We show that atoms in an optical lattice can respond cooperatively to resonant incident light and that such a response can be employed for precise control and manipulation of light on a subwavelength scale. Specific collective excitation modes of the system that result from strong light-mediated dipole-dipole interactions can be addressed by tailoring the spatial phase profile of the incident light. We demonstrate how the collective response can be used to produce optical excitations at well-isolated sites on the lattice.

Non-Hermitian Hamiltonians satisfying parity-time ($\mathcal{PT}$) symmetry reveal unusual physical phenomena related to exceptional points, where the onset of $\mathcal{PT}$ symmetry breaking occurs. Here, by permitting dispersive variations in the $\mathcal{PT}$-symmetric potential along the propagation axis of a wave, we show that it is possible to obtain $\mathcal{PT}$-induced exceptional points of spatiospectral separation. As an example, we demonstrate “rainbow nonreciprocity” using a $\mathcal{PT}$-symmetric chirped optical potential.

We describe the back action of microwave-photon detection via a Josephson photomultiplier (JPM), a superconducting qubit coupled strongly to a high-quality microwave cavity. The back-action operator depends qualitatively on the duration of the measurement interval, resembling the regular photon annihilation operator at short interaction times and approaching a variant of the photon subtraction operator at long times. The optimal operating conditions of the JPM differ from those considered optimal for processing and storing of quantum information, in that a short $T_2$ of the JPM suppresses the cavity dephasing incurred during measurement. Understanding this back action opens the possibility of performing multiple JPM measurements on the same state, hence performing efficient state tomography.

Recently a high precision spectroscopic investigation of the $EF{}^1{\Sigma }_g^{+}$–$X{}^1{\Sigma }_g^{+}$ system of molecular hydrogen was reported yielding information on QED and relativistic effects in a sequence of rotational quantum states in the $X{}^1{\Sigma }_g^{+}$ ground state of the H${}_2$ molecule [Salumbides et al., Phys. Rev. Lett. 107, 043005 (2011)]. The present paper presents a more detailed description of the methods and results. Furthermore, the paper serves as a stepping stone towards a continuation of the previous study by extending the known level structure of the $EF{}^1{\Sigma }_g^{+}$ state to highly excited rovibrational levels through Doppler-free two-photon spectroscopy. Based on combination differences between vibrational levels in the ground state, and between three rotational branches ($O$, $Q$, and $S$ branches) assignments of excited $EF{}^1{\Sigma }_g^{+}$ levels, involving high vibrational and rotational quantum numbers, can be unambiguously made. For the higher $E{F}^1{\Sigma }_g^{+}$ levels, where no combination differences are available, calculations were performed using the multichannel quantum defect method, for a broad class of vibrational and rotational levels up to $J=19$. These predictions were used for assigning high-$J$ $EF$ levels and are found to be accurate within 5 cm${}^{-1}$.

We derive a general expression for the spontaneous-emission-induced transition rates between the atomic states of atoms trapped in far-detuned optical potentials, taking into account both their internal and external degrees of freedom. We apply this result to atoms trapped in deep optical lattices. For one- and two-dimensional lattices, we quantify the correlations among excitations in the tightly confined directions, the loosely trapped directions, and the internal state of the atom. We emphasize the differences in excitations between red- and blue-detuned lattices.

Using the time-dependent density-matrix renormalization-group method, we calculate transport properties of an interacting Fermi gas in an optical lattice with a confining trap after a sudden displacement of the trap center. In the regime of attractive interactions, the dipolar motion after the displacement can be classified into underdamped oscillations and overdamped relaxations, depending on the interaction strength. These numerical calculations are consistent with experimental results. In the regime of repulsive interactions, we predict a revival of the oscillations of the center of mass when the interaction strength is increased. This unique feature can be considered as a dynamical signature for the emergence of a Mott plateau for an interacting trapped Fermi gas in an optical lattice.

We introduce the concept of local parity-time symmetric ($\mathcal{PT}$) invariance in optical waveguides (or cavity) structures. Starting from a Lagrangian formalism, we establish the connection between light dynamics in these configurations and the seemingly different physics of “supersymmetric” parametric oscillators. Using this powerful tool, we present analytical solutions for optical beam propagation in local $\mathcal{PT}$-invariant coupled systems and we show that the intensity tunneling between the two channels critically depends on the initial conditions. For unbalanced inputs, symmetric as well as asymmetric power evolution can be observed depending on the excitation channel. On the other hand, under certain physical conditions, our analysis predicts that for a modal $\mathcal{PT}$-symmetric input, a unidirectional fractional phase exchange can take place. Few cases where analytical solutions cease to exist are also investigated numerically. Finally, by exploiting the supersymmetric nature of the oscillator equations, we show that under certain initial conditions, one can obtain the propagation dynamics of field amplitudes that “resides” on the supersymmetric eigenfunctions of the system—a phenomenon we call resonant propagation.

We present theoretical and experimental results on spatial quantum correlations induced by multiple scattering of nonclassical light. A continuous-mode quantum theory is derived that enables determining the spatial quantum correlation function from the fluctuations of the total transmittance and reflectance. Utilizing frequency-resolved quantum noise measurements, we observe that the strength of the spatial quantum correlation function can be controlled by changing the quantum state of an incident bright squeezed-light source. Our results are found to be in excellent agreement with the developed theory and form a basis for future research on, e.g., quantum interference of multiple quantum states in a multiple scattering medium.

We propose an experimental and theoretical study of the dynamical regimes observed when the two modes of a two-frequency solid-state laser are coupled by frequency-shifted optical feedback. The detuning associated to the feedback is close to the frequency of the laser relaxation oscillations. Special attention is devoted to the dynamics of the phase of the beat note between the modes relative to the phase of an external radio-frequency reference. In particular, we analyze in detail the transition from phase locking to the external reference to phase drift. This transition occurs through a window of frequency locking without phase locking [Thévenin, Romanelli, Vallet, Brunel, and Erneux, Phys. Rev. Lett. 107, 104101 (2011)]. Furthermore, a large variety of dynamical behaviors, such as weak intensity modulation, self-pulsing, and chaos, are experimentally found. These results are successfully reproduced by a theoretical model based on coupled rate equations. The extension of the model to the description of other active media such as semiconductor lasers is briefly outlined.

Recent work has examined how undecidable problems can arise in quantum-information science. We augment this by introducing three undecidable problems stated in terms of tensor networks. These relate to ideas of Penrose about the physicality of a spin network representing a physical process, closed timelike curves, and Boolean relation theory. Seemingly slight modifications of the constraints on the topology or the tensor families generating the networks lead to problems that transition from decidable to undecidable to even always satisfiable.

Here, we report a measurement scheme for determining an absorption profile with an accuracy imposed solely by photon shot noise. We demonstrate the power of this technique by measuring the absorption of cesium vapor with an uncertainty at the 2-ppm level. This extremely high signal-to-noise ratio allows us to directly observe the homogeneous line-shape component of the spectral profile, even in the presence of Doppler broadening, by measuring the spectral profile at a frequency detuning more than 200 natural linewidths from the line center. We then use this tool to discover an optically induced broadening process that is quite distinct from the well-known power broadening phenomenon.

We show that the dipole-dipole interaction between two Rydberg atoms can give rise to long-range molecules. The binding potential arises from two states that converge to different separated-atom asymptotes. These states interact weakly at large distances, but start to repel each other strongly as the van der Waals interaction turns into a resonant dipole-dipole interaction with decreasing separation between the atoms. This mechanism leads to the formation of an attractive well for one of the potentials. If the two separated-atom asymptotes come from the small Stark splitting of an atomic Rydberg level, which lifts the Zeeman degeneracy, the depth of the well and the location of its minimum are controlled by the external electric field. We discuss two different geometries that result in a localized and a donut-shaped potential, respectively.

Transverse pattern formation in an optical cavity containing a cloud of cold two-level atoms is discussed. We show that density modulation becomes the dominant mechanism as the atomic temperature is reduced. Indeed, for low but easily achievable temperatures the internal degrees of freedom of the atoms can be neglected, and the system is well described by treating them as linear dielectric particles. A linear stability analysis predicts the instability threshold and the spatial scale of the emergent pattern. Numerical simulations in two transverse dimensions confirm the instability and predict the spontaneous formation of honeycomb and hexagonal density structures, respectively, for the blue and red detuned cases.

Recent progress in the development of superconducting circuits has enabled the realization of interesting sources of nonclassical radiation at microwave frequencies. Here, we discuss field quadrature detection schemes for the experimental characterization of itinerant microwave photon fields and their entanglement correlations with stationary qubits. In particular, we present joint state tomography methods of a radiation field mode and a two-level system. Including the case of finite quantum detection efficiency, we relate measured photon field statistics to generalized quasiprobability distributions and statistical moments for one-channel and two-channel detection. We also present maximum-likelihood methods to reconstruct density matrices from measured field quadrature histograms. Our theoretical investigations are supported by the presentation of experimental data, for which microwave quantum fields beyond the single-photon and Gaussian level have been prepared and reconstructed.

Decoherence-free subspaces (DFSs) provide a strategy for protecting the dynamics of an open system from decoherence induced by the system-environment interaction. So far, DFSs have been primarily studied in the framework of Markovian master equations. In this work, we study decoherence-free (DF) states in the general setting of a non-Markovian fermionic environment. We identify the DF states by diagonalizing the nonunitary evolution operator for a two-level fermionic system attached to an electron reservoir. By solving the exact master equation, we show that DF states can be stabilized dynamically.

We revisit the “field entanglement in noninertial frames” scenario extensively studied in previous literature and demonstrate that the geometric discord, a proposed measure of generalized quantum correlations, decays to zero in the infinite acceleration limit. This is in contrast with previous research showing that the acceleration-induced degradation of quantum discord was not strong enough to extinguish discord in this limit. We argue that our finding has two opposing but noncontradicting implications. On the one hand, the usable quantum correlations in the large acceleration regime appear severely limited for any protocols making use of geometric discord as a figure of merit. On the other hand, our result indicates, in corroboration with other recent work, that the geometric discord is not a faithful measure of quantum correlations, especially in the case of continuous-variable systems, due to the existence of states which have finite quantum discord but are nevertheless infinitesimally close (in the Hilbert-Schmidt norm) to a zero-discord state.

As quantum key distribution becomes a mature technology, it appears clearly that some assumptions made in the security proofs cannot be justified in practical implementations. This might open the door to possible side-channel attacks. We examine several discrepancies between theoretical models and experimental setups in the case of continuous-variable quantum key distribution. We study in particular the impact of an imperfect modulation on the security of Gaussian protocols and show that approximating the theoretical Gaussian modulation with a discrete one is sufficient in practice. We also address the issue of properly calibrating the detection setup and in particular the value of the shot noise. Finally, we consider the influence of phase noise in the preparation stage of the protocol and argue that taking this noise into account can improve the secret key rate because this source of noise is not controlled by the eavesdropper.

Any imperfections in a practical quantum key distribution (QKD) system may be exploited by an eavesdropper (Eve) to collect information about the key without being discovered. For example, without the decoy-state method, Eve can perform the photon-number-splitting (PNS) attack and get full information without introducing any perturbation, since weak laser pulses are widely used in practical systems instead of single-photon sources. However, the decoy-state method against PNS attack itself may introduce another loophole while closing the loophole of multiphoton pulses. In this paper, a fatal loophole of practical decoy-state plug-and-play QKD systems has been exploited and a wavelength-selected photon-number-splitting (WSPNS) attack scheme against plug-and-play QKD systems with the decoy-state method is proposed. Theoretical analysis shows that the eavesdropper can get full information about the key generated between the legitimate parties just like the PNS attack was performed in plug-and-play QKD systems without the decoy-state method.

We present investigations of the formation rate and collisional stability of lithium Feshbach molecules in an ultracold three-component mixture composed of two resonantly interacting fermionic ${}^6$Li spin states and bosonic ${}^{174}$Yb. We observe long molecule lifetimes ($>100$ ms) even in the presence of a large ytterbium bath and extract reaction rate coefficients of the system. We find good collisional stability of the mixture in the unitary regime, opening new possibilities for studies and probes of strongly interacting quantum gases in contact with a bath species.

We demonstrate the spatial separation of the prototypical indole(H${}_2$O) clusters from the various species present in the supersonic expansion of mixtures of indole and water. The major molecular constituents of the resulting molecular beam are H${}_2$O, indole, indole(H${}_2$O), and indole(H${}_2$O)${}_2$. It is a priori not clear whether such floppy systems are amenable to strong manipulation using electric fields. Here, we have exploited the cold supersonic molecular beam and the electrostatic deflector to separate indole(H${}_2$O) from the other molecular species as well as the helium seed gas. The experimental results are quantitatively explained by trajectory simulations, which also demonstrate that the quantum-state selectivity of the process leads to samples of indole(H${}_2$O) in low-lying rotational states. The prepared clean samples of indole(H${}_2$O) are ideally suited for investigations of the stereodynamics of this complex system, including time-resolved half-collision and diffraction experiments of fixed-in-space clusters. Our findings clearly demonstrate that the hydrogen bonded indole(H${}_2$O) complex behaves as a rigid molecule under our experimental conditions and that it can be strongly deflected.