We propose a method for efficiently harnessing the quadratic optical response of two-dimensional graphenelike materials by theoretically investigating second-harmonic generation from a current biased sheet placed within a planar active microcavity. We show that, by tuning the cavity to resonate at the second-harmonic frequency, a highly efficient frequency doubling process is achieved (several orders of magnitude more efficient than the free-standing sheet). The efficiency of the process is not due to phase matching, which is forbidden by the localization of the nonlinear quadratic response on the two-dimensional atomic layered material, but stems from the interplay between the two-dimensional planar geometry of the nonlinear medium and the field oscillation within the active cavity near its threshold. The suggested method can easily be extended to different wave interactions and nonlinearities, and therefore it can represent a basic tool for efficiently exploiting nonlinear optical properties of two-dimensional materials.

The measurement-device-independent quantum key distribution (MDI-QKD) protocol is proposed to remove the detector side channel attacks, while its security relies on the assumption that the encoding systems are perfectly characterized. In contrast, the MDI-QKD protocol based on the Clauser-Horne-Shimony-Holt inequality (CHSH-MDI-QKD) weakens this assumption, which only requires the quantum state to be prepared in the two-dimensional Hilbert space and the devices are independent. In experimental realizations, the weak coherent state, which is always used in QKD systems due to the lack of an ideal single-photon source, may be prepared in the high-dimensional space. In this paper, we investigate the decoy-state CHSH-MDI-QKD protocol with $s(3\le s\le 5)$ intensities, including one signal state and $s-1$ decoy states, and we also consider the finite-size effect on the decoy-state CHSH-MDI-QKD protocol with five intensities. Simulation results show that this scheme is very practical.

In a local realist model, physical properties are defined prior to and independent of measurement and no physical influence can propagate faster than the speed of light. Proper experimental violation of a Bell inequality would show that the world cannot be described with such a model. Experiments intended to demonstrate a violation usually require additional assumptions that make them vulnerable to a number of “loopholes.” In both pulsed and continuously pumped photonic experiments, an experimenter needs to identify which detected photons belong to the same pair, giving rise to the coincidence-time loophole. Here, via two different methods, we derive Clauser-Horne- and Eberhard-type inequalities that are not only free of the fair-sampling assumption (thus not being vulnerable to the detection loophole), but also free of the fair-coincidence assumption (thus not being vulnerable to the coincidence-time loophole). Both approaches can be used for pulsed as well as for continuously pumped experiments. Moreover, as they can also be applied to already existing experimental data, we finally show that a recent experiment [Giustina et al., Nature (London) 497, 227 (2013)] violated local realism without requiring the fair-coincidence assumption.

We report photoassociation spectroscopy of ultracold ${}^{86}\mathrm{Sr}$ atoms near the intercombination line and provide theoretical models to describe the obtained bound-state energies. We show that using only the molecular states correlating with the ${}^1{S}_0+{}^3{P}_1$ asymptote is insufficient to provide a mass-scaled theoretical model that would reproduce the bound-state energies for all isotopes investigated to date: ${}^{84}\mathrm{Sr},{}^{86}\mathrm{Sr}$, and ${}^{88}\mathrm{Sr}$. We attribute that to the recently discovered avoided crossing between the ${}^1{S}_0+{}^3{P}_1$ $0_u^{+}$ $\left({}^3{\Pi }_u\right)$ and ${}^1{S}_0+{}^1{D}_2$ $0_u^{+}$ $\left({}^1{\Sigma }_u^{+}\right)$ potential curves at short range and we build a mass-scaled interaction model that quantitatively reproduces the available $0_u^{+}$ and $1_u$ bound-state energies for the three stable bosonic isotopes. We also provide isotope-specific two-channel models that incorporate the rotational (Coriolis) mixing between the $0_u^{+}$ and $1_u$ curves which, while not mass scaled, are capable of quantitatively describing the vibrational splittings observed in experiment. We find that the use of state-of-the-art ab initio potential curves significantly improves the quantitative description of the Coriolis mixing between the two $-8$-GHz bound states in ${}^{88}\mathrm{Sr}$ over the previously used model potentials. We show that one of the recently reported energy levels in ${}^{84}\mathrm{Sr}$ does not follow the long-range bound-state series and theorize on the possible causes. Finally, we give the Coriolis-mixing angles and linear Zeeman coefficients for all of the photoassociation lines. The long-range van der Waals coefficients $C_6\left(0_u^{+}\right)=3868\left(50\right)$ a.u. and $C_6\left(1_u\right)=4085\left(50\right)$ a.u. are reported.

The velocity of detectable information (signal velocity) in a medium capable of supporting abnormal (superluminal or negative) group velocities is calculated. This is carried out by tracking the time instant at which the signal-to-noise ratio (SNR) at the detector output reaches a predetermined threshold while considering the total classical and quantum noise of the channel in addition to the detector noise. Furthermore, the method of steepest descent is incorporated to systematically study various forms of pulse reshaping associated with superluminal propagation and its effect on SNR. By studying the behavior of SNR as a function of both space and time, the present analysis predicts the existence of a cutoff distance beyond which signal velocity of a superluminal pulse is delayed as compared to a companion pulse traveling the same distance in vacuum. Finally, the interplay between the relative strength of the medium-generated noise and the detector noise and its effect on signal velocity is discussed.

In a quantum memory scheme with the Raman process, the read process encounters noise from four-wave mixing (FWM), which can destroy the nonclassical properties of the generated quantum fields. Here we demonstrate experimentally that the noise from FWM can be greatly suppressed by simply reducing the FWM transition channels with a circularly polarized read beam while at the same time retaining relatively high retrieval efficiency.

Above the light line, guided modes cannot be perpetually sustained into a photonic crystal membrane owing to its periodic modulation. The resulting leaky waves are nowadays largely exploited in the context of integrated optics. We develop here a model affording insight into the mechanism that allows the obtention of resonances endowed with extremely high quality factors. As a matter of fact, the model indicates that the quality factor can take arbitrarily high values and that this phenomenon results from the coupling between guided and radiated modes. The obtained tight control over the emission spectrum of a photonic crystal membrane is employed to design a spectrally and spatially selective absorber.

We investigate quantum coherences in the presence of noise by entangling the spin and path degrees of freedom of the output neutron beam from a noisy three-blade perfect crystal neutron interferometer. We find that in the presence of dephasing noise on the path degree of freedom the entanglement of the output state reduces to 0, however the quantum discord remains nonzero for all noise values. Hence even in the presence of strong phase noise nonclassical correlations persist between the spin and the path of the neutron beam. This indicates that measurements performed on the spin of the neutron beam will induce a disturbance on the path state. We calculate the effect of the spin measurement by observing the changes in the observed contrast of the interferometer for an output beam postselected on a given spin state. In doing so we demonstrate that these measurements allow us to implement a quantum eraser and a which-way measurement of the path taken by the neutron through the interferometer. While strong phase noise removes the quantum eraser, the spin-filtered which-way measurement is robust to phase noise. We experimentally demonstrate this disturbance by comparing the contrasts of the output beam with and without spin measurements of three neutron interferometers with varying noise strengths. This demonstrates that even in the presence of noise that suppresses path coherence and spin-path entanglement, a neutron interferometer still exhibits uniquely quantum behavior.

We demonstrate and characterize the extraction of genuine tripartite entanglement from the vacuum of a periodic cavity field. That is, three probe quantum systems (detectors) can become both bipartitely and tripartitely entangled without coming into causal contact, by means of interaction with a common quantum field. We do this by using an oscillator-detector model that forgoes the need for perturbation theory and which instead is solved exactly. We find that the extraction of tripartite entanglement is considerably easier than that of bipartite. As a secondary result, we also compare a periodic cavity with one that has Dirichlet boundary conditions. We find that the extraction of both bipartite and tripartite entanglement is more easily achieved using the former case.

Recent developments in laser sources allow one to shape the precise electric-field waveform oscillation at the subcycle level. These waveforms may then be used to drive and control ultrafast nonlinear phenomena at the attosecond timescale. By utilizing numerical solutions of time-dependent Schrödinger equations and exact solutions of a simple quantum-mechanical system, we show that an atom driven by such sources exhibit coherent history-dependent effects. These manifest themselves in “macroscopic” quantities such as the yield in multicolor, strong-field ionization. We argue that weakly bound, metastable electronic states may enable the dependence on the system history even in long-duration, relatively weak driving waveforms.

We report on the delayed statistical electron emission of photoexcited fullerene anions ${\mathrm{C}}_n{}^{-}$ ($n=60$, 70, 78, 84, 90). Kinetic energy release distributions have been measured by a velocity-map imaging spectrometer for delays of about 100 ns after the nanosecond laser excitation. They show significant variations as a function of the molecular size. Their analysis in the frame of the detailed balance requires the description of the reverse process, namely the electron attachment to the neutral fullerene. For this purpose, the quantum Vogt and Wannier's formalism is used, which allows introducing an empirical expression for the intramolecular vibrational redistribution (IVR). Information on the IVR has been deduced from the fit of the experimental spectra and has been interpreted using the molecular symmetries. In particular ${\mathrm{C}}_{60}$ has an original behavior related to its icosahedral symmetry.

We propose a scheme to enhance molecular orientation by combing an intense electrostatic field and a four-color laser field. We show that molecular orientation by the combined field can be obtained under a laser-field-free condition, and the maximal orientation degree can be enhanced by comparing with the sum of that individually created by the electrostatic field and the four-color laser field. Our results show that the orientation enhancement results from the larger asymmetry of the four-color laser field because of the existence of the electrostatic field. Furthermore, we also discuss the dependence of the orientation enhancement on the carrier-envelope phase, laser intensity, and pulse duration of the four-color laser field and the molecular rotational temperature.

We report an experimental study of peak and phase-space density of a two-stage magneto-optical trap (MOT) of ${}^6\mathrm{Li}$ atoms, which exploits the narrower $2S_{1/2}\to 3P_{3/2}$ ultraviolet (UV) transition at 323 nm following trapping and cooling on the more common $D2$ transition at 671 nm. The UV MOT is loaded from a red MOT and is compressed to give a high phase-space density up to $3\times {10}^{-4}$. Temperatures as low as $33\mu \mathrm{K}$ are achieved on the UV transition. We study the density limiting factors and in particular find a value for the light-assisted collisional loss coefficient of $1.3\pm 0.4\times {10}^{-10}$ ${\mathrm{cm}}^3$/s for low repumping intensity.

We analyze the equilibration process between two either fermionic or bosonic reservoirs containing ultracold atoms with a fixed total number of particles that are weakly connected via a few-level quantum system. We allow for both the temperatures and particle densities of the reservoirs to evolve in time. Subsequently, linearizing the resulting equations enables us to characterize the equilibration process and its time scales in terms of equilibrium reservoir properties and linear-response transport coefficients. Additionally, we investigate the use of such a device as particle transistor or particle capacitor and analyze its efficiency.

The diluted Kondo lattice model is investigated at strong antiferromagnetic local exchange couplings $J$, where almost-local Kondo clouds drastically restrict the motion of conduction electrons, giving rise to the possibility of quantum localization of conduction electrons for certain geometries of impurity spins. This localization may lead to the formation of local magnetic moments in the conduction-electron system, and the inverse indirect magnetic exchange (IIME) provided by virtual excitations of the Kondo singlets couples those local moments to the remaining electrons. Exemplarily, we study the one-dimensional two-impurity Kondo model with impurity spins near the chain ends, which supports the formation of conduction-electron magnetic moments at the edges of the chain for sufficiently strong $J$. Employing degenerate perturbation theory as well as analyzing spin gaps numerically by means of the density-matrix renormalization group, it is shown that the low-energy physics of the model can be well captured within an effective antiferromagnetic Ruderman–Kittel–Kasuya–Yosida-like two-spin model (“RKKY from IIME”) or within an effective central-spin model, depending on edge-spin distance and system size.

Nonclassicality filters provide a universal method to visualize the nonclassicality of arbitrary quantum states of light through negativities of a regularized Glauber-Sudarshan $P$ function, also denoted as nonclassicality quasiprobability. Such filters are introduced and analyzed for optimizing the experimental certification of nonclassical effects. An analytic filter is constructed which preserves the full information on the quantum state. For balanced homodyne detection, the number of data points is analyzed to get the negativities of the nonclassicality quasiprobability with high statistical significance. The method is applied to different scenarios, such as phase-randomized squeezed vacuum states, single-photon-added thermal states, and heralded state engineering with array detectors. The generalization to visualize quantum correlations of multimode radiation fields is also considered.

We investigate the unextendible maximally entangled bases (UMEBs) in ${\mathbb{C}}^d\otimes {\mathbb{C}}^d$ and present a 30-number UMEB construction in ${\mathbb{C}}^6\otimes {\mathbb{C}}^6$. For the higher-dimensional case, we show that for a given $N$-number UMEB in ${\mathbb{C}}^d\otimes {\mathbb{C}}^d$, there is an $\tilde{N}$-number, $\tilde{N}={\left(qd\right)}^2-(d^2-N)$, UMEB in ${\mathbb{C}}^{qd}\otimes {\mathbb{C}}^{qd}$ for any $q\in \mathbb{N}$. As an example, for ${\mathbb{C}}^{12n}\otimes {\mathbb{C}}^{12n}$ systems, we show that there are at least two sets of UMEBs which are not equivalent.

Time-frequency Schmidt (TFS) modes of ultrafast quantum states are naturally compatible with high-bit-rate integrated quantum communication networks. Thus they offer an attractive alternative for the realization of high-dimensional quantum optics. Here, we present a quantum pulse gate based on dispersion-engineered ultrafast frequency conversion in a nonlinear optical waveguide, which is a key element for harnessing the potential of TFS modes. We experimentally retrieve the modal spectral-temporal structure of our device and demonstrate a single-mode operation fidelity of 80%, which is limited by experimental shortcomings. In addition, we retrieve a conversion efficiency of 87.7% with a high signal-to-noise ratio of 8.8 when operating the quantum pulse gate at the single-photon level.

Expansion and amplification of weak randomness play a crucial role in many security protocols. Using quantum devices, such procedure is possible even without trusting the devices used, by utilizing correlations between outcomes of parts of the devices. We show here how to extract random bits with an arbitrarily low bias from single arbitrarily weak min-entropy block source in a device independent setting. To do this we use Mermin devices that exhibit superclassical correlations. The number of devices used scales polynomially in the length of the random sequence $n$. Our protocol is robust; it can tolerate devices that malfunction with probability decreasing polynomially in $n$ at the cost of minor increase in the number of devices used.