Weak-value amplification has been successfully used to amplify the sensitivity of small detector signals, which paved the way towards new developments for precise parameter estimation methods. In general, the scheme is based on the idea that, between pre- and postselection, the measured system and the pointer are coupled in such a way that the interaction depends linearly on the coupling constant, the so-called linear regime. In this article, we propose an improvement to this technique that is based on the quadratic regime of interaction to increase the precision of such measurements. We demonstrate that by employing a purely real (imaginary) weak value in this regime, it is possible to decrease considerably the standard deviation of the final pointer observable in the position (momentum) space, when compared to the traditional linear regime, rendering a precision enhancement. By using an all-optical implementation, we obtained experimental data that confirm our theoretical predictions.

Gaussian quantum states hold special importance in the continuous variable regime. In quantum information science, the understanding and characterization of central resources such as entanglement may strongly rely on the knowledge of the Gaussian or non-Gaussian character of the quantum state. However, the quantum measurement associated with the spectral photocurrent of light modes consists of a mixture of quadrature observables. Within the framework of two recent papers [Phys. Rev. A 88, 052113 (2013) and Phys. Rev. Lett. 111, 200402 (2013)], we address here how the statistics of the spectral photocurrent relates to the character of the Wigner function describing those modes. We show that a Gaussian state can be misidentified as non-Gaussian and vice versa, a conclusion that forces the adoption of tacit a priori assumptions to perform quantum state reconstruction. We experimentally analyze the light beams generated by the optical parametric oscillator operating above threshold to show that the data strongly supports the generation of Gaussian states of the field, validating the use of necessary and sufficient criteria to characterize entanglement in this system.

Controlling the non-Markovian dynamics of open quantum systems is essential in quantum information technology since it plays a crucial role in preserving quantum memory. Albeit in many realistic scenarios the quantum system can simultaneously interact with composite environments, this condition remains little understood, particularly regarding the effect of the coupling between environmental parts. We analyze the non-Markovian behavior of a qubit interacting at the same time with two coupled single-mode cavities which in turn dissipate into memoryless or memory-keeping reservoirs. We show that increasing the control parameter, that is the two-mode coupling, allows for triggering and enhancing a non-Markovian dynamics for the qubit starting from a Markovian one in the absence of coupling. Surprisingly, if the qubit dynamics is non-Markovian for the zero control parameter, increasing the latter enables multiple transitions from non-Markovian to Markovian regimes. These results hold independently on the nature of the reservoirs. This work highlights that suitably engineering the coupling between parts of a compound environment can efficiently harness the quantum memory, stored in a qubit, based on non-Markovianity.

We investigate the thermal Casimir force between two parallel plates made of different isotropic materials which are separated by a uniaxial anisotropic film. Numerical computations of the Casimir pressure at $T=300$ K are performed using the complete Lifshitz formula adapted for an anisotropic intervening layer and in the nonrelativistic limit. It is shown that the standard (nonrelativistic) theory of the van der Waals force is not applicable in this case, because the effects of retardation contribute significantly even for film thicknesses of a few nanometers. We have also obtained simple analytic expressions for the classical Casimir free energy and pressure for large film thicknesses (high temperatures). Unlike the case of isotropic intervening films, for two metallic plates the classical Casimir free energy and pressure are shown to depend on the static dielectric permittivities of an anisotropic film. One further interesting feature is that the classical limit is achieved at much shorter separations between the plates than for a vacuum gap. Possible applications of the obtained results are discussed.

The transmission of electrons with incident energies of 300, 500, and 800 eV through insulating polyethylene terephthalate (PET) nanocapillaries for small tilt angles near 0° was studied. The exiting electrons showed elastic and inelastic behaviors with the inelasticity increasing with the tilt angle. This paper is an extension of our earlier work for electrons on PET reported by Das et al. [Phys. Rev. A 76, 042716 (2007)], but the present study goes further by investigating the transmission for very small tilt angles near 0° with the small step sizes revealing distinct regions of behavior. The work performed here was conducted for two samples with different capillary diameters and pore densities: sample 1 with 100-nm diameter capillaries in a foil of 12-µm thickness and a pore density of $5\times {10}^8/{\mathrm{cm}}^2$, and sample 2 with 200-nm diameter capillaries with the same foil thickness and a pore density of $5\times {10}^7/{\mathrm{cm}}^2$. Two distinct regions of transmission depending on the sample tilt angle for both samples corresponding to direct and guided electrons as well as the transition region between them were observed. The angular centers and widths of the transmission for each angle investigated were found and are discussed in terms of the tilt angle variation in the foil. For the widths, comparison with the inherent angular width of the electron spectrometer is made. Energy loss of electrons was studied from the centroids of the energy spectra with these energies corresponding to the peak centers of the angular profiles. The broadness of the energy distributions due to inelasticity was investigated from the full width at half maxima (in eV) of the spectra. The results for both samples agreed well despite the differences in capillary diameters and pore densities. The results are in partial agreement with the previous work performed for PET nanocapillaries by Das et al. [Phys. Rev. A 76, 042716 (2007)], but differences are also found.

We combine multichannel electronic structure theory with quantum optimal control to derive femtosecond-time-scale Raman pulse sequences that coherently populate a valence excited state. For a neon atom, Raman target populations of up to 13% are obtained. Superpositions of the ground and valence Raman states with a controllable relative phase are found to be reachable with up to 4.5% population and arbitrary phase control facilitated by the pump pulse carrier-envelope phase. Analysis of the optimized pulse structure reveals a sequential mechanism in which the valence excitation is reached via a fast (femtosecond) population transfer through an intermediate resonance state in the continuum rather than avoiding intermediate-state population with simultaneous or counterintuitive (stimulated Raman adiabatic passage) pulse sequences. Our results open a route to coupling valence excitations and core-hole excitations in molecules and aggregates that locally address specific atoms and represent an initial step towards realization of multidimensional spectroscopy in the xuv and x-ray regimes.

The fragmentation pathways and dynamics of ethylene molecules after core ionization are explored using coincident measurements of the Auger electron and fragment ions by employing the cold target recoil-ion momentum spectroscopy method. The influence of several factors on the dynamics and kinematics of the dissociation is studied. These include propensity rules, ionization mechanisms, symmetry of the orbitals from which the Auger electrons originate, multiple scattering, conical intersections, interference, and possible core-hole localization for the double ionization of this polyatomic molecule. Energy correlation maps allow probing the multidimensional potential energy surfaces and, in combination with our multiconfiguration self-consistent field calculations, identifying the populated electronic states of the dissociating dication. The measured angular distributions of the Auger electrons in the molecular frame further support and augment these assignments. The deprotonation and molecular hydrogen ion elimination channels show a nearly isotropic Auger electron angular distribution with a small elongation along the direction perpendicular to the molecular axis. For the symmetric breakup the angular distributions show a clear influence of multiple scattering on the outgoing electrons. The lowest kinetic energy release feature of the symmetric breakup channel displays a fingerprint of entangled Auger and photoelectron motion in the angular emission pattern identifying this transition as an excellent candidate to probe core-hole localization at a conical intersection of a polyatomic molecule.

The periodically $\delta$-kicked quantum linear rotor is known to experience nonclassical bounded energy growth due to quantum dynamical localization in angular momentum space. We study the effect of random deviations of the kick period in simulations and experiments. This breaks the energy and angular momentum localization and increases the rotational alignment, which is the analog of the onset of Anderson localization in one-dimensional chains.

We directly visualize the spatiotemporal evolution of a unidirectional rotating molecular rotational wave packet. Excited by two time-delayed polarization-skewed ultrashort laser pulses, the cigar- or disk-shaped rotational wave packet is impulsively kicked to unidirectionally rotate as a quantum rotor which afterwards disperses and exhibits field-free revivals. The rich dynamics can be coherently controlled by varying the timing or polarization of the excitation laser pulses. The numerical simulations very well reproduce the experimental observations and intuitively revivify the thoroughgoing evolution of the molecular rotational wave packet of unidirectional spin.

The feasibility of complete photoionization experiments, in which the full set of photoionization matrix elements is determined, using multiphoton ionization schemes with polarization-shaped pulses has recently been demonstrated [P. Hockett et al., Phys. Rev. Lett. 112, 223001 (2014)]. Here we extend our previous work to discuss further details of the numerics and analysis methodology utilized and compare the results directly to new tomographic photoelectron measurements, which provide a more sensitive test of the validity of the results. In so doing we discuss in detail the physics of the photoionization process and suggest various avenues and prospects for this coherent multiplexing methodology.

Photoelectron interferograms, manifested in photoelectron angular distributions (PADs), are high-information, coherent observables. In order to obtain the maximum information from angle-resolved photoionization experiments it is desirable to record the full, three-dimensional (3D), photoelectron momentum distribution. Here we apply tomographic reconstruction techniques to obtain such 3D distributions from multiphoton ionization of potassium atoms, and fully analyze the energy and angular content of the 3D data. The PADs obtained as a function of energy indicate good agreement with previous 2D data and detailed analysis [Hockett et al., Phys. Rev. Lett. 112, 223001 (2014)] concerning the main spectral features, but also indicate unexpected symmetry breaking in certain regions of momentum space, thus revealing additional continuum interferences which cannot otherwise be observed. These observations reflect the presence of additional ionization pathways and, most generally, illustrate the power of maximum-information measurements of coherent observables for quantum metrology of complex systems.

The thermodynamics of the inhomogeneous one-dimensional repulsive fermionic Hubbard model with parabolic confinement is studied by a density-functional-theory approach, based on Mermin's generalization to finite temperatures. A local-density approximation (LDA), based on exact results for the homogeneous model, is used to approximate the correlation part in the Helmholtz free-energy, comprising the thermodynamic Bethe ansatz LDA (TBALDA). The general presentation of the method is given and some properties of the homogeneous model that are relevant to the DFT approach are analyzed. Extensive comparison between TBALDA and numerical exact diagonalization results for thermodynamic properties of small inhomogeneous chains is discussed. In the remaining, a classical thermodynamic treatment of the confined system is developed with the focus on global properties of large systems. It is observed that, depending on the temperature and specific volume, the system can have its temperature increased under isentropic expansion. Such unusual behavior becomes more pronounced with the increase of the intrasite interaction $U$ and can be understood considering the peculiar behavior of the correlation entropy per site of the Hubbard model around half-filling. A related nonmonotonic variation of the temperature under isentropic change of $U$ is also studied. At the end, the dependence of the double occupancy on temperature and volume per particle is discussed.

We propose a theoretical idea to reach the $p$-wave superfluid phase in an ultracold Fermi gas. The key of our idea is that the pairing symmetry of a Fermi superfluid is fully dominated by the symmetry of the superfluid order parameter, which is essentially given by the product of a pair amplitude and a pairing interaction. Noting this, in our proposal, we first prepare a $p$-wave pair amplitude not by using a $p$-wave interaction, but by using the phenomenon that a $p$-wave pair amplitude is induced in an $s$-wave superfluid Fermi gas with antisymmetric spin-orbit interaction. In this case, although the system is still in the $s$-wave superfluid state with the $s$-wave superfluid order parameter, when one suddenly replaces the $s$-wave interaction by an appropriate $p$-wave one (which is possible in cold Fermi gases by using a Feshbach resonance technique), the product of the $p$-wave interaction and the $p$-wave pair amplitude that has already been prepared in the spin-orbit-coupled $s$-wave superfluid state immediately gives a finite $p$-wave superfluid order parameter. Thus, at least just after this manipulation, the system is in the $p$-wave superfluid state, being characterized by the artificially produced $p$-wave superfluid parameter. In this paper, to assess our idea, we evaluate the $p$-wave pair amplitude in a spin-orbit-coupled $s$-wave superfluid Fermi gas at $T=0$. We determine the region where a large $p$-wave pair amplitude is obtained in the phase diagram with respect to the strengths of the $s$-wave pairing interaction and the spin-orbit coupling. We also discuss the accessibility of this optimal region on the viewpoint of the superfluid phase transition temperature. Since the achievement of a $p$-wave superfluid Fermi gas is one of the most crucial issues in cold-atom physics, our proposal would be useful for this exciting challenge.

We study Dicke superradiance as collective and coherent absorption and (time-delayed) emission of photons from an ensemble of ultracold atoms in an optical lattice. Since this process depends on the coherence properties of the atoms (e.g., superfluidity), it can be used as a probe for their quantum state. In analogy to pump-probe spectroscopy in solid-state physics, this detection method facilitates the investigation of nonequilibrium phenomena and is less invasive than time-of-flight experiments or direct (projective) measurements of the atom number (or parity) per lattice site, which both destroy properties of the quantum state such as phase coherence.

We investigate the effect of next-nearest-neighbor hopping on the zero-temperature Drude weight or superfluidity in mesoscopic one-dimensional systems of (a) a single particle with quasidisorder (Aubry-André model) and (b) hard-core bosons with nearest-neighbor interaction. We show that there is an interaction-induced enhancement of the Drude weight when the next-nearest-neighbor hopping is frustrated for the many-body system. The observed nonmonotonic behavior of the Drude weight occurs because the repulsive interactions first suppress the frustration in the system, leading to a rise in the Drude weight, whereas at much larger interaction strengths the charge density wave fluctuations set in the insulator and the Drude weight drops. The present paper reveals a scenario in which a persistent flow enhancement is plausible in the presence of kinetic frustration and repulsive interactions.

The irradiation of a dilute cloud of cold atoms with a coherent light field produces a random intensity distribution known as laser speckle. Its statistical fluctuations contain information about the mesoscopic scattering processes at work inside the disordered medium. Following up on earlier work by Assaf and Akkermans [Phys. Rev. Lett. 98, 083601 (2007)], we analyze how static speckle-intensity correlations are affected by an internal Zeeman degeneracy of the scattering atoms. It is proven on general grounds that the speckle correlations cannot exceed the standard Rayleigh law. On the contrary, because which-path information is stored in the internal atomic states, the intensity correlations suffer from strong decoherence and become exponentially small in the diffusive regime applicable to an optically thick cloud.

We present an experimental and theoretical investigation of spontaneous pattern formation in the transverse section of a single retroreflected laser beam passing through a cloud of cold rubidium atoms. In contrast to previously investigated systems, the nonlinearity at work here is that of a two-level atom, which realizes the paradigmatic situation considered in many theoretical studies of optical pattern formation. In particular, we are able to observe the disappearance of the patterns at high intensity due to the intrinsic saturable character of two-level atomic transitions.

Generating nonclassical states of photons such as polarization-entangled states on a monolithic chip is a crucial step towards practical applications of optical quantum information processing such as quantum computing and quantum key distribution. Here we demonstrate two polarization-entangled photon sources in a single monolithic semiconductor waveguide. The first source is achieved through the concurrent utilization of two spontaneous parametric down-conversion (SPDC) processes, namely, type-0 and type-I SPDC processes. The chip can also generate polarization-entangled photons via the type-II SPDC process, enabling the generation of both copolarized- and cross-polarized polarization-entangled photons in the same device. In both cases, polarization entanglement is generated directly on the chip without the use of any off-chip compensation, interferometry, or bandpass filtering. This enables direct, chip-based generation of both Bell states $\left(\right|H,H\rangle +|V,V\rangle )/\sqrt{2}$ and $\left(\right|H,V\rangle +|V,H\rangle )/\sqrt{2}$ simultaneously utilizing the same pump source. In addition, based on compound semiconductors, this chip can be directly integrated with it own pump laser. This technique ushers an era of self-contained, integrated, electrically pumped, room-temperature polarization-entangled photon sources.