Photon correlations are a cornerstone of quantum optics. Recent works [E. del Valle, New J. Phys. 15, 025019 (2013); A. Gonzalez-Tudela et al., New J. Phys. 15, 033036 (2013); C. Sanchez Muñoz et al., Phys. Rev. A 90, 052111 (2014)] have shown that by keeping track of the frequency of the photons, rich landscapes of correlations are revealed. Stronger correlations are usually found where the system emission is weak. Here, we characterize both the strength and signal of such correlations, through the introduction of the “frequency-resolved Mandel parameter.” We study a plethora of nonlinear quantum systems, showing how one can substantially optimize correlations by combining parameters such as pumping, filtering windows and time delay.

We introduce a model of a dual-core optical waveguide with opposite signs of the group-velocity dispersion in the two cores, and a phase-velocity mismatch between them. The coupler is embedded into an active host medium, which provides for the linear coupling of a gain-loss type between the two cores. The same system can be derived, without phenomenological assumptions, by considering the three-wave propagation in a medium with the quadratic nonlinearity, provided that the depletion of the second-harmonic pump is negligible. This linear system offers an optical realization of the charge-parity symmetry, while the addition of the intracore cubic nonlinearity breaks the symmetry. By means of direct simulations and analytical approximations, it is demonstrated that the linear system generates expanding Gaussian states, while the nonlinear one gives rise to broad oscillating solitons, as well as a general family of stable stationary gap solitons.

We theoretically study the central wavelength tuning of a laser pulse propagating in a hollow-core fiber filled with prealigned nitrogen gas. The simulation results show that the double pump pulses, suitable pump pulse energy, gas pressure, and temperature can extend the wavelength shift range, and the spectral blueshift or redshift can be controlled by tuning the time delay between the pump and probe pulses. It is also shown that several hundred nanometers wavelength tuning is obtained by the double pump pulses with appropriate parameters.

We propose and analyze a robust quantum-state transfer protocol by the use of a combination of coherent quantum coupling and decoherence-free subspaces in a coupled quantum spin chain. Under decoherence, an arbitrary unknown quantum state embedded in a decoherence-free subspace can be perfectly transferred through a noisy channel being weakly coupled to two end registers. The method protects quantum information from both the channel noise and the environmental decoherence. A special case of utilizing two physical qubits to encode one logical qubit is considered as an example and the robustness is confirmed by numerical performances.

We analyze the scattering rate of an impurity atom in a Fermi sea as a function of momentum and temperature in the BCS-BEC crossover. The cross section is calculated using a microscopic multichannel theory for the Feshbach resonance scattering, including finite range and medium effects. We show that pair correlations significantly increase the cross section for strong interactions close to the unitarity regime. They give rise to a molecule pole of the cross section at negative energy on the BEC side of the resonance, which smoothly evolves into a resonance at positive scattering energy with a nonzero imaginary part on the BCS side. The resonance is the analog of superfluid pairing for the corresponding population balanced system. Using Fermi liquid theory, we show that the low temperature scattering rate of the impurity atom is significantly increased due to these pair correlations for low momenta. We demonstrate that finite range and mass imbalance effects are significant for the experimentally relevant ${}^6\mathrm{Li}-{}^{40}\mathrm{K}$ mixture, and we finally discuss how the scattering rate can be measured using radio-frequency spectroscopy and Bose-Fermi mixtures.

We show how two circular polarization components of a linearly polarized pulse, propagating through a coherently driven dilute atomic vapor, can be well resolved in a time domain by weak measurement. The slower group velocity of one of the components due to electromagnetically induced transparency leads to a differential group delay between the two components. For a low number density, this delay may not be large enough to temporally resolve the two components. We show how this can be enhanced in terms of the mean time of arrival of the output pulse through a postselected polarizer. We demonstrate the idea with all the analytical and numerical results, with a specific example of alkali atoms.

We study spectral singularities and their application in determining the threshold gain coefficient $g^{(E/M)}$ for oblique transverse electric or magnetic (TE/TM) modes of an infinite planar slab of homogenous optically active material. We show that $g^{\left(E\right)}$ is a monotonically decreasing function of the incidence angle $\theta$ (measured with respect to the normal direction to the slab), while $g^{\left(M\right)}$ has a single maximum, ${\theta }_c$, where it takes an extremely large value. We identify ${\theta }_c$ with the Brewster's angle and show that $g^{\left(E\right)}$ and $g^{\left(M\right)}$ coincide for $\theta =0$ (normal incidence), tend to zero as $\theta \to {90}^{\circ }$, and satisfy $g^{\left(E\right)}<g^{\left(M\right)}$ for $0<\theta <{90}^{\circ }$. We therefore conclude that lasing and coherent perfect absorption are always more difficult to achieve for the oblique TM waves and that they are virtually impossible for the TM waves with $\theta \approx {\theta }_c$. We also give a detailed description of the behavior of the energy density and the Poynting vector for spectrally singular oblique TE and TM waves. This provides an explicit demonstration of the parity-invariance of these waves and shows that the energy density of a spectrally singular TM wave with $\theta >{\theta }_c$ is smaller inside the gain region than outside it. The converse is true for the TM waves with $\theta <{\theta }_c$ and all TE waves.

We demonstrate that the performance of a quantum annealer on hard random Ising optimization problems can be substantially improved using quantum annealing correction (QAC). Our error correction strategy is tailored to the D-Wave Two device. We find that QAC provides a statistically significant enhancement in the performance of the device over a classical repetition code, improving as a function of problem size as well as hardness. Moreover, QAC provides a mechanism for overcoming the precision limit of the device, in addition to correcting calibration errors. Performance is robust even to missing qubits. We present evidence for a constructive role played by quantum effects in our experiments by contrasting the experimental results with the predictions of a classical model of the device. Our work demonstrates the importance of error correction in appropriately determining the performance of quantum annealers.

We study spin-dependent heat transport in quantum gases, focusing on transport phenomena related to pure spin currents and spin-dependent temperatures. Using the Boltzmann equation, we compute the coupled spin-heat transport coefficients as a function of temperature and interaction strength for energy-dependent $s$-wave scattering. We address the issue of whether spin-dependent temperatures can be sustained on a time and length scale relevant for experiments by computing the spin-heat relaxation time and diffusion length. We find that the time scale for spin-heat relaxation time diverges at low temperatures for both bosons and fermions, indicating that the concept of spin-heat accumulation is well defined for degenerate gases. For bosons, we find power-law behavior on approach to Bose condensation above the critical temperature, as expected from the theory of dynamical critical phenomena.

With the assistance of a cyclic three-level artificial atom of a superconducting flux quantum circuit interacting with a two-mode superconducting transmission-line resonator, we study theoretically the degenerate microwave parametric down-conversion (PDC) in a circuit-QED system. By adiabatically eliminating the excited states of the three-level artificial atom, we obtain an effective microwave PDC Hamiltonian for the two resonator modes (i.e., the fundamental and second-harmonic modes). The corresponding PDC efficiency in our model can be much larger than that in the similar circuit-QED system based on a single two-level superconducting qubit [K. Moon and S. M. Girvin, Phys. Rev. Lett. 95, 140504 (2005)]. Furthermore, we investigate the squeezing and bunching behavior of the fundamental resonator mode resulting from the coherent driving to the second-harmonic one.

We study the optical response of cold rubidium atoms driven into the four-level Y configuration exhibiting two high Rydberg levels in the regime of electromagnetically induced transparency (EIT). Atoms excited to either Rydberg level interact with each other just via self-blockade potentials (I) or also via cross blockade potentials (II). Numerical results show a few interesting quantum phenomena on the transmitted properties of a weak probe field owing to controlled single and double Rydberg blockade. In case (I), it is viable to switch between single-photon outputs with vanishing (invariable) two-photon (three-photon) correlation and photon-pair outputs with vanishing (invariable) three-photon (two-photon) correlation. Such output switch can be easily done by modulating frequencies and intensities of two strong coupling fields to create a degenerate EIT window or two separated EIT windows. In case (II), we find that two-photon and three-photon correlations decrease together at a degenerate EIT window center while increasing together between two separated EIT windows. Such consistent changes are observed because both correlations are modified by the identical polarizability degradation though depending on single and double Rydberg blockade, respectively.

We develop a scheme for generating a universal qubit cluster state using probabilistic Bell measurements without the need for feed-forward. Borrowing ideas from percolation theory, we numerically show that using unambiguous Bell measurements that succeed with 75% success probability one could build a cluster state with an underlying pyrochlore geometry such that the probability of having a spanning cluster approaches unity in the limit of infinite lattice size. The initial resources required for the generation of a universal state in our protocol are three-qubit cluster states that are within experimental reach and are a minimal resource for a Bell-measurement-based percolation proposal. Since single and multiphoton losses can be detected in Bell measurements, our protocol raises the prospect of a fully error-robust scheme.

We report measurements and calculations of the differential cross sections for elastic scattering of low-energy electrons by pentane, ${\mathrm{C}}_5{\mathrm{H}}_{12}$. The incident energies measured are at 1, 1.5, 2, 3, 5, 10, 15, 20, 30, 50, and 100 eV, and the calculations covered energies up to 100 eV. The range of experimental scattering angles is from 5° to 130°. We compare our experimental and theoretical values to each other and to available experimental and theoretical data for linear $n$-alkanes.

We show that large spin fermions have very rich spin structures. The local spin order of a spin-$f$ Fermi gas is a linear combination of $2f$ (particle-hole) angular momentum states, $L=1,...,2f$. $L=1,2$ represent ferromagnetic and nematic spin order, while $L\ge 3$ are higher spin orders that have no analog in spin-1/2 systems. Each $L$ spin sector is characterized as $L$ pairs of antipodal points on a sphere. Model calculations show that some of these spin orders have the symmetry of Platonic solid, and many of them have non-Abelian line defects.

We provide a simplified yet rigorous presentation of the ideas from Bombín's paper (arXiv:1311.0879v3). Our presentation is self-contained, and assumes only basic concepts from quantum error correction. We provide an explicit construction of a family of color codes in arbitrary dimensions and describe some of their crucial properties. Within this framework, we explicitly show how to transversally implement the generalized phase gate $R_n=\text{diag}(1,e^{2\pi i/2^n})$, which deviates from the method in the aforementioned paper, allowing an arguably simpler proof. We describe how to implement the Hadamard gate $H$ fault tolerantly using code switching. In three dimensions, this yields, together with the transversal controlled-not (cnot), a fault-tolerant universal gate set $\{H,cnot,R_3\}$ without state distillation.

A class of $k$-particle observables in a two-mode system of Bose particles is characterized by typicality: if the state of the system is sampled out of a suitable ensemble, an experimental measurement of that observable (almost) always yields the same result. We investigate the general features of typical observables and the criteria to determine typicality and finally focus on the case of density correlation functions, which are related to spatial distribution of particles and interference.

The nonclassicality, entanglement, and dimensionality of a noisy twin beam are determined using a characteristic function of the beam written in the Fock basis. One-to-one correspondence between the negativity quantifying entanglement and the nonclassicality depth is revealed. Twin beams, which are either entangled or nonclassical (independent of their entanglement), are observed only for the limited degrees of noise, which degrades their quantumness. The dimensionality of the twin beam quantified by the participation ratio is compared with the dimensionality of entanglement determined from the negativity. Partitioning of the degrees of freedom of the twin beam into those related to entanglement and to noise is suggested. Both single-mode and multimode twin beams are analyzed. Weak nonclassicality based on integrated-intensity quasidistributions of multimode twin beams is studied. The relation of the model to the experimental twin beams is discussed.

We propose a scheme for realizing fractional quantum Hall states of light. In our scheme, photons of two polarizations are coupled to different atomic Rydberg states to form two flavors of Rydberg polaritons that behave as an effective spin. An array of optical cavity modes overlapping with the atomic cloud enables the realization of an effective spin-$1/2$ lattice. We show that the dipolar interaction between such polaritons, inherited from the Rydberg states, can be exploited to create a flat, topological band for a single spin-flip excitation. At half filling, this gives rise to a photonic (or polaritonic) fractional Chern insulator—a lattice-based, fractional quantum Hall state of light.

Extending the longitudinal range of plasma channels produced by femtosecond annular (ring) Gaussian beams in atmosphere is numerically investigated. We find that the length of a stable plasma channel induced by the annular Gaussian beam, comparing with a Gaussian beam under the same initial condition, has a significant improvement, which can be well understood from two aspects. The dynamic complementation caused by the optical system composing of an axicon and a diverging lens can extend the plasma channel greatly, and the special initial transverse distribution of ring-Gaussian pulse leads to pulse splitting and redistribution of the pulse energy before the formation of optical filament. The ultrashort ring-Gaussian beam perhaps offers a new and an efficient route towards the generation of extended optical filament.