The angular correlation functions (ACFs) of a pair of Lyman-α photons emitted from H fragments in the photodissociation of a hydrogen molecule are measured at a 33.66-eV incident photon energy and at hydrogen gas pressures of approximately 0.1 and 1 Pa. The ACFs are measured for both opposite and nonopposite arrangements of the two photon detectors. It turns out that the experimental ACFs involve neither the contribution of the reactions $\mathrm{H}\left(n=2\right)+{\mathrm{H}}_2$ nor the contribution of the cascade from $\mathrm{H}(n$ ⩾ 3) to H($2p)$ fragments. Thus the experimental ACFs are those for primary H($2p)$ pairs following the photodissociation of ${\mathrm{H}}_2$. The experimental ACFs are compared with (i) the theoretical ACF for entangled pairs of H($2p)$ atoms, where the magnetic quantum number of each hydrogen atom is indefinite, and (ii) the theoretical ACF for H($2p)$ pairs with definite magnetic quantum number of each hydrogen atom relative to the internuclear axis [the former entangled state of H($2p)$ pairs is a sum of the latter pair states with definite magnetic quantum number]. In the theoretical ACF in (ii), the disentanglement in H($2p)$ pairs during the dissociation is considered. The experimental ACFs show a similar tendency in angular dependence to the theoretical ACF for entangled H($2p)$ pairs. However, there still remains a considerable difference in the variation magnitude between those experimental and theoretical ACFs. The experimental ACFs show the reverse tendency in angular dependence to the theoretical ACF for H($2p)$ pairs with definite magnetic quantum number of each hydrogen atom relative to the internuclear axis. We thus conclude that the pair of H($2p)$ atoms in the photodissociation of ${\mathrm{H}}_2$ is unlikely to be in the definite states of magnetic quantum number of each hydrogen atom relative to the internuclear axis, i.e., unlikely to be in the components of the entangled state of H($2p)$ pairs.

We investigate whether nonadiabatic effects, rather than an initial longitudinal momentum spread, can explain the additional final momentum spread measured in strong-field ionization experiments with ultrafast laser pulses. We find that, when used consistently, a well-known nonadiabatic theory which includes an initial velocity offset yields results similar to adiabatic theory. By “consistent use” we mean that nonadiabatic theory is used also for field strength calibration of the experiment. The additional momentum spread can be accounted for by including an initial longitudinal momentum spread, as was done previously in the adiabatic case. Interestingly, when the experimental intensity is calibrated using a common in situ calibration method based on adiabatic assumptions, the nonadiabatic theory improves upon the adiabatic theory. This result highlights the uncertainty associated with using theory-based calibration methods, which are the most common way of calibrating experimental data in attosecond science.

We theoretically propose a control scheme of temporal wave-packet separation for oriented molecules, based on nonresonant dynamic Stark effect (DSE) in the dipole limit. The proposed scheme exploits the fact that diabatic coupling between the lowest two potential-energy curves (PECs) depends on diatomic internuclear distance. In the control process nonresonant DSE shifts the PECs, moves the avoided crossing position, and thus changes transition probability from one PEC to another. In the scheme linearly polarized single-cycle THz pulses are employed as control fields and are applied to the molecules oriented along the field polarization. In this paper the proposed scheme is applied to the temporal wave-packet separation of the binary mixture of alkali-halide isotopologues ${}^{133}\mathrm{CsI}$ and ${}^{135}\mathrm{CsI}$. We assume that before applying the control pulse one of the isotopologues is oriented along the field polarization direction and the other is oriented along the opposite direction (isotope-dependent orientation) and then they are electronically excited from the ground-state PEC to the excited-state counterpart. Numerical wave-packet propagations under some control pulses reveal that a THz pulse yields a temporal wave-packet separation of about 2 ps between the two isotopologue photodissociations. We also consider the cases where the molecular axis is not parallel to the field polarization direction and carry out the same wave-packet calculations. It is found, as a result, that the temporal wave-packet separation is comparable with the parallel case, indicating that even when the isotope-dependent orientation right before control is not perfectly achieved, the proposed control scheme works well. This result suggests that the present separation scheme for oriented molecules is more robust against imperfect orientation than the molecular separation technique using the dependence of electronic transition probability on the field polarization direction relative to the molecular axis.

We use a gray molasses operating on the $D_1$ atomic transition to produce degenerate quantum gases of ${}^6\mathrm{Li}$ with a large number of atoms. This sub-Doppler cooling phase allows us to lower the initial temperature of $10{}^9$ atoms from 500 to 40 $\mu \mathrm{K}$ in 2 ms. We observe that $D_1$ cooling remains effective into a high-intensity infrared dipole trap where two-state mixtures are evaporated to reach the degenerate regime. We produce molecular Bose-Einstein condensates of up to 5 $\times$ $10{}^5$ molecules and weakly interacting degenerate Fermi gases of $7\times {10}^5$ atoms at $T/T_F<0.1$ with a typical experimental duty cycle of 11 s.

We experimentally study the spin dynamics of mesoscopic ensembles of ultracold magnetic spin-3 atoms located in two separated wells of an optical dipole trap. We use a radio-frequency sweep to selectively flip the spin of the atoms in one of the wells, which produces two separated spin domains of opposite polarization. We observe that these engineered spin domains are metastable with respect to the long-range magnetic dipolar interactions between the two ensembles. The absence of intercloud dipolar spin-exchange processes reveals a classical behavior, in contrast to previous results with atoms loaded in an optical lattice. When we merge the two subsystems, we observe spin-exchange dynamics due to contact interactions which enable the determination of the $s$-wave scattering length of ${}^{52}\mathrm{Cr}$ atoms in the $S=0$ molecular channel $a_0=13.5_{-10.5}^{+11}{a}_B$ (where $a_B$ is the Bohr radius).

We study mixtures of a population-imbalanced, strongly interacting Fermi gas and of a Bose-Einstein condensed gas at zero temperature. In the homogeneous case, we find that the Chandrasekhar-Clogston critical polarization for the onset of instability of Fermi superfluidity is enhanced due to the interaction with the bosons. Predictions for the critical polarization are also given in the trapped case, with a special focus on the situation of equal Fermi-Bose and Bose-Bose coupling constants, where the density of fermions becomes flat in the center of the trap. This regime can be realized experimentally using Feshbach resonances and is well suited to investigate the emergence of exotic configurations, such as the occurrence of spin domains or the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase.

Observing nonlinear optical quantum effects or implementing quantum information protocols using nonlinear optics requires moving to ever-smaller input light intensities. However, low light intensities generally mean weak optical nonlinearities, which are inadequate for many applications. Here we calculate the performance of four-wave mixing in various optical fibers for the case where one of the input beams is a single photon. We show that in tapered chalcogenide glass fibers (microwires) a single photon plus strong pump beam can produce a pair of photons with a probability of 0.1%, much higher than in previous work on bulk and wave-guided crystal sources. Such a photon converter could be useful for creating large entangled photon states, for performing a loophole-free test of Bell's inequalities, and for quantum communication.

We show how nitrogen-vacancy centers can be used to determine the amplitude, phase, and frequency of unknown weak monochromatic and multichromatic oscillating magnetic fields using only the periodic dynamical decoupling and Carr-Purcell-Meiboom-Gill sequences. The effect of decoherence on the measurement of the magnetic field parameters is explicitly analyzed, and we take into account the fact that different pulse sequences suppress decoherence to different extents. Since the sensitivity increases with increasing sensing time while it decreases due to decoherence and limited photon detection efficiency, we use the Fisher information matrix in order to optimize the number of pulses that should be used.

We report the experimental reconstruction of the statistical properties of an ultrafast pulsed type II parametric down-conversion source in a periodically poled potassium titanyl phosphate waveguide at telecom wavelengths, with almost perfect photon-number correlations. We use a photon-number-resolving time-multiplexed detector based on a fiber-optical setup and a pair of avalanche photodiodes. By resorting to a germane data-pattern tomography, we assess the properties of the nonclassical light states with unprecedented precision.

As recently discovered [T. Karpiuk et al., Phys. Rev. Lett. 109, 190601 (2012)], Anderson localization in a bulk disordered system triggers the emergence of a coherent forward scattering (CFS) peak in momentum space, which twins the well-known coherent backscattering (CBS) peak observed in weak localization experiments. Going beyond the perturbative regime, we address here the long-time dynamics of the CFS peak in a one-dimensional random system and we relate this novel interference effect to the statistical properties of the eigenfunctions and eigenspectrum of the corresponding random Hamiltonian. Our numerical results show that the dynamics of the CFS peak is governed by the logarithmic level repulsion between localized states, with a time scale that is, with good accuracy, twice the Heisenberg time. This is in perfect agreement with recent findings based on the nonlinear sigma model. In the stationary regime, the width of the CFS peak in momentum space is inversely proportional to the localization length, reflecting the exponential decay of the eigenfunctions in real space, while its height is exactly twice the background, reflecting the Poisson statistical properties of the eigenfunctions. It would be interesting to extend our results to higher dimensional systems and other symmetry classes.

The expansion dynamics of bosonic gases in optical lattices has recently been the focus of increasing attention, both experimental and theoretical. We consider, by means of numerical Bethe ansatz, the expansion dynamics of initially confined wave packets of two interacting bosons on a lattice. We show that a correspondence between the asymptotic expansion velocities and the projection of the evolved wave function over the bound states of the system exists, clarifying the existing picture for such situations. Moreover, we investigate the role of the lattice in this kind of evolution.

We present a derivation that maps the original problem of a many-body open quantum system (OQS) coupled to a harmonic oscillator reservoir into that of a many-body OQS coupled to a lattice of harmonic oscillators. The present method is particularly suitable to analyze the dynamics of atoms arranged in a periodic structure and coupled with the electromagnetic field within a photonic crystal. It allows to solve the dynamics of a many-body OQS with methods alternative to the commonly used master, stochastic Schrödinger, and Heisenberg equations, and thus to reach regimes well beyond the weak coupling and Born-Markov approximations.

Optical breathing solitons are known to display well-resolved cycles where the phase of pulse compression is followed by pulse stretching. Here we show that, in the extreme regimes where the soliton pulse width approaches the field cycle, the field waveform dynamics can drastically differ from this textbook scenario. We demonstrate that such extremely short soliton transients can develop optical shock waves, which seed parametric amplification, facilitating, along with ionization nonlinearity, soliton compression to subcycle pulse widths. This pulse compression scenario is shown to enable the generation of sub-quarter-cycle multigigawatt optical field waveforms in the mid infrared.

Cold-atom experiments in optical lattices offer a versatile platform to realize various topological quantum phases. A key challenge in those experiments is to unambiguously probe the topological order. We propose a method to directly measure the characteristic topological invariants (order) based on the time-of-flight imaging of cold atoms. The method is generally applicable to detection of topological band insulators in one, two, or three dimensions characterized by integer topological invariants. Using detection of the Chern number for the two-dimensional anomalous quantum Hall states and the Chern-Simons term for the three-dimensional chiral topological insulators as examples, we show that the proposed detection method is practical, and robust to typical experimental imperfections such as limited imaging resolution, inhomogeneous trapping potential, and disorder in the system.

We have demonstrated the storage of electromagnetic pulses by exploiting a spin-echo quantum memory protocol. Therein, we have used an electron-nuclear spin ensemble of tetracyanoethylene anion radicals in toluene which has a natural periodic structure of narrow electron-spin resonance (ESR) hyperfine lines. Robust storage up to three temporal modes has been achieved with storage time $\sim 1$ $\mu \mathrm{s}$ at room temperature on a conventional pulsed ESR spectrometer. On-demand retrieval of the stored field has been realized by holding the electron spins in a dephased state during the storage time. A longer storage time and the highest overall finesse $\sim 20$ of the ESR spectrum have been attained by using multipulse dynamical decoupling of the electron spins from the noisy environment. The obtained experimental results suggest that the electron-nuclear spin ensemble of radicals in liquid should be a promising system for the room-temperature quantum memory.

I present a theory of environmental noise spectroscopy via dynamical decoupling of a qubit at an optimal working point. Considering a sequence of $n$ pulses and pure dephasing due to quadratic coupling to Gaussian distributed noise $\xi \left(t\right)$, I use the linked-cluster (cumulant) expansion to calculate the coherence decay. Solutions allowing for reconstruction of spectral density of noise are given. For noise with correlation time shorter than the time scale on which coherence decays, the noise filtered by the dynamical decoupling procedure can be treated as effectively Gaussian at large $n$, and well-established methods of noise spectroscopy can be used to reconstruct the spectrum of ${\xi }^2\left(t\right)$ noise. On the other hand, for noise of dominant low-frequency character $(1/f^{\beta }$ noise with $\beta >1)$, an infinite-order resummation of the cumulant expansion is necessary, and it leads to an analytical formula for coherence decay having a power-law tail at long times. In this case, the coherence at time $t$ depends both on spectral density of $\xi \left(t\right)$ noise at $\omega =n\pi /t$, and on the effective low-frequency cutoff of the noise spectrum, which is typically given by the inverse of the data acquisition time. Simulations of decoherence due to purely transverse noise show that the analytical formulas derived in this paper apply in this often encountered case of an optimal working point, provided that the number of pulses is not very large, and that the longitudinal qubit splitting is much larger than the transverse noise amplitude.

In this paper we investigate the entanglement properties of the class of $\pi$–locally-maximally-entangleable $(\pi$-LME) states, which are also known as the real equally weighted states or the hypergraph states. The $\pi$-LME states comprise well-studied classes of quantum states (e.g., graph states) and exhibit a large degree of symmetry. Motivated by the structure of LME states, we show that the capacity to (efficiently) determine if a $\pi$-LME state is entangled would imply an efficient solution to the Boolean satisfiability problem. More concretely, we show that this particular problem of entanglement detection, phrased as a decision problem, is $\mathsf{NP}$-complete. The restricted setting we consider yields a technically uninvolved proof, and illustrates that entanglement detection, even when quantum states under consideration are highly restricted, still remains difficult.

A central field model is used to study the two-photon positron annihilation spectrum for the rare gas atoms He, Ne, Ar, Kr, and Xe at energies close to thermal. Correlation effects are incorporated with a semiempirical polarization potential. The $\gamma$ spectrum is given, with values reported for individual subshells. The predicted full widths at half-maximum (FWHM) for all systems are typically $5\%–20\%$ larger than the experimental values reported using the positron trap at the University of California, San Diego, while, with the exception of neon, generally being $2\%–10\%$ smaller than the FWHMs measured at University College London. The detailed spectrum for xenon is reported and the likelihood of core annihilation's making a measurable contribution to the observed Doppler spectrum is discussed. The $\gamma$ spectra are found to be insensitive to variations in the scattering potential and whether the target is represented by a Hartree-Fock or a Dirac-Fock wave function. The model potential used in the solution of the positron-atom Schrödinger equation provides a reasonable fit to recent total elastic cross-section measurements reported by the Australian National University and University of Trento groups.

We study dynamical conductivity of strongly correlated bosons loaded in an optical lattice with restricted geometry in which gauge fields are present. We show that dynamics influenced by the uniform synthetic magnetic field combined with layered lattice structures changes into rich insulator-metallic behavior in the strongly correlated regime. Especially, the amplitude of optical conductivity for a given frequency is a nonmonotonous function of the number of layers $L$. In particular, conductivity for frequency corresponding to on-site interaction energy can abruptly vanish for a special number of applied layers. Moreover, such an insulating behavior is stable in the whole range of parameters in the Mott phase. This robustness arises from the complex gaplike behavior or from Dirac-like physics reflected in the quasiparticle energy spectra. Furthermore we show that a large interlayer tunneling anisotropy destabilizes the absence of conducting state. We also investigate the critical conductivity on the Mott-insulator–superfluid phase boundary and show the correspondence between the number of Hofstadter subbands and the number of layers. The obtained results also reveal that the value of critical conductivity gradually goes to zero when a three-dimensional system is approached. The experimental setup for generation of layered optical lattices is also proposed.