Polarization transfers from an electron spin to a nuclear spin are essential for various physical tasks, such as dynamic nuclear polarization in nuclear magnetic resonance and quantum information processing on hybrid electron-nuclear spin systems. We present time-optimal schemes for electron-nuclear polarization transfers which improve on conventional approaches, and we thereby establish an important class of faster controls. We highlight how time-optimal polarization transfers and their optimality are related to the time optimality of unitary transformations. Moreover, our work develops generally applicable analytic methods for analyzing the limits in controlling quantum systems.

We investigate continuous-time quantum walks of two fermionic atoms loaded in one-dimensional optical lattices with on-site interaction and subjected to a Zeeman field. The quantum walks are accompanied by spin-flipping processes. We calculate the time-dependent density distributions of the two fermions with opposite spins which are initially positioned at the center site by means of an exact numerical method. Besides the usual fast linear expansion behavior, we find an interesting spin-flipping-induced localization in the time evolution of density distributions. We show that the fast linear expansion behavior can be restored by simply ramping up the Zeeman field or further increasing the spin-flipping strength. The intrinsic origin of this exotic phenomenon is attributed to the emergence of a flat band in the single-particle spectrum of the system. Furthermore, we investigate the effect of on-site interaction on the dynamics of the quantum walkers. The two-particle correlations are calculated and the signal of localization is also shown therein. A simple potential experimental application of this interesting phenomenon is proposed.

High-contrast matter-wave interferometry is essential in various fundamental quantum mechanical experiments as well as for technical applications. Thereby, contrast and sensitivity are typically reduced by decoherence and dephasing effects. While decoherence accounts for a general loss of quantum information in a system due to entanglement with the environment, dephasing is due to collective time-dependent external phase shifts, which can be related to temperature drifts, mechanical vibrations, and electromagnetic oscillations. In contrast to decoherence, dephasing can, in principle, be reversed. Here, we demonstrate in experiment and theory a method for the analysis and reduction of the influence of dephasing noise and perturbations consisting of several external frequencies in an electron interferometer. This technique uses the high spatial and temporal resolution of a delay-line detector to reveal and remove dephasing perturbations by second-order correlation analysis. It allows matter-wave experiments under perturbing laboratory conditions and can be applied, in principle, to electron, atom, ion, neutron, and molecule interferometers.

We study in detail a system of two interferometers aimed at detecting extremely faint phase fluctuations. This system can represent a breakthrough for detecting a faint correlated signal that would remain otherwise undetectable even using the most sensitive individual interferometric devices, as in the case of so-called holographic noise. The signature of this kind of noise emerges as a correlation between the output signals of the interferometers. On the other hand, when holographic noise is absent one expects uncorrelated signals since the time-averaged fluctuations due to shot noise and other independent contributions vanish (though limiting the overall sensitivity). We show how injecting quantum light in the free ports of the interferometers can reduce the photon noise of the system beyond the shot noise, enhancing the resolution in the phase-correlation estimation. We analyze the use of both the two-mode squeezed vacuum and two independent squeezed states. Our results confirm the benefit of using squeezed beams together with strong coherent beams in interferometry. We also investigate the possible use of the two-mode squeezed vacuum, discovering interesting and unexplored areas of application of bipartite entanglement, in particular the possibility of reaching in principle a surprising uncertainty reduction.

We consider a system of dynamically-modulated photonic resonator lattice undergoing photonic transition and show that in the ultrastrong-coupling regime such a lattice can exhibit nontrivial topological properties, including topologically nontrivial band gaps, and the associated topologically robust one-way edge states. Compared with the same system operating in the regime where the rotating wave approximation is valid, operating the system in the ultrastrong-coupling regime results in a one-way edge mode that has a larger bandwidth and is less susceptible to loss. Also, in the ultrastrong-coupling regime, the system undergoes a topological insulator-to-metal phase transition as one varies the modulation strength. This phase transition has no counterpart in systems satisfying the rotating wave approximation, and its nature is directly related to the nontrivial topology of the quasienergy space.

The Rabi model plays a fundamental role in understanding light-matter interaction. It reduces to the Jaynes-Cummings model via the rotating-wave approximation, which is applicable only to the cases of near resonance and weak coupling. However, recent experimental breakthroughs in upgrading light-matter coupling order require understanding the physics of the full quantum Rabi model (QRM). Despite the fact that its integrability and energy spectra have been exactly obtained, the challenge to formulate an exact wave function in a general case still hinders physical exploration of the QRM. Here we unveil a ground-state phase diagram of the QRM, consisting of a quadpolaron and a bipolaron as well as their changeover in the weak-, strong-, and intermediate-coupling regimes, respectively. An unexpected overweighted antipolaron is revealed in the quadpolaron state, and a hidden scaling behavior relevant to symmetry breaking is found in the bipolaron state. An experimentally accessible parameter is proposed to test these states, which might provide novel insights into the nature of the light-matter interaction for all regimes of the coupling strengths.

We experimentally investigate a cavity in which two resonant meta-atoms are embedded. Its corresponding response spectrum to an incident microwave shows an electromagnetically-induced-transparency–like (EIT-like) phenomenon when the meta-atoms are arranged in the sequence node-antinode but not in the sequence antinode-node. We carry out in situ measurements of the cavity field to bring physical insight into this asymmetric response, and use the transfer matrix method joined with coupled-mode theory to give a quantitative analysis. Finally, we demonstrate the unidirectional EIT-like behavior in a two-port design. Our results may benefit novel nonreciprocal devices in the future by exploring the nonlinearity of the meta-atoms with this asymmetric arrangement.

The use of distributed amplifiers may have some potential advantages for the transmission of quantum information through optical fibers. In addition to the quantum noise introduced by the amplifiers, entanglement between atoms in the amplifying media and the optical field corresponds to which-path information that can further reduce the coherence. Here we analyze the effects of decoherence in a phase-insensitive distributed amplifier by using perturbation theory to calculate the state of the entire system including the atomic media. For an initial coherent state, tracing over the atomic states allows the reduced density matrix of the field to be expressed as a mixture of squeezed states with a reduced spread in photon number and an increased phase uncertainty. The amplifier noise and decoherence can be interpreted as being due to entanglement with the environment rather than the amplification of vacuum fluctuation noise.

We propose and investigate a type of optical waveguide made by an array of atoms without involving conventional Bragg scattering or total internal reflection. A finite chain of atoms collectively coupled through their intrinsic resonance supports a propagating mode with minimal radiative loss when the array spacing $a$ is around $0.6{\lambda }_0/2\pi$ where ${\lambda }_0$ is the wavelength of the nearly resonant optical transition. We find that the transportation is robust with respect to position fluctuation and remains possible when the atoms are placed on a circle. Our result paves the way to implement the subwavelength transportation of light in integrated optical circuits with cold atoms.

Probabilities for atomic electron excitation (shake-up) and ionization (shake-off) are studied following the beta-decay process ${}^6\text{He}\to {}^6\text{Li}^{+}+e^{-}+{\overline{\nu }}_e$, and in particular, recoil-induced contributions to the shake-off probability are calculated within the nonrelativistic sudden approximation. A pseudostate expansion method together with Stieltjes imaging is used to represent the complete two-electron spectrum of final ${}^6\text{Li}^{+},{}^6\text{Li}^{2+}$, and ${}^6\text{Li}^{3+}$ states. Results for the recoil correction show a $7\sigma$ disagreement with the experiment of Carlson et al. [Phys. Rev. 129, 2220 (1963)]. A variety of sum rules, including a newly derived Thomas-Reich-Kuhn oscillator strength sum rule for dipole recoil terms, provides tight constraints on the accuracy of the results. Calculations are performed for the helium $1s2s{}^{3}S$ metastable state, as well as for the $1s^2{}^{1}S$ ground state. Our results would reduce the recoil-induced correction to the measured electroneutrino coupling constant $a_{e\mu }$ from the apparent 0.6% used in the experiments to 0.09%.

Recently, entanglement concentration was explicitly shown to be irreversible. However, it is still not clear what kind of states can be reversibly converted in the asymptotic setting by local operations and classical communication (LOCC) when neither the initial nor the target state is maximally entangled. We derive the necessary and sufficient condition for the reversibility of LOCC conversions between two bipartite pure entangled states in the asymptotic setting. In addition, we show that conversion can be achieved perfectly with only local unitary operation under such condition except for special cases. Interestingly, our result implies that an error-free reversible conversion is asymptotically possible even between states whose copies can never be locally unitarily equivalent with any finite numbers of copies, although such a conversion is impossible in the finite setting. In fact, we show such an example. Moreover, we establish how to overcome the irreversibility of LOCC conversion in two ways. As for the first method, we evaluate how many copies of the initial state are to be lost to overcome the irreversibility of LOCC conversion. The second method is to add a supplementary state appropriately, which also works for local unitary conversion unlike the first method. Especially, for the qubit system, any nonmaximally pure entangled state can be a universal resource for the asymptotic reversibility when copies of the state are sufficiently many. More interestingly, our analysis implies that far-from-maximally entangled states can be better than nearly maximally entangled states as this type of resource. This fact brings new insight to the resource theory of state conversion.

In the relativistic description of atomic systems in external fields, the total momentum and the external electric field couple to the angular momentum of the individual particles. Therefore, the motional state of an ion in a particle trap influences measurements of internal observables such as energy levels or the $g$ factor. We calculate the resulting relativistic shift of the Larmor frequency and the corresponding $g$-factor correction for a bound electron in a hydrogenlike ion in the $1 S$ state due to the ion moving in a Penning trap and show that it is negligible at the current precision of measurements. We also show that the analogous energy shift for measurements with an ion in the ground state of a Paul trap vanishes in leading order.

In this work, we calculate dynamic polarizabilities and hyperfine-structure $A$ and $B$ constants of a few low-lying states for ${\mathrm{Sc}}^{2+}$. The sum-over-states technique is applied to calculate the polarizabilities of the $3d {}^2{D}_{3/2},3d {}^2{D}_{5/2}$, and $4s {}^2{S}_{1/2}$ states. The most important and correlation sensitive part of the sum is calculated using a highly correlated relativistic coupled-cluster theory. The remaining part of the sum is calculated using a lower-order many-body perturbation theory and the Dirac-Fock theory. Present dynamic polarizabilities are important to investigate the Stark shifts in the $4s {}^2{S}_{1/2}–3d {}^2{D}_{5/2}$ and $4s {}^2{S}_{1/2}–3d {}^2{D}_{3/2}$ clock transitions of ${\mathrm{Sc}}^{2+}$. Magic wavelengths for zero Stark shifts corresponding to these transitions are found in the vacuum-ultraviolet region. The coupled-cluster theory is used to estimate the hyperfine $A$ and $B$ constants with a very high accuracy.

We use the classical trajectory Monte Carlo method to analyze charge exchange processes between fully stripped projectiles with $\mathrm{H}\left(1s\right)$ in a screened environment at impact energies in the range 18 eV/amu to 10 keV/amu. For $\mathrm{Fe}^{26+}$ projectiles, $(n,l)$-state selective charge exchange and line emission cross sections are presented for Debye screening lengths from 15 a.u. up to the unscreened case limit. At low-impact energies, a strong dependence of the hardness ratio on the Debye screening length is found. We show that such strong dependence also evidences itself for $\mathrm{Ne}^{10+},\mathrm{P}^{15+},\mathrm{Ar}^{18+}$, and $\mathrm{Kr}^{36+}$ projectiles. Clear indications of this dependence are noticeable in the photonic spectra even for large screening lengths ($\sim 100$ a.u.).

Extreme events in the form of pulses of extraordinary intensity (sometimes also called optical rogue waves) are easily observed in the chaotic regime of an all-solid-state laser with a saturable absorber if the Fresnel number of the cavity is high. This result suggests that the nonlinear interaction among transverse modes is an essential ingredient in the formation of extreme events in this type of laser, but there is no theoretical description of the phenomenon yet. We report here a set of experimental results on the regularities of these extreme events in order to provide a basis for the development of such a description. Among these results, we point out here (i) the decay of the correlation across the transversal section of the laser beam, and (ii) the appearance of extreme events even if the time elapsed since the previous pulse is relatively short (in terms of the average interpulse time interval), which indicates the existence of some unknown mechanism of energy storage. We hypothesize that this mechanism is related to the imperfect depletion of the gain by some of the transversal modes. We also present evidence in support of this hypothesis.

By analyzing the decaying intensity, leaking out a high-finesse cavity previously “filled” by a cw laser source (using the cavity ring-down spectroscopy technique), we observed frequency beating between what we think are two orthogonal eigenpolarization states of the intracavity electromagnetic field. The time decay (ring down) is analyzed by varying the angle of the polarization analyzer located in front of the detector. A full modeling of the observed signal is proposed. It is based on the Jones matrix formalism required for modeling the cavity behavior following a rotated phase shifter. The full transfer function is first established in the frequency domain, and then Fourier transformed to recover the temporal response. The same optical cavity, i.e., constituted of the same set of mirrors, is used at two different wavelengths ($\sim 800$ and $\sim 880$ nm). It demonstrates the differences in behavior between a high-finesse cavity $\left(\sim 400000\right)$ and a lower finesse cavity $\left(\sim 50000\right)$. Beating frequency, characteristics time, and beat amplitude are mainly discussed versus the analyzer angle. A cavity birefringence of $\sim 1.6\times {10}^{-5}$ rad, resulting from the mirror birefringence is suggested. If the current analysis is in agreement with pulsed CRDS experiments (polarimetry) obtained in an isotropic moderate-finesse cavity, it differs from a recent work report on a high-finesse cavity associated with a source mode locking [Phys. Rev. A 85, 013837 (2012)].

We consider specific signatures of squeezing for time-modulated light fields and propose the scheme of an optical parametric oscillator driven by a continuously modulated pump field. The application of a periodically modulated driving field instead of a continuous wave field drastically improves the degree of quadrature integral squeezing. This quantity goes below the standard limit of 50% relative to the level of vacuum fluctuations. We develop semiclassical and quantum theories of an optical parametric oscillator under the influence of a pump field with harmonically modulated amplitude for all operational regimes, including numerical simulations at the threshold point. The results can be directly applied in time-resolved quantum communication protocols.

The orthogonality of cat and displaced cat states, underlying Heisenberg limited measurement in quantum metrology, is studied in the limit of a large number of states. The mesoscopic superposition of the generalized cat state is correlated with the corresponding state overlap function, controlled by the sub-Planck structures arising from phase-space interference. The asymptotic expression of this overlap function is evaluated, and the validity of large phase-space support and distinguishability of the constituent states, in which context the asymptotic limit is achieved, are discussed in detail. For a large number of coherent states, uniformly located on a circle, the overlap function significantly matches the diffraction pattern for a circular ring source with uniform angular strength. This is in accordance with the van Cittert–Zernike theorem, where the overlap function, similar to the mutual coherence function, matches a diffraction pattern. The physical situation under consideration is delineated in phase space by utilizing the Husimi $Q$ function.

We study the nonlinear interaction of a cold sample of ${}^{88}\mathrm{Sr}$ atoms coupled to a single mode of a low finesse optical cavity in the so-called bad cavity limit, and we investigate the implications for applications to laser stabilization. The atoms are probed on the weak intercombination line $|5s^2{}^1{S}_0\rangle -|5s5p{}^3{P}_1\rangle$ at 689 nm in a strongly saturated regime. Our measured observables include the atomic induced phase shift and absorption of the light field transmitted through the cavity represented by the complex cavity transmission coefficient. We demonstrate high signal-to-noise-ratio measurements of both quadratures—the cavity transmitted phase and absorption—by employing frequency modulation (FM) spectroscopy (noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy). We also show that when FM spectroscopy is employed in connection with a cavity locked to the probe light, observables are substantially modified compared to the free-space situation in which no cavity is present. Furthermore, the nonlinear dynamics of the phase dispersion slope is experimentally investigated, and the optimal conditions for laser stabilization are established. Our experimental results are compared to state-of-the-art cavity QED theoretical calculations.