local perturbation within the ring. Spectrum oscillation arising from complex coupling can be implemented by uniform rotation of the ring, but with certain significant differences that are explained.

We explore the interspecies interaction quench dynamics of ultracold spin-polarized few-body mass-balanced Fermi-Fermi mixtures confined in a double well with an emphasis on the beyond Hartree-Fock correlation effects. It is shown that the ground state of particle-imbalanced mixtures exhibits a symmetry breaking of the single-particle density for strong interactions in the Hartree-Fock limit, which is altered within the many-body approach. Quenching the interspecies repulsion towards the strongly interacting regime, the two species phase separate within the Hartree-Fock approximation while remaining miscible in the many-body treatment. Despite their miscible character on the one-body level, the two species are found to be strongly correlated and exhibit a phase separation on the two-body level that suggests the antiferromagneticlike behavior of the few-body mixture. For particle-balanced mixtures we show that an intrawell fragmentation (filamentation) of the density occurs both for the ground state and upon quenching from weak to strong interactions, a result that is exclusively caused by the presence of strong correlations. By inspecting the two-body correlations, a phase separation of the two species is unveiled, being a precursor towards an antiferromagnetic state. Finally, we simulate in situ single-shot measurements and showcase how our findings can be retrieved by averaging over a sample of single-shot images.

We explore the photon-controlled phonon statistics in a hybrid spin-optomechanical system. Here the proposed phonon blockade is based on the nonlinearity of system induced by the strong spin-mechanical coupling. Our approach involves introducing an ancillary quadratic optomechanical coupling (i.e., the cavity field quadratically coupled to mechanical motion) and simultaneously considering the cavity field with a single photon. By injecting the auxiliary photons, one can not only enhance the spin-resonator interaction, but also realize fully switchable phonon blockade. This work offers an approach to manipulate phonon statistics with photons, which should inspire the engineering of new single-phonon quantum devices and have potential applications in the future hybrid photon-phonon quantum networks.

We analyze an efficient scheme to enhance the Goos-Hänchen (GH) shifts of the reflected and transmitted beams in a cavity containing two-level atomic medium. A broadband squeezed vacuum field is injected into the cavity to interact with the atomic medium. In the bad cavity limit, the Bloch equations for the atomic operators are identical to those in the free space, but with the modification of the system parameters. Using experimentally achievable parameters, we identify the conditions under which the squeezed vacuum allows us to enhance the GH shifts of the reflected and transmitted beams beyond what is achievable in the absence of the squeezed vacuum. The enhanced GH shifts originate from the coherent population oscillations controlled by the squeezed vacuum field. Furthermore, we also find that the GH shifts of the reflected and transmitted beams depend sensitively on the relative phase between the control field and the squeezed vacuum field. Subsequently, we propose a scheme for such a configuration of the GH shift as a family of hypersensitized displacement sensors. Based on the numerical analysis, the detection sensitivity and minimum detectable value for the tiny displacement can reach approximately $2340\mu \mathrm{m}/\mathrm{nm}$ and 14.4 pm, respectively.

We discuss both the spectrum and the dynamics of cavity QED in the presence of dissipation beyond the standard perturbative treatment of losses. Using the dynamical polaron ansatz and matrix-product state simulations, we discuss the case where both light-matter $g$ coupling and system-bath interaction are in the ultra-strong-coupling regime. We provide a critical $g$ for the onset of Rabi oscillations. Besides, we demonstrate that the qubit is dressed by the cavity and dissipation. Such a dressing governs the dynamics and, thus, it can be measured. Finally, we sketch an implementation for our theoretical ideas within circuit QED technology.

A generalized quantum Rabi Hamiltonian with both one- and two-photon terms has emerged in the circuit quantum electrodynamics system for a decade. The usual parity symmetry is broken naturally in the simultaneous presence of both couplings, which complicates analytical treatments, even in the rotating-wave approximations. In this paper, we propose an adiabatic approximation to this generic model by using Bogoliubov operators and obtain a very concise analytical solution for both eigenvalues and eigenstates. Although the adiabatic approximation is only exact in the vanishing limit of the qubit frequency, the results for some physical observables nevertheless agree well with the numerical ones in a wide parameter regime. In the rotating-wave approximations, we also derive an analytical eigensolution. Two dominant Rabi frequencies are found in the Rabi oscillations of this generalized model. We also apply the present analytical theory to the vacuum Rabi splitting. It is found that some different phenomena emerge just because of the presence of the additional two-photon coupling term.

We propose a vectorial perturbation theory for axially symmetric, generally nonspherical whispering gallery resonators made of isotropic and anisotropic optical materials. It is based on analysis of the leading terms in the coupled equations for independent light-field components, as derived from Maxwell's equations, and true boundary conditions. Strong localization of the whispering gallery modes (WGMs) near the resonator rim, controlled by the azimuth modal number $m$, is the main prerequisite for our analysis. The theory gives high-precision expressions for the WGM frequencies and modal functions, including the evanescent effects. One of important applications of the theory is analysis of anticrossings of the WGM resonances in anisotropic resonators detected in experiments. Simple relations for the frequency avoidance gaps during the anticrossings are derived and compared with experimental data obtained in lithium-niobate-based WGM resonators. We show also that the vectorial effects substantially restrict the field of applicability of the scalar WGM models.

Many quantum algorithms, such as the Harrow-Hassidim-Lloyd (HHL) algorithm, depend on oracles that efficiently encode classical data into a quantum state. The encoding of the data can be categorized into two types: analog encoding, where the data are stored as amplitudes of a state, and digital encoding, where they are stored as qubit strings. The former has been utilized to process classical data in an exponentially large space of a quantum system, whereas the latter is required to perform arithmetics on a quantum computer. Quantum algorithms such as HHL achieve quantum speedups with a sophisticated use of these two encodings. In this work, we present algorithms that convert these two encodings to one another. While quantum digital-to-analog conversions have implicitly been used in existing quantum algorithms, we reformulate it and give a generalized protocol that works probabilistically. On the other hand, we propose a deterministic algorithm that performs a quantum analog-to-digital conversion. These algorithms can be utilized to realize high-level quantum algorithms such as a nonlinear transformation of amplitudes of a quantum state. As an example, we construct a “quantum amplitude perceptron,” a quantum version of the neural network that hence has a possible application in the area of quantum machine learning.

Einstein-Podolsky-Rosen (EPR) steering is the explicit demonstration of the fact that the measurements of one party can influence the quantum state held by another distant party and do so even if the measurements themselves are untrusted. This has been shown to allow one-sided device-independent quantum information tasks between two remote parties. However, in general, advanced multiparty protocols for generic quantum technologies, such as quantum secret sharing and blind quantum computing for quantum networks, demand multipartite quantum correlations of graph states shared between more than two parties. Here, we show that when one part of a quantum multidimensional system composed of a two-colorable graph state (e.g., cluster and Greenberger-Horne-Zeilinger states) is attacked by an eavesdropper using a universal cloning machine, only one of the copy subsystems can exhibit multipartite EPR steering but not both. Such a no-sharing restriction secures both state sources and channels against cloning-based attacks for generic quantum networking tasks, such as distributed quantum information processing, in the presence of uncharacterized measurement apparatuses.

We investigate the ability of correlation measures to witness non-Markovian open-quantum-system dynamics. It is shown that the mutual information and any entanglement measure between the system and an ancilla do not witness all non-Markovian dynamics. A correlation measure is introduced, and it is proven that, in an enlarged setting with two ancillary systems, this measure detects almost all non-Markovian dynamics, except possibly a zero-measure set of dynamics that is nonbijective in finite time intervals. Our proof is constructive and provides different initial states detecting the non-Markovian evolutions. These states are all separable and some are arbitrarily close to a product state. A further advantage of the introduced correlation measure is that it can be efficiently computed through semidefinite programming.

We propose a sufficient entanglement criterion for the multipartite systems based on quantum Fisher information. It complements the well-established positive partial transpose criterion in the sense that it is also able to detect bound entangled states. Moreover, it improves the well-known upper bound on the quantum Fisher information of separable multiqudit states introduced by Toth et al. [G. Toth and T. Vertesi, Phys. Rev. Lett. 120, 020506 (2018)] and hence detects a greater number of metrologically useful entangled states. This criterion is formulated as a simple inequality which must be fulfilled by the quantum Fisher information of separable states of multipartite systems. Violation of this inequality by a state reveals its entanglement and accordingly confirms its usefulness for quantum metrology. The efficiency of this criterion for detection of entanglement is illustrated through some examples.