N00N states—maximally path-entangled states of $N$ photons—exhibit spatial interference patterns sharper than any classical interference pattern. This is known as superresolution. However, even given perfectly efficient number-resolving detectors, the detection efficiency of all previous measurements of such interference would decrease exponentially with the number of photons in the N00N state, often leading to the conclusion that N00N states are unsuitable for spatial measurements. A technique known as the “optical centroid measurement” has been proposed to solve this and has been experimentally verified for photon pairs; here we present the first extension beyond two photons, measuring the superresolution fringes of two-, three-, and four-photon N00N states. Moreover, we compare the N00N-state interference to the corresponding classical superresolution interference. Although both provide the same increase in spatial frequency, the visibility of the classical fringes decreases exponentially with the number of detected photons. Our work represents an essential step forward for quantum-enhanced measurements, overcoming what was believed to be a fundamental challenge to quantum metrology.

New tools and approaches of quantum optics offer a unique opportunity to generate light pulses carrying a precise number of photons. Accurate control over the light pulses helps to improve the characterization of photoinduced processes. Here, we study interaction of a specialized light source which provides flashes containing just one photon, with retinal rod cells of Xenopus laevis toads. We provide unambiguous proof of the single-photon sensitivity of rod cells without relying on the statistical modeling. We determine their quantum efficiencies without the use of any precalibrated detectors and obtain the value of $(29\pm 4.7)\%$. Our approach provides the path for future studies and applications of quantum properties of light in phototransduction, vision, and photosynthesis.

Laser-induced tunnel ionization from a coherent superposition of electronic states in ${\mathrm{Ar}}^{+}$ is studied in a kinematically complete experiment. Within a pump-probe scheme a spin-orbit wave packet is launched through the first ionization step from the neutral species. The multielectron coherent wave packet is probed as a function of time by the second pulse which ionizes the system to ${\mathrm{Ar}}^{++}$. By measuring delay-dependent electron momentum distributions we directly image the evolution of the nonstationary multielectron wave function. Comparing the results with simulations we test common assumptions about electron momentum distributions and the tunneling process itself.

We simultaneously measure the gravitationally induced phase shift in two Raman-type matter-wave interferometers operated with laser-cooled ensembles of ${}^{87}\mathrm{Rb}$ and ${}^{39}\mathrm{K}$ atoms. Our measurement yields an Eötvös ratio of ${\eta }_{\mathrm{Rb},\mathrm{K}}=(0.3\pm 5.4)\times {10}^{-7}$. We briefly estimate possible bias effects and present strategies for future improvements.

The critical nuclear charge $Z_c$ required to bind a nucleus plus two electrons in a heliumlike atom has recently been an area of active study, resulting in a disagreement with earlier calculations and with the value obtained from the radius of convergence $1/Z^{*}$ of a $1/Z$ expansion of the energy. In order to resolve the disagreement, have performed high-precision variational calculations in Hylleraas coordinates. With the double basis set method, we have been able to obtain good convergence for $Z$ very close to $Z_c$, which together with the Hellmann-Feynman theorem yields the value $Z_c=0.91102822407725573(4)$, corresponding to $1/Z_c=1.09766083373855980(5)$. This value is in agreement with the value obtained by Baker et al. [Phys. Rev. A 41, 1247 (1990)]. A significant feature of the results is that the outer electron remains localized near the nucleus, even at $Z=Z_c$, and the bound state evidently changes smoothly into a shape resonance for $Z<Z_c$. A qualitative polarization potential is proposed to account for the resonance, and the radial distribution function for the electron density is calculated.

We report a high-accuracy measurement of the differential static scalar polarizability $\mathrm{\Delta }{\alpha }_0$ of the $5s{}^2{S}_{1/2}-4d{}^2{D}_{5/2}$ transition of the ${}^{88}{\mathrm{Sr}}^{+}$ ion. The high accuracy is obtained by comparing the micromotion-induced positive scalar Stark shift to the negative time-dilation shift. Measurement of the trap drive frequency where these shifts cancel is used to determine $\mathrm{\Delta }{\alpha }_0$ without the need to determine the electric field. $\mathrm{\Delta }{\alpha }_0$ is a critical parameter for the operation of frequency standards as it determines the blackbody radiation frequency shift coefficient, the largest source of uncertainty in the ${}^{88}{\mathrm{Sr}}^{+}$ ion clock. The measured value of $\mathrm{\Delta }{\alpha }_0$ is $-4.7938(71)\times 10^{-40}\mathrm{J}{\mathrm{m}}^2/{\mathrm{V}}^2$. Taking into account the dynamic correction, the blackbody shift at 300 K is 0.247 99(37) Hz. The contribution of the blackbody shift coefficient to the uncertainty of the ion standard has been reduced by a factor of 24, from $2\times 10^{-17}$ to $8.3\times 10^{-19}$. The revised total uncertainty of our reference standard is $1.2\times 10^{-17}$, limited by the blackbody field evaluation. An additional benefit of the low uncertainty of $\mathrm{\Delta }{\alpha }_0$ is the ability to suppress, by a factor of about 200, the net micromotion frequency shifts.

Holonomic phases—geometric and topological—have long been an intriguing aspect of physics. They are ubiquitous, ranging from observations in particle physics to applications in fault tolerant quantum computing. However, their exploration in particles sharing genuine quantum correlations lacks in observations. Here, we experimentally demonstrate the holonomic phase of two entangled photons evolving locally, which, nevertheless, gives rise to an entanglement-dependent phase. We observe its transition from geometric to topological as the entanglement between the particles is tuned from zero to maximal, and find this phase to behave more resiliently to evolution changes with increasing entanglement. Furthermore, we theoretically show that holonomic phases can directly quantify the amount of quantum correlations between the two particles. Our results open up a new avenue for observations of holonomic phenomena in multiparticle entangled quantum systems.

We report cross sections for pickup of guest molecules on neutral argon and water clusters with the mean sizes in the range from $\overline{N}=50$ to 600. The experiments are supported by molecular dynamics simulations and analytical models based on the interaction potentials. The cross sections for argon clusters are consistent with their assumed spherical shape and follow approximately the theoretically justified $N^{1/3}$ dependence. On the other hand, the cross sections of water clusters depart from this dependence and are considerably larger starting from $\overline{N}\ge 300$. We interpret this increase of cross section by the occurrence of highly irregular shapes of water clusters produced in the supersonic expansion of water vapor under the conditions of the large cluster generation.

We have used picosecond THz pulses to induce transient field-free orientation of OCS molecules. Coherent optical Raman excitation prepares the molecules in rotational superposition states prior to THz irradiation, substantially enhancing the degree of orientation. The time-dependent alignment and orientation are characterized via Coulomb explosion in an intense probe laser. The degree of OCS orientation is an order of magnitude larger than previously observed following THz irradiation and is achieved with a significantly smaller THz field.The field-free orientation level is comparable to that generated using pulsed, two-color laser fields but is obtained with negligible target ionization.

The ionization dynamics of He nanodroplets irradiated with intense femtosecond extreme ultraviolet pulses of up to ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ power density have been investigated by photoelectron spectroscopy. Helium droplets were resonantly excited to atomiclike $2p$ states with a photon energy of 21.4 eV, below the ionization potential ($I_p$), and directly into the ionization continuum with 42.8 eV photons. While electron emission following direct ionization above $I_p$ is well explained within a model based on a sequence of direct electron emission events, the resonant excitation provides evidence of a new, collective ionization mechanism involving many excited atomiclike $2p$ states. With increasing power density the direct photoline due to an interatomic Coulombic decay disappears. It indicates that ionization occurs due to energy exchange between at least three excited atoms proceeding on a femtosecond time scale. In agreement with recent theoretical work the novel ionization process is very efficient and it is expected to be important for many other systems.

We generalize the Power-Zineau-Woolley transformation to obtain a canonical Hamiltonian of cavity quantum electrodynamics for arbitrary geometry of boundaries. This Hamiltonian is free from the $A$-square term and the instantaneous Coulomb interaction between distinct atoms. The single-mode models of cavity QED (Dicke, Tavis-Cummings, Jaynes-Cummings) are justified by a term by term mapping to the proposed microscopic Hamiltonian. As one straightforward consequence, the basis of no-go argumentations concerning the Dicke phase transition with atoms in electromagnetic fields dissolves.