We show that quantum Zeno dynamics can be mimicked by the isolated evolution of an unobserved system in an effective potential. Monitoring frequently whether a particle remains in a region of space leads to the same wave-packet dynamics as placing the region on top of a potential barrier and letting the particle evolve on its own, without external couplings. We focus on very frequent but not continuous observation so that the particle abandons the initial region with some finite probability. The height of the barrier relative to the surroundings for a high frequency $\nu$ of the observations being mimicked is found numerically to be $h\nu /2$, where $h$ is Planck's constant.

We report a destabilization mechanism of localized solutions in spatially extended systems which is induced by delayed feedback. Considering a model of a wide-aperture laser with a saturable absorber and delayed optical feedback, we demonstrate the appearance of multiple coexistent laser cavity solitons. We show that at large delays apart from the drift and phase instabilities the soliton can exhibit a delay-induced modulational instability associated with the translational neutral mode. The combination of drift and modulational instabilities produces a zigzagging motion of the solitons, which are either periodic, with the period close to the delay time, or chaotic, with low-frequency fluctuations in the direction of the soliton motion. The same type of modulational instability is demonstrated for localized solutions of the cubic-quintic complex Ginzburg-Landau equation.

In this work, we extensively study the problem of broadcasting of quantum correlations (QCs). This includes broadcasting of quantum entanglement as well as correlations that go beyond the notion of entanglement (QCsbE). It is quite well known from the “no-broadcasting theorem” that perfect broadcasting of QCs is not possible. However, it does not rule out the possibility of partial broadcasting of QCs where we can get lesser correlated states from a given correlated state. In order to have a holistic view of broadcasting, we investigate this problem by starting with a most general representation of two qubit mixed states in terms of the Bloch vectors. As a cloning transformation we have used universal symmetric optimal Buzek-Hillery (BH) cloner both locally and nonlocally. Unlike entanglement, we find that it is impossible to broadcast QCsbE optimally. Lastly, we generalize these results for any symmetric or asymmetric cloning machines as well. This result brings out a fundamental difference between the correlations defined from the perspective of entanglement and the correlations measure which claims to go beyond entanglement.

We develop generalization of the fixed-phase diffusion Monte Carlo method for Hamiltonians which explicitly depends on particle spins such as for spin-orbit interactions. The method is formulated in a zero-variance manner and is similar to the treatment of nonlocal operators in commonly used static-spin calculations. Tests on atomic and molecular systems show that it is very accurate, on par with the fixed-node method. This opens electronic structure quantum Monte Carlo methods to a vast research area of quantum phenomena in which spin-related interactions play an important role.

Dielectronic recombination (DR) is a key atomic process that affects the spectroscopic diagnostic modeling of tungsten, most of whose ionization stages will be found somewhere in the ITER fusion reactor: in the edge, divertor, or core plasma. Accurate DR data are sparse while complete DR coverage is unsophisticated (e.g., average-atom or Burgess General Formula), as illustrated by the large uncertainties that currently exist in the tungsten ionization balance. To this end, we present a series of partial final-state-resolved and total DR rate coefficients for ${\text{W}}^{73+}$ to ${\text{W}}^{56+}$ tungsten ions. This is part of a wider effort within The Tungsten Project to calculate accurate dielectronic recombination rate coefficients for the tungsten isonuclear sequence for use in collisional-radiative modeling of finite-density tokamak plasmas. The recombination rate coefficients have been calculated with autostructure using $\kappa \text{-averaged}$ relativistic wave unctions in level resolution (intermediate coupling) and configuration resolution (configuration average). Comparison with previous calculations of total DR rate coefficients for ${\text{W}}^{63+}$ and ${\text{W}}^{56+}$ yield agreement to within 20% and 10%, respectively, at peak temperature. It is also seen that the Jüttner correction to the Maxwell distribution has a significant effect on the ionization balance of tungsten at the highest charge states, changing both the peak abundance temperatures and the ionization fractions of several ions.

The process of capture of a molecular ensemble into rotational resonance in the optical centrifuge is investigated. The adiabaticity and phase-space incompressibility are used to find the resonant capture probability in terms of two dimensionless parameters $P_{1,2}$ characterizing the driving strength and the nonlinearity, and related to three characteristic time scales in the problem. The analysis is based on the transformation to action-angle variables and the single resonance approximation, yielding reduction of the three-dimensional rotation problem to one degree of freedom. The analytic results for capture probability are in good agreement with simulations. The existing experiments satisfy the validity conditions of the theory.

We propose a class of variational Gaussian wave functions to describe Fröhlich polarons at finite momenta. Our wave functions give polaron energies that are in excellent agreement with the existing Monte Carlo results for a broad range of interactions. We calculate the effective mass of polarons and find smooth crossover between weak- and intermediate-coupling strength. Effective masses that we obtain are considerably larger than those predicted by the mean-field method. A prediction based on our variational wave functions is a special pattern of correlations between host atoms that can be measured in time-of-flight experiments.

The terahertz frequency range is lacking an experimental implementation of Hanbury Brown and Twiss photon correlation measurements with sub-cycle time resolution and high sensitivity. Such a technique would be needed, for example, to observe photon pairs predicted to be released in a nonadiabatic modulation of an ultrastrongly coupled light-matter system. In this paper, we propose a room-temperature measurement of photon correlations in the THz range based on electro-optic sampling. We apply this technique to a THz quantum cascade laser and measure below and above threshold first- and second-order degree of coherence with a subcycle temporal resolution of 146 fs. The sensitivity of the proposed measurement scheme is so far limited to $\sim 1500$ photons. This technique can, in principle, be extended for ultrahigh bandwidth single-photon sensitivity in a wide range of frequencies.

Measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks, is the most promising solution to the security issues in practical quantum key distribution systems. Although several experimental demonstrations of MDI-QKD have been reported, they all make one crucial but not yet verified assumption, that is, there are no flaws in state preparation. Such an assumption is unrealistic and security loopholes remain in the source. Here we present a MDI-QKD experiment with the modulation error taken into consideration. By applying the loss-tolerant security proof by Tamaki et al. [Phys. Rev. A 90, 052314 (2014)], we distribute secure keys over fiber links up to 40 km with imperfect sources, which would not have been possible under previous security proofs. By simultaneously closing loopholes at the detectors and a critical loophole—modulation error in the source, our work shows the feasibility of secure QKD with practical imperfect devices.

We report diffusion Monte Carlo results for the ground state of unpolarized spin-1/2 fermions in a cylindrical container and properties of the system with a vortex-line excitation. The density profile of the system with a vortex line presents a nonzero density at the core. We calculate the ground-state energy per particle, the superfluid pairing gap, and the excitation energy per particle. These simulations can be extended to calculate the properties of vortex excitations in other strongly interacting systems such as superfluid neutron matter using realistic nuclear Hamiltonians.

Quantum simulations based on near-resonance Bose gases are limited by their short lifetimes due to severe atom losses. In addition to this, the recently predicted thermodynamical instability adds another constraint on accessing the resonant Bose gases. In this paper, we offer a potential solution by proposing long-lived resonant Bose gases in both two and three dimensions, where the conventional few-body losses are strongly suppressed. We show that the thermodynamical properties as well as the lifetimes of these strongly interacting systems are universal and independent of short-range physics.

Inspired by the idea of ghost imaging, we propose a ghost scattering scheme to study light scattering with incoherent light sources through the nonlocal correlation measurement of the differential scattering cross-section fluctuations in two different optical paths. We present a rigorous formal theory to describe the ghost scattering process. Also we have derived a simple and closed-form ghost scattering formula within the first-order Born approximation which is particularly suited for weak scatterers. We find that the scattering information of a test scatterer can be obtained by using only a single-pixel detector in the corresponding optical path through the nonlocal correlation measurement with the help of another reference path.

We propose a feasible approach to generate the desired light with controllable photon bunchings by adjusting the atomic superpositions in a driven cavity. Under the large detuning limit, i.e., the cavity is far resonance with the inside atom(s), we show that the photons in the cavity are always bunchings. Typically, when the effective dispersive interaction equals the detuning between the driving and cavity fields, we find that the value of second-order correlation $g^{\left(2\right)}\left(0\right)$ inverses to the probability of the superposed atomic state. This suggests that such a value could be arbitrarily large, and thus the bunchings of the photons could be significantly enhanced.

We propose and experimentally demonstrate a scheme to generate optically controlled delays based on off-resonant Raman absorption. Dispersion in a transparency window between two neighboring, optically activated Raman absorption lines is used to reduce the group velocity of broadband 765 nm pulses. We implement this approach in a potassium titanyl phosphate (KTP) waveguide at room temperature, and demonstrate Raman-induced delays of up to 140 fs for a 650-fs duration, 1.8-THz bandwidth, pulse. Our approach should be applicable to single-photon signals, offers wavelength tunability, and is a step toward processing ultrafast photons.

We introduce a quantum-optical notion of nonclassicality that we call the process output nonclassicality for multimode quantum channels. The motivation comes from an information-theoretic point of view and the emphasis is on the output states of a channel. We deem a channel to be “classical” if its outputs are always classical irrespective of the input, i.e., if the channel is nonclassicality breaking, and nonclassical otherwise. Our condition is stronger than the one considered by Rahimi-Keshari et al., [Phys. Rev. Lett. 110, 160401 (2013)] and we compare the two approaches. Using our framework we then quantify the nonclassicality of a quantum process by introducing a noise-robustness type of measure that we call the nonclassicality depth of a channel. It characterizes a certain threshold noise beyond which a given channel outputs only classical states. We achieve this by generalizing a prescription by Lee [Phys. Rev. A 44, R2775 (1991)] to multimode states and then by extension to multimode channels.

We study the violation of the Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) inequality for entangled SU(1,1) coherent states of the form proposed by Perelomov. Specifically, we examine Bell-CHSH violations by such states in the case in which distant observers Alice and Bob perform local, noncompact, SU(1,1) transformations characterized by hyperbolic angles on each of the subsystems and subsequently measure dichotomic observables, namely SU(1,1) parity operators. We find significant violations over a broad range of hyperbolic angles.

It is shown that adiabatic cycles excite a quantum particle, which is confined in a one-dimensional region and is initially in an eigenstate. During the cycle, an infinitely sharp wall is applied and its strength and position are varied. After the completion of the cycle, the state of the particle arrives at another eigenstate. It is also shown that we may vary the final adiabatic state by choosing the parameters of the cycle. With a combination of these adiabatic cycles, we can connect an arbitrary pair of eigenstates. Hence, these cycles may be regarded as the basis of the adiabatic excitations. A detailed argument is provided for the case that the particle is confined by an infinite square well. This is an example of exotic quantum holonomy in Hamiltonian systems.

In a circuit consisting of two or more resonators, the intercavity crosstalk is inevitable, which could create some problems, such as degrading the performance of quantum operations and the fidelity of various quantum states. The focus of this work is to propose a crosstalk-insensitive method for simultaneously coupling multiple pairs of resonators, which is important in large-scale quantum information processing and communication in a network consisting of resonators or cavities. In this work, we consider $2N$ resonators of different frequencies, which are coupled to a three-level quantum system (qutrit). By applying a strong pulse to the coupler qutrit, we show that an effective Hamiltonian can be constructed for simultaneously coupling multiple pairs of resonators. The main advantage of this proposal is that the effect of inter-resonator crosstalks is greatly suppressed by using resonators of different frequencies. In addition, by employing the qutrit-resonator dispersive interaction, the intermediate higher-energy level of the qutrit is virtually excited and thus decoherence from this level is suppressed. This effective Hamiltonian can be applied to implement quantum operations with photonic qubits distributed in different resonators. As one application of this Hamiltonian, we show how to simultaneously generate multiple Einstein-Podolsky-Rosen pairs of photonic qubits distributed in $2N$ resonators. Numerical simulations show that it is feasible to prepare two high-fidelity EPR photonic pairs using a setup of four one-dimensional transmission line resonators coupled to a superconducting flux qutrit with current circuit QED technology.

We conduct a theoretical study of magnetically induced Feshbach resonances and near-threshold bound states in isotopic NaK pairs. Our calculations accurately reproduce Feshbach spectroscopy data on $\mathrm{Na}{}^{40}\mathrm{K}$ and explain the origin of the observed multiplets in the $p$ wave [Phys. Rev. A 85, 051602(R) (2012)]. We apply the model to predict scattering and bound state threshold properties of the boson-boson $\mathrm{Na}{}^{39}\mathrm{K}$ and $\mathrm{Na}{}^{41}\mathrm{K}$ systems. We find that the $\mathrm{Na}{}^{39}\mathrm{K}$ isotopic pair presents broad magnetic Feshbach resonances and favorable ground-state features for producing nonreactive polar molecules by two-photon association. Broad $s$-wave resonances are also predicted for $\mathrm{Na}{}^{41}\mathrm{K}$ collisions.

We propose a smooth pseudopotential for the contact interaction acting between ultracold atoms confined to two dimensions. The pseudopotential reproduces the scattering properties of the repulsive contact interaction up to 200 times more accurately than a hard disk potential, and in the attractive branch gives a tenfold improvement in accuracy over the square well potential. Furthermore, the potential enables diffusion Monte Carlo simulations of the ultracold gas to be run 15 times quicker than was previously possible.