We address the interplay between dimension and quantum anomaly on the breathing mode frequency of a strongly interacting Fermi gas harmonically trapped at zero temperature. Using a beyond-mean-field Gaussian pair fluctuation theory, we employ periodic boundary conditions to simulate the dimensionality of the system and impose a local-density approximation, with two different schemes, to model different trapping potentials in the tightly confined axial direction. By using a sum-rule approach, we compute the breathing mode frequency associated with a small variation of the trapping frequency along the weakly confined transverse direction and describe its behavior as a function of the dimensionality, from two to three dimensions, and of the interaction strength. We compare our predictions with previous calculations on the two-dimensional breathing mode anomaly and discuss their possible observation in ultracold Fermi gases of ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$ atoms.

We consider the thermodynamic potential of a dilute Fermi gas with a contact interaction, at both finite temperature $T$ and nonzero effective magnetic fields $\mathbf{H}$, and derive the equation of state analytically using second-order perturbation theory. Special attention is paid to the nonanalytic dependence of $\mathrm{\Omega }$ on temperature $T$ and (effective) magnetic field $\mathbf{H}$, which exhibits a crossover behavior as the ratio of the two is continuously varied. This nonanalyticity is due to the particle-hole pair excitation being always gapless and long ranged. The nonanalytic crossover found in this paper can therefore be understood as an analog of the Ginzburg-Landau critical scaling, albeit only at the subleading order. We extend our results to an ${\mathcal{N}}_c$-component Fermi gas with an $\mathrm{SU}\left({\mathcal{N}}_c\right)$-symmetric interaction and point out possible enhancement of the crossover behavior by a large ${\mathcal{N}}_c$.

The atmospheric turbulence is the main factor that influences quantum properties of propagating optical signals and may sufficiently degrade the performance of quantum communication protocols. The probability distribution of transmittance (PDT) for free-space channels is the main characteristic of the atmospheric links. Applying the law of total probability, we derive the PDT by separating the contributions from turbulence-induced beam wandering and beam-spot distortions. As a result, the obtained PDT varies from log-negative Weibull to truncated log-normal distributions depending on the channel characteristics. Moreover, we show that the method allows one to consistently describe beam tracking, a procedure which is typically used in practical long-distance free-space quantum communication. We analyze the security of decoy-state quantum key exchange through the turbulent atmosphere and show that beam tracking does not always improve quantum communication.

We propose a scheme called difference weak measurement, and we illustrate its advantages for measuring a small longitude phase shift in high precision. Compared to the standard interferometry and standard weak measurement schemes, the proposed scheme has a much higher resolution in the presence of various practical imperfections, such as alignment error and light intensity variation error. Moreover, we highlight the advantage of utilizing a complex weak value, where its imaginary part can reduce the harmful effect induced by channel decoherence. Finally, we propose a closed-loop scenario to solve the narrow dynamic range problem that exists in the current weak measurement schemes. The difference weak measurement scheme simultaneously fulfills the requirements of high precision, wide dynamic range, and strong robustness, which makes it a powerfully practical tool for phase-shift measurement and other metrological tasks.

The set of correlations between particles in multipartite quantum systems is larger than those in classical systems. Nevertheless, it is subject to restrictions by the underlying quantum theory. In order to better understand the structure of this set, a possible strategy is to divide all correlations into two components, depending on the question of whether they involve an odd or an even number of particles. For pure multiqubit states we prove that these two components are inextricably interwoven and often one type of correlations completely determines the other. As an application, we prove that all pure qubit states with an odd number of qubits are uniquely determined among all mixed states by the odd component of the correlations. In addition, our approach leads to invariants under the time evolution with Hamiltonians containing only odd correlations and can simplify entanglement detection.

We present a classical model for high-order harmonic generation during the propagation of an intense laser pulse through an atomic gas. Numerical simulations of the model show excellent quantitative agreement with the corresponding quantum model for the blueshift and intensity reduction of the propagating laser pulse over experimentally realistic propagation distances. We observe a significant extension of the high-order harmonic cutoff due to propagation effects. A phase-space analysis of our classical model uncovers the mechanism behind this extension.

In every experimental test of a Bell inequality, we are faced with the problem of inefficient detectors. How we treat the events when no particle is detected has a big influence on the properties of the test. In this work, we study this influence. We show that the choice of postprocessing can change the critical detection efficiency, the equivalence between different inequalities, or the applicability of the nonsignaling principle. We also consider the problem of choosing the optimal postprocessing strategy. We show that this is a nontrivial problem and that different strategies are optimal for different ranges of detector efficiencies.

Recently, the quantumness of local correlations arising from separable states in the context of a Bell scenario has been studied and linked with superlocality [Phys. Rev. A 95, 032120 (2017)]. Here we investigate the quantumness of unsteerable correlations in the context of a given steering scenario. Generalizing the concept of superlocality, we define as supercorrelation the requirement for a larger dimension of the preshared randomness to simulate the correlations than that of the quantum states that generate them. Since unsteerable states form a subset of Bell local states, it is an interesting question whether certain unsteerable states can be supercorrelated. Here, we answer this question in the affirmative. In particular, the quantumness of certain unsteerable correlations can be pointed out by the notion of superunsteerability, the requirement for a larger dimension of the classical variable that the steering party has to preshare with the trusted party for simulating the correlations than that of the quantum states which reproduce them. This provides a generalized approach to quantify the quantumness of unsteerable correlations in convex operational theories.

We present a way to realize a multiplex-controlled phase gate of $n-1$ control qubits simultaneously controlling one target qubit, with $n$ qubits distributed in $n$ different cavities. This multiqubit gate is implemented by using $n$ qutrits ( three-level natural or artificial atoms) placed in $n$ different cavities, which are coupled to an auxiliary qutrit. Here, the two logic states of a qubit are represented by the two lowest levels of a qutrit placed in a cavity. We show that this $n$-qubit controlled phase gate can be realized using only $2n+2$ basic operations, i.e., the number of required basic operations only increases linearly with the number $n$ of qubits. Since each basic operation employs the qutrit-cavity or qutrit-pulse resonant interaction, the gate can be fast implemented when the number of qubits is not large. Numerical simulations show that a three-qubit controlled phase gate, which is executed on three qubits distributed in three different cavities, can be high-fidelity implemented by using a circuit QED system. This proposal is quite general and can be applied to a wide range of physical systems, with atoms, NV centers, quantum dots, or various superconducting qutrits distributed in different cavities. Finally, this method can be applied to implement a multiqubit controlled phase gate with atoms using a cavity. A detailed discussion on implementing a three-qubit controlled phase gate with atoms and one cavity is presented.

Continuous-variable systems realized in quantum optics play a major role in quantum information processing, and it is also one of the promising candidates for a scalable quantum computer. We introduce a resource theory for continuous-variable systems relevant to universal quantum computation. In our theory, easily implementable operations—Gaussian operations combined with feed-forward—are chosen to be the free operations, making the convex hull of the Gaussian states the natural free states. Since our free operations and free states cannot perform universal quantum computation, genuine non-Gaussian states—states not in the convex hull of Gaussian states—are the necessary resource states for universal quantum computation together with free operations. We introduce a monotone to quantify the genuine non-Gaussianity of resource states, in analogy to the stabilizer theory. A direct application of our resource theory is to bound the conversion rate between genuine non-Gaussian states. Finally, we give a protocol that probabilistically distills genuine non-Gaussianity—increases the genuine non-Gaussianity of resource states—only using free operations and postselection on Gaussian measurements, where our theory gives an upper bound for the distillation rate. In particular, the same protocol allows the distillation of cubic phase states, which enable universal quantum computation when combined with free operations.

We observed efficient sub-Doppler cooling of ${}^{85}\mathrm{Rb}$ atoms under a nonzero magnetic field in a moving optical lattice formed in a bichromatic magneto-optical trap. Trap laser detunings were biased or set differently for the counterpropagating laser beams so that the atoms could be trapped where the magnetic field was nonzero. We investigated the center position and the temperature of the atomic cloud. We found that the sub-Doppler cooling effect, known to decrease as the nonzero magnetic field increases in a magneto-optical trap, would in fact increase under a particular trap-laser detuning difference. We first derived the condition under theoretical considerations and then verified it experimentally by conducting nondestructive measurement of atomic temperature.

Based on a one-dimensional double-well superlattice with a unit filling of ultracold atoms per site, we propose a scheme to generate scalable entangled states in the superlattice through symmetry-protected resonant lattice shaking. Our scheme utilizes periodic lattice modulations with a specific two-body exchange symmetry to entangle two atoms in each unit cell with respect to their orbital degree of freedom, and the complete atomic system in the superlattice becomes a cluster of bipartite entangled atom pairs. To demonstrate this we perform ab initio quantum dynamical simulations using the multilayer multiconfiguration time-dependent Hartree method for mixtures, which accounts for all correlations among the atoms. The proposed clusters of bipartite entanglements manifest as an essential resource for various quantum applications, such as measurement-based quantum computation. The lattice shaking scheme to generate this cluster possesses advantages such as a high scalability, fast processing speed, rich controllability on the target entangled states, and accessibility within current experimental techniques.

We calculate the force of a near-resonant guided light field of an ultrathin optical fiber on a two-level atom. We show that, if the atomic dipole rotates in the meridional plane, the magnitude of the force of the guided light depends on the field propagation direction. The chirality of the force arises as a consequence of the directional dependencies of the Rabi frequency of the guided driving field and the rate of the spontaneous emission from the atom. This provides a unique method for controlling atomic motion in the vicinity of an ultrathin fiber.

We consider linear and nonlinear coherent perfect absorbers (CPAs) in multidimensional geometries and construct explicitly the respective perfectly absorbed solutions. The multidimensional CPAs have the structure of the so-called simple layers which represent the generalization of the point $\delta$ function potential to higher dimensions. The considered examples include broadband CPAs confined to a straight line (in a two-dimensional setting) and to a plane (in the three-dimensional space), CPAs for topological vortices on an absorbing circle, as well as axially symmetric CPAs on a sphere. Additionally, it is shown that a paraxial beam propagating along a surface nonlinear CPA embedded in the three-dimensional space can be stable against perturbations. The results are interpreted in applications to optical and acoustic systems.

We investigate both theoretically and experimentally four-wave mixing (FWM) in hot potassium vapor, generated by a copropagating pump and probe in an off-resonant double-$\mathrm{\Lambda }$ system, and present conditions when this atomic system is (1) a strong phase-insensitive parametric amplifier and (2) a source of large-amplitude squeezing. Theoretically, nonperturbative numerical calculations of optical Bloch-Maxwell equations have been solved for a four-level atomic system of K in order to derive the atomic polarization and then amplitudes of propagating optical waves, pump, probe, and conjugate. For potassium, to our knowledge, there are no such comparisons of theoretical and experimental results of gains of twin beams under the large range of FWM parameters as presented here. Results have shown that one-photon detuning has to be slightly larger than the Doppler broadened transition for large gains and strong squeezing. The gain is particularly large for small red two-photon detuning ($-2–6\mathrm{MHz}$) and high K density ($5.5–10\times {10}^{12}{\mathrm{cm}}^{-3}$). Following experimentally and theoretically determined relation between gains, probe transmissions, and squeezing in Rb and Cs, we have found parameters of FWM when maximum squeezing in hot K vapor is expected.

Frequent measurements can modify the decay of an unstable quantum state with respect to the free dynamics given by Fermi's golden rule. In a landmark article [A. G. Kofman and G. Kurizki, Nature (London) 405, 546 (2000)], Kofman and Kurizki concluded that in quantum decay processes, acceleration of the decay by frequent measurements, called the quantum anti-Zeno effect (AZE), appears to be ubiquitous, while its counterpart, the quantum Zeno effect, is unattainable. However, up to now there have been no experimental observations of the AZE for atomic radiative decay (spontaneous emission) in free space. In this work, making use of analytical results available for hydrogenlike atoms, we find that in free space, only non-electric-dipole transitions should present an observable AZE, revealing that this effect is consequently much less ubiquitous than first predicted. We then propose an experimental scheme for AZE observation, involving the electric quadrupole transition between $D_{5/2}$ and $S_{1/2}$ in the alkali-earth ions ${\mathrm{Ca}}^{+}$ and ${\mathrm{Sr}}^{+}$. The proposed protocol is based on the stimulated Raman adiabatic passage technique, which acts like a dephasing quasimeasurement.

Analyzing weak microwave signals in the GHz regime is a challenging task if the signal level is very low and the photon energy widely undefined. A superconducting qubit can detect signals in the low photon regime, but due to its discrete level structure, it is only sensitive to photons of certain energies. With a multilevel quantum system (qudit) in contrast, the unknown signal frequency and amplitude can be deduced from the higher level ac Stark shift. The measurement accuracy is given by the signal amplitude, its detuning from the discrete qudit energy level structure, and the anharmonicity. We demonstrate an energy sensitivity in the order of ${10}^{-3}$ with a measurement range of more than $1\mathrm{GHz}$. Here, using a transmon qubit, we experimentally observe shifts in the transition frequencies involving up to three excited levels. These shifts are in good agreement with an analytic circuit model and master equation simulations. For large detunings, we find the shifts to scale linearly with the power of the applied microwave drive. Exploiting the effect, we demonstrated a power meter which makes it possible to characterize the microwave transmission from source to sample.

Charge-exchange collisions of ${\mathrm{C}}^{6+}$ and ${\mathrm{O}}^{8+}$ ions with hydrogen and krypton atoms followed by radiative emissions are examined using the two-center basis generator method in the low- and intermediate-energy regimes. Capture cross sections are obtained within the independent-electron model and are used in a radiative cascade model to yield Lyman line-emission counts. The main focus of this analysis is on single-electron capture with the inclusion of autoionizing double capture in krypton collisions. Because hydrogen and krypton have the same first ionization potential, it is expected from the classical overbarrier model that the capture selectivity of these two atoms is the same, making krypton a good surrogate for hydrogen to study collision-induced radiative emissions in the laboratory. However, the present analysis shows that the subshell distributions with respect to the impact energy in krypton collisions can differ from hydrogen collisions and, hence, from predictions of the classical overbarrier model. Based on the comparison of present results with previous measurements, this difference in capture behavior of the two target atoms does not appear problematic for the Lyman line-emission results for ${\mathrm{C}}^{6+}$ collisions, but does appear to affect these results for ${\mathrm{O}}^{8+}$ collisions.

Time-resolved pump-probe measurements of Xe, pumped at 133 nm and probed at 266 nm, are presented. The pump pulse prepared a long-lived hyperfine wave packet in the Xe $5p^5\left({}^{2}P_{1/2}^{\circ }\right)6s{}^2{[1/2]}_1^{\circ }$ manifold ($E=77185{\mathrm{cm}}^{-1}=9.57\mathrm{eV}$). The wave packet was monitored via single-photon ionization and velocity map photoelectron images were measured. The images provide angle- and time-resolved data which, when obtained over a large time window (900 ps), constitute a precision quantum-beat spectroscopy measurement of the hyperfine state splittings. Additionally, analysis of the full photoelectron image stack provides a quantum-beat imaging modality, in which the Fourier components of the photoelectron images correlated with specific beat components can be obtained. This may also permit the extraction of isotope-resolved photoelectron images in the frequency domain, in cases where nuclear spins (hence beat components) can be uniquely assigned to specific isotopes (as herein), and also provides phase information relating to the ionization dynamics. The information content of both raw and inverted image stacks is investigated, suggesting the utility of the Fourier analysis methodology in cases where images cannot be inverted.