A number of unassigned lines in the absorption spectrum of diatomic hydrogen are attributed to nominally forbidden transitions from the ground state to the $nf$ manifold of states (Rydberg electron with $\ell =3$ orbital momentum). They appear via weak $\ell -$ mixing interactions leading to local level perturbations. Our analysis is based on multichannel quantum defect theory and uses known theoretical information from the literature. The upper levels of most of these transitions are known to give rise to molecular fluorescence, and they are shown to be singlet-triplet mixed. We conclude that the well-known metastable $c{}^3{\mathrm{\Pi }}_u^{-}$ state can be populated via one-photon absorption of uv photons followed by cascade emission $4f\to 3d\to 2p$.

We analyze the interplay of adiabatic rotation and Rashba spin-orbit coupling on the BCS-BEC evolution of a harmonically trapped Fermi gas in two dimensions under the assumption that vortices are not excited. First, by taking the trapping potential into account via both the semiclassical and exact quantum-mechanical approaches, we firmly establish the parameter regime where the noninteracting gas forms a ring-shaped annulus. Then, by taking the interactions into account via the BCS mean-field approximation, we study the pair-breaking mechanism that is induced by rotation, i.e., the Coriolis effects. In particular, we show that the interplay allows for the possibility of creating either an isolated annulus of rigidly rotating normal particles that is disconnected from the central core of nonrotating superfluid pairs or an intermediate mediator phase where the superfluid pairs and normal particles coexist as a partially rotating gapless superfluid.

We report an experimental demonstration of atom-pattern engineering via Rabi oscillations. We make use of interferometric Raman beams to drive a microwave transition for laser-cooled atoms confined within an optical dipole trap. Up to nine fringes are patterned within a half wavelength of the Raman standing wave. We also demonstrate spatial atom localization by using two Raman pulses together with optical depletion. Our work may be useful in atom-pattern engineering for quantum information and simulation applications.

Optical emitters strongly coupled to photons propagating in one-dimensional waveguides are a promising platform for optical quantum information processing. Here, we present a theoretical study of the scattering of two indistinguishable photons on a single two-level atom in a Hong-Ou-Mandel setup. By computing the dynamics, we can describe the system at any time of the scattering event. This allows us to highlight the one-to-one correspondence between the saturation of the atom and the effective interaction induced between the photons. Furthermore, we discuss the integrability of the atomic beam splitter and provide an intuitive picture for the correlations observed between the outgoing photons.

The generalized theory of Stokes-Mueller polarimetry is employed to develop the third-order optical polarimetry framework for third-harmonic generation (THG). The outgoing and incoming radiations are represented by 4-element and 16-element column vectors, respectively, and the intervening medium is represented by a $4\times 16$ triple Mueller matrix. Expressions for the THG Stokes vector and the Mueller matrix are provided in terms of coherency and correlation matrices and expanded by four-dimensional $\gamma$ matrices that are analogs of Pauli matrices. Useful expressions of triple Mueller matrices are presented for cylindrically symmetric and isotropic structures. In addition, the relation between third-order susceptibilities and the measured triple Mueller matrix is provided. This theoretical framework can be applied for structural investigations of crystalline materials, including biological structures.

Starting from a microscopic model, we investigate the optical spectra of molecules in strongly coupled organic microcavities examining how they might self-consistently adapt their coupling to light. We consider both rotational and vibrational degrees of freedom, focusing on features which can be seen in the peak in the center of the spectrum at the bare excitonic frequency. In both cases we find that the matter-light coupling can lead to a self-consistent change of the molecular states, with consequent temperature-dependent signatures in the absorption spectrum. However, for typical parameters, these effects are much too weak to explain recent measurements. We show that another mechanism which naturally arises from our model of vibrationally dressed polaritons has the right magnitude and temperature dependence to be at the origin of the observed data.

Paramagnetic Faraday rotation is a quantum nondemolition measurement method that can generate spin squeezing and improve the measurement precision of a collective spin component beyond the standard quantum limit. In practice, a constant bias magnetic field is used to drive the spin precessing at sufficiently high frequency in order to lift the signal out of low-frequency technical noises. However, continuous measurement of precessing spins introduces back-action noise (BAN) due to the light-shift effect. Two types of back-action-evading (BAE) measurement of collective spin components have been demonstrated recently: continuous measurement of a two-ensemble system and stroboscopic measurement of a single ensemble. Here we propose another single ensemble BAE measurement by periodically modulating the bias field with $\pi$ pulses. Our theoretical calculation shows that under experimental settings where pulse-field modulation does not introduce significant decoherences, the proposed method can suppress the BAN and generate spin squeezing faster than the stroboscopic one at the same probe light power. Moreover, if it is combined with synchronous stroboscopic probing, light-shift BAN can be completely eliminated.

We investigate the generic aspects of quantum coherence guided by the concentration of measure phenomenon. We find the average relative entropy of coherence of pure quantum states sampled randomly from the uniform Haar measure and show that it is typical, i.e., the probability that the relative entropy of coherence of a randomly chosen pure state is not equal to the average relative entropy of coherence (within an arbitrarily small error) is exponentially small in the dimension of the Hilbert space. We find the dimension of a random subspace of the total Hilbert space such that all pure states that reside on it almost always have at least a fixed nonzero amount of the relative entropy of coherence that is arbitrarily close to the typical value of coherence. Further, we show, with high probability, every state (pure or mixed) in this subspace also has the coherence of formation at least equal to the same fixed nonzero amount of the typical value of coherence. Thus, the states from these random subspaces can be useful in the relevant coherence consuming tasks like catalysis in the coherence resource theory. Moreover, we calculate the expected trace distance between the diagonal part of a random pure quantum state and the maximally mixed state and find that it does not approach to zero in the asymptotic limit. This establishes that randomly chosen pure states are not typically maximally coherent (within an arbitrarily small error). Additionally, we find the lower bound on the relative entropy of coherence for the set of pure states whose diagonal parts are at a fixed most probable distance from the maximally mixed state.

Motivated by the advances of quantum Darwinism and recognizing the role played by redundancy in identifying the small subset of quantum states with resilience characteristic of objective classical reality, we explore the implications of redundant records for consistent histories. The consistent histories formalism is a tool for describing sequences of events taking place in an evolving closed quantum system. A set of histories is consistent when one can reason about them using Boolean logic, i.e., when probabilities of sequences of events that define histories are additive. However, the vast majority of the sets of histories that are merely consistent are flagrantly nonclassical in other respects. This embarras de richesses (known as the set selection problem) suggests that one must go beyond consistency to identify how the classical past arises in our quantum universe. The key intuition we follow is that the records of events that define the familiar objective past are inscribed in many distinct systems, e.g., subsystems of the environment, and are accessible locally in space and time to observers. We identify histories that are not just consistent but redundantly consistent using the partial-trace condition introduced by Finkelstein as a bridge between histories and decoherence. The existence of redundant records is a sufficient condition for redundant consistency. It selects, from the multitude of the alternative sets of consistent histories, a small subset endowed with redundant records characteristic of the objective classical past. The information about an objective history of the past is then simultaneously within reach of many, who can independently reconstruct it and arrive at compatible conclusions in the present.

Multipartite quantum nonlocality is an important diagnostic tool and resource for both researches in fundamental quantum mechanics and applications in quantum information protocols. Shared reference frames among all parties are usually required for experimentally observing quantum nonlocality, which is not possible in many circumstances. Previous results have shown violations of bipartite Bell inequalities with approaching unit probability, without shared reference frames. Here we experimentally demonstrate genuine multipartite quantum nonlocality without shared reference frames, using the Svetlichny inequality. A significant violation probability of 0.58 is observed with a high-fidelity three-photon Greenberger-Horne-Zeilinger state. Furthermore, when there is one shared axis among all the parties, which is the usual case in fiber-optic or earth-satellite links, the experimental results demonstrate the genuine three-partite nonlocality with certainty. Our experiment will be helpful for applications in multipartite quantum communication protocols.

While the novel applications of weak values have recently attracted wide attention, weak measurement, the usual way to extract weak values, suffers from risky approximations and severe quantum noises. In this paper, we show that the weak-value information can be obtained exactly in strong measurement with postselections, via measuring the coupling-deformed pointer observables, i.e., the observables selected according to the coupling strength. With this approach, we keep all the advantages claimed by weak-measurement schemes and at the same time solve some widely criticized problems thereof, such as the questionable universality, systematical bias, and drastic inefficiency.

We generalize the control power of a perfect controlled teleportation of an entangled three-qubit pure state, suggested by Li and Ghose [Phys. Rev. A 90, 052305 (2014)], to the control power of a general controlled teleportation of a multiqubit pure state. Thus, we define the minimal control power, and calculate the values of the minimal control power for a class of general three-qubit Greenberger-Horne-Zeilinger (GHZ) states and the three-qubit $W$ class whose states have zero three-tangles. Moreover, we show that the standard three-qubit GHZ state and the standard three-qubit $W$ state have the maximal values of the minimal control power for the two classes, respectively. This means that the minimal control power can be interpreted as not only an operational quantity of a three-qubit quantum communication but also a degree of three-qubit entanglement. In addition, we calculate the values of the minimal control power for general $n$-qubit GHZ states and the $n$-qubit $W$-type states.

We provide an explanation of recent experimental results of Xue et al. [P. Xue, R. Zhang, H. Qin, X. Zhan, Z. H. Bian, J. Li, and B. C. Sanders, Phys. Rev. Lett. 114, 140502 (2015)], where full revivals in a time-dependent quantum walk model with a periodically changing coin are found. Using methods originally developed for “electric” walks with a space-dependent, rather than a time-dependent, coin, we provide a full explanation of the observations of Xue et al. We extend the analysis from periodic time dependence to quasiperiodic behavior with periods incommensurate to the step size. Spectral analysis, one of the principal tools for the study of electric walks, fails for time-dependent systems, but we find qualitative propagation behavior of the time-dependent system in close analogy to the electric case.

We present high-resolution spectroscopy of ${\mathrm{Rb}}_{\text{2}}$ ultralong-range Rydberg molecules bound by mixed singlet-triplet electron-neutral atom scattering. The mixing of the scattering channels is a consequence of the hyperfine interaction in the ground-state atom, as predicted recently by Anderson et al. [Phys. Rev. A 90, 062518 (2014)]. Our experimental data enable the determination of the effective zero-energy singlet $s$-wave scattering length for Rb. We show that an external magnetic field can tune the contributions of the singlet and the triplet scattering channels and therefore the binding energies of the observed molecules. This mixing of molecular states via the magnetic field results in observed shifts of the molecular line which differ from the Zeeman shift of the asymptotic atomic states. Finally, we calculate molecular potentials using a full diagonalization approach including the $p$-wave contribution and all orders in the relative momentum $k$, and compare the obtained molecular binding energies to the experimental data.

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just a few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, $J=1\leftarrow 0$ and $J=2\leftarrow 1$ of ${\mathrm{OD}}^{-}$ anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598 596.08(19) MHz for $J=1\leftarrow 0$ and 1 196 791.57(27) MHz for $J=2\leftarrow 1$ of ${\mathrm{OD}}^{-}$, in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.

Autoionization decay from doubly excited states of ${\mathrm{Ne}}^{6+}[1s^{2}3lnl$ ($n=3,4,5$)] (symmetric configurations) as well as the Coster-Kronig transition from doubly excited states of ${\mathrm{Ne}}^{6+}\left[1s^{2}2pnl(n⩾7)\right]$ (asymmetric configurations) are observed in the transfer ionization reaction channel of 80 keV/u ${\mathrm{Ne}}^{8+}$-He collisions. It has been predicted that the formation of symmetric configurations results from uncorrelated double-electron capture processes [Z. Chen and C. D. Lin, Phys. Rev. A 48, 1298 (1993)], and the formation of asymmetric configurations probably results from correlated double-electron capture caused by the dynamical electron-correlation effects which are attributed to small internuclear distances. However, previous experimental measurements were not able to obtain information about the dependence on internuclear distance which can be reflected in the transversal recoil-ion momentum. In this work, we measure the recoil-ion momentum in coincidence with the ejected electron velocity and find that the observed transverse recoil-ion momentum is smaller for the formation of symmetric ${\mathrm{Ne}}^{6+}\left[1s^{2}3lnl(n=3,4,5)\right]$ states than the formation of asymmetric ${\mathrm{Ne}}^{6+}\left[1s^{2}2pnl(n⩾7)\right]$ states. Since large momentum transfer occurs for small internuclear distances (strong electron-electron interactions) and small momentum transfer occurs for large internuclear distances (weak electron-electron interactions), the results indicate that dynamical electron correlation is important for the formation of the asymmetric states.

An integral equation approach to the weak-field asymptotic theory (WFAT) of tunneling ionization is developed. An integral representation for the exact partial amplitudes of ionization into parabolic channels is derived. The WFAT expansion for the ionization rate follows immediately from this relation. Integral representations for the coefficients in the expansion are obtained. The integrals accumulate where the ionizing orbital has large amplitude and are not sensitive to its behavior in the asymptotic region. Hence, these formulas enable one to reliably calculate the WFAT coefficients even if the orbital is represented by an expansion in Gaussian basis, as is usually the case in standard software packages for electronic structure calculations. This development is expected to greatly simplify the implementation of the WFAT for polyatomic molecules, and thus facilitate its growing applications in strong-field physics.

We investigate ionization of neon atoms by an isolated attosecond pump pulse in the presence of two coherent extreme ultraviolet or x-ray probe fields. The probe fields are tuned to a core-valence transition in the residual ion and induce spectral shearing of the photoelectron distributions. We show that the photoelectron-ion coincidence signal contains an interference pattern that depends on the temporal structure of the attosecond pump pulse and the stimulated core-valence transition. Many-body perturbation theory is used to compute “atomic response times” for the processes and we find strikingly different behavior for stimulation to the outer-core hole $\left(2p\leftrightarrow 2s\right)$ and stimulation to the inner-core hole $\left(2p\leftrightarrow 1s\right)$. The response time of the inner-core transition is found to be comparable to that of state-of-the-art laser-based characterization techniques for attosecond pulses.

A technique is proposed to cool Fermi gases to the regime of quantum degeneracy based on the expected inhibition of spontaneous emission due to the Pauli principle. The reduction of the linewidth for spontaneous emission originates a corresponding reduction of the Doppler temperature, which under specific conditions may give rise to a runaway process through which fermions are progressively cooled. The approach requires a combination of a magneto-optical trap as a cooling system and an optical dipole trap to enhance quantum degeneracy. This results in expected Fermi degeneracy factors $T/T_F$ comparable to the lowest values recently achieved, with potential for a direct implementation in optical lattices. The experimental demonstration of this technique should also indirectly provide a macroscopic manifestation of the Pauli exclusion principle at the atomic physics level.