The ground-state Hanle effect (GSHE) in alkali-metal atomic vapor using a single circularly polarized light wave underlies one of the most reliable and simple techniques of modern atomic magnetometry. This effect causes a narrow (subnatural-width) resonance in the light wave intensity passing through the vapor cell. The GSHE-based sensors typically operate in the so-called spin-exchange relaxation-free (SERF) regime to reduce the resonance linewidth. However, this regime requires a relatively high temperature of vapors (approximately equal to $150{}^{\circ }\mathrm{C}$), leading to high heat release and power consumption of the sensor head. In addition, without applying special measures, the SERF regime significantly limits the dynamic range of measurements. Here we study a pump-probe scheme involving a single elliptically polarized light wave and a polarimetric detection technique. The wave is in resonance with two adjacent optical transitions in the cesium $D_1$ line ($\lambda \approx 894.5$ nm) due to their overlap in the presence of a buffer gas (130 Torr neon). Using a small ($V\approx 0.1{\mathrm{cm}}^3$) glass vapor cell, we demonstrate the possibility to observe subnatural-width resonances with a high contrast-to-width ratio (up to 45%/mG) in low-temperature ($60{}^{\circ }\mathrm{C}$) operation due to strong light-induced circular dichroism in the medium. Based on the $\mathrm{\Lambda }$ scheme of atomic energy levels, we obtain explicit analytical expressions for the shape of the resonance line. The model reveals a linewidth narrowing effect due to openness of the level scheme. The observed bright features are unusual for magneto-optical atomic spectroscopy as openness is commonly considered to be an undesirable effect that degrades resonance characteristics. Measuring the noise voltage, we estimate the sensitivity of the magnetic-field measurements to be 1.8 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ with a sensitivity of 60 $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ in the photon-shot-noise limit. In general, the results contribute to the theory of GSHE resonances and also can be applied to development of a low-temperature high-sensitivity miniaturized magnetic-field sensor with an extended dynamic range.

We discuss finite-temperature damping of rotons in elongated Bose-condensed dipolar gases, which are in the Thomas-Fermi regime in the tightly confined directions. The presence of many branches of excitations, which can participate in the damping process, is crucial for the Landau damping and results in significant increase of the damping rate. It is found, however, that even rotons with energies close to the roton gap may remain fairly stable in systems with the roton gap as small as $1\mathrm{nK}$.

A single paraxial beam reflection at a plane dielectric interface, configured appropriately, can lead to the formation of a polarization singularity in the inhomogeneously polarized output beam field for any central angle of incidence. In this paper we derive the necessary condition to realize this effect. We explore the phase singularity characteristics associated with this polarization-singular field and explore the dynamics of the singularities due to controlled variations of the input polarization. The simulation-generated exact field information lead to the exploration of the unique Goos-Hänchen, Imbert-Fedorov, and spin shifts of the optical-singular fields and the anticipation of an exact mathematical characterization of spin-orbit interaction phenomena involved therein. The formation of a phase singularity independent of a polarization singularity is explained subsequently. Interrelating these seemingly unconnected beam-field phenomena and generic optical singularities can lead to a significant and fundamental understanding of the inhomogeneously polarized beam field; additionally, our singularity generation method can find potential application in experimental characterization of the involved dielectric media.

The characterization of physical systems relies on the observable properties which are measured, and how such measurements are performed. Here we analyze two ways of assigning a description to a quantum system assuming that we only have access to coarse-grained properties. More specifically, we compare the maximum entropy principle method, with the Bayesian-inspired recently proposed average assignment map method [Correia et al., Phys. Rev. A 103, 052210 (2021)]. Despite the fact that the assigned descriptions by both methods respect the measured constraints, and that they share the same conceptual foundations, the descriptions differ in scenarios that go beyond the traditional system-environment structure. The average assignment map is thus shown to be a more sensible choice for the ever more prevalent scenario of complex quantum systems. We discuss the physics behind such a difference, and further exploit it in a quantum thermodynamics process.

We propose a family of noncontextuality inequalities and show that they can be used for certification of multiqubit quantum systems. Our scheme, unlike those based on nonlocality, does not require spatial separation between the subsystems, yet it makes use of certain compatibility relations between measurements. Moreover, it is scalable in the sense that the number of expectation values that are to be measured to observe contextuality scales polynomially with the number of qubits that are being certified. In a particular case we also show our scheme to be robust to errors and experimental imperfections. Our results seem promising as far as certification of physical setups is concerned in scenarios where spatial separation between the subsystems cannot be guaranteed.

Quantum steering, which lies between Bell nonlocality and entanglement, is a characteristic quantum correlation. Its application in quantum information tasks is worth pursuing. In this paper, we investigate the application of quantum steering as resource of quantum teleportation. It is found that two-qubit states violating the three-setting linear steering inequality are always useful for teleportation. Some steerable states which obey the Bell–Clauser-Horne-Shimony-Holt (CHSH) inequality could be used for teleportation. Two-qubit states that are optimal for quantum teleportation for a fixed amount of steering are present. The optimal states achieve the maximal average fidelity and also exhibit zero fidelity deviation.

The photoionization-excitation spectrum of Ne i, in which an electron in the $1s$ shell is ionized and the $2p$ orbital electron simultaneously excited to the $np$ orbital, is investigated in detail for complex satellite states ${\mathrm{Ne}}^{+}$ $1s^{\text{–}1}2p^{\text{–}1}{(}^{1,3}P)np$ $(n=3,4,5,6,7)$. Then, the multiple Auger decay process is studied using the multistep method, including cascade, knock-out, and shake-off mechanisms, based on perturbation theory for the satellite states ${\mathrm{Ne}}^{+}$ $1s^{\text{–}1}2p^{\text{–}1}\left({}^{1,3}\mathrm{P}\right)np$ $(n=3,4)$ with two holes. The major single Auger decay can be described as a spectator process that valence electrons remain in the original orbitals. For double Auger decay, direct and cascade processes are considered, in which the direct mechanism is dominated by the spectator process, while the cascade mechanism is mainly from the participator process. For triple Auger decay, the important contribution is from the process consisting of direct double Auger and the subsequent single Auger electron emissions. The calculated ion yields of ${\mathrm{Ne}}^{2+}$, ${\mathrm{Ne}}^{3+}$, and ${\mathrm{Ne}}^{4+}$ for multiple Auger process of satellite states ${\mathrm{Ne}}^{+}$ $1s^{\text{–}1}2p^{\text{–}1}{(}^{1,3}P)np$ $(n=3,4)$ are consistent with experimental values [Hikosaka et al., Phys. Rev. A 97, 023405 (2018)].

We study the ground-state phase diagram of population balanced and imbalanced ultracold atomic Fermi gases with a short-range attractive interaction throughout the crossover from BCS to Bose-Einstein condensation (BEC), in a two-dimensional (2D) optical lattice comprised of a 2D array of 1D lines. We find that the mixing of lattice and continuum dimensions, together with population imbalance, has an extraordinary effect on pairing and the superfluidity of atomic Fermi gases. In the balanced case, the superfluid ground state prevails over the majority of the phase space. However, for relatively small lattice hopping integral $t$ and large lattice constant $d$, a pair density wave (PDW) emerges unexpectedly at intermediate coupling strength, and the nature of the in-plane and overall pairing changes from particlelike to holelike in the BCS and unitary regimes, associated with an abnormal increase in the Fermi volume with the pairing strength. In the imbalanced case, the stable polarized superfluid phase shrinks to only a small portion of the entire phase space spanned by $t, d$, imbalance $p$, and interaction strength $U$, mainly in the bosonic regime of low $p$, moderately strong pairing, relatively large $t$, and small $d$. Due to the Pauli exclusion between paired and excessive fermions within the confined momentum space, a PDW phase emerges and the overall pairing evolves from particlelike into holelike, as the pairing strength grows stronger in the BEC regime. In both cases, the ground-state property is largely governed by the Fermi surface topology. These findings are very different from the cases of a pure 3D continuum, a 3D lattice, or a 1D optical lattice.

We investigate the particle transition dynamics in two-component Bose-Einstein condensates dominated by pair-particle transitions by introducing weak periodic perturbations to the initial states. We show that the transitions can be well controlled by varying the perturbation period while maintaining the external fields and nonlinear parameters during the transition process. In particular, there exists a critical period at which the transition efficiency is approximately $100\%$, with a numerical error of 0.72%. Analytic Akhmediev breather solutions and linear stability analyses show that modulation instability is the underlying mechanism for these transition dynamics. Furthermore, we establish a quantitative relation between the transition efficiency and phase shifts induced by the modulation instability. This relation can be used to measure the phase shift of the plane-wave background in well-known Akhmediev breathers.

Experiments near the lock-in region in maximally dissipative non-Hermitian systems, e.g., conventional laser gyroscopes near the deadband, have run up against the Petermann limit, where excess noise exactly cancels any scale-factor enhancement resulting in no overall enhancement in precision. As a result, one might be tempted to conclude that exceptional points (EPs) generally cannot be used to increase the precision of laser sensors. Indeed, using a linear eigenmode analysis we show that the Petermann limit applies not just to maximally dissipative systems, but for any type of EP, owing to the fact that EPs are rotationally invariant. It turns out, however, that this restriction comes from the assumption of linearity. We find that nonlinearity breaks the rotation symmetry such that the different types of EPs are no longer equivalent above threshold. In particular, EPs in conservatively coupled systems can lead to an increase in the fundamental precision beyond the Petermann limit as a result of gain saturation. Importantly, we find that only one mode lases under these conditions. We show that the beat note can be recovered by interference with an auxiliary mode, but that this has consequences for the quantum and classical noise that depend on the recovery scheme. Thus, it remains to be seen whether practical experiments can be designed that can take advantage of this enhancement.

A technique is introduced to calculate the mean path length of light rays diffusely incident on a refractive object. It uses the phase portrait from billiard theory to determine the criteria for which chords are accessible for a given refractive index. The mean path length is given as an integral over the lengths of chords accessible via refraction, which implicitly accounts for total internal reflections. We demonstrate this method by calculating the mean path length in ellipses and spheroids. The mean path length is given by a double integral for the ellipse and a triple integral for the spheroid, which may be evaluated numerically, and also allows us to deduce simple series expansions for low eccentricity. These results give analytic expressions for the orientation averaged absorption cross sections in the geometric optics limit.

We present an analytical quantum theoretic model for nonresonant molecular two-photon absorption (TPA) of broadband, spectrally multimode squeezed vacuum, including low-gain (isolated entangled photon pairs) and high-gain (bright squeezed vacuum or BSV) regimes. The results are relevant to the potential use of entangled-light TPA as a spectroscopic and imaging method. We treat the scenario that the exciting light is spatially single mode and is nonresonant with all intermediate molecular states. In the case of high gain, we find that in the case that the linewidth of the final molecular state is much narrower than the bandwidth of the exciting light, bright squeezed vacuum is found to be equally (but no more) effective in driving TPA as is a quasimonochromatic coherent-state (classical) pulse of the same temporal shape, duration, and mean photon number. Therefore, in this case the sought-for advantage of observing TPA at extremely low optical flux is not provided by broadband bright squeezed vacuum. In the opposite case that the final-state linewidth is much broader than the bandwidth of the BSV exciting light, we show that the TPA rate is proportional to the second-order intensity autocorrelation function at zero time delay $g^{\left(2\right)}\left(0\right)$, as expected. We derive and evaluate formulas describing the transition between these two limiting cases, that is, including the regime where the molecular linewidth and optical bandwidth are comparable, as is often the case in experimental studies. We also show that for $g^{\left(2\right)}\left(0\right)$ to reach the idealized form $g^{\left(2\right)}\left(0\right)=3+1/\overline{n}$, with $\overline{n}$ being the mean number of photons per temporal mode, it is required to compensate the dispersion inherent in the nonlinear-optical crystal used to generate the BSV.

Apart from the Bell nonlocality, which deals with the correlations generated from the local input-output statistics, quantum theory exhibits another kind of nonlocality that involves the indistinguishability of the locally preparable set of multipartite states. While Bell-type nonlocality cannot be activated from a given local correlation via local operations and shared randomness, it is already reported that the latter kind of nonlocality can be activated from a “local,” i.e., locally distinguishable set of states. However, recently it is shown that a stronger notion of such a nonlocality, which deals with elimination instead of discrimination, can be activated from locally preparable bipartite states of dimension $7\times 8$. The present work observes that the same notion can be demonstrated even in lower dimensional multipartite systems. Importantly, the possibly strongest version of such an activation is further depicted here, where none of the transformed product states can be eliminated, even if all but one of the parties come together.

Can classical systems grasp quantum dynamics executed in an untrusted quantum device? Metger and Vidick answered this question affirmatively by proposing a computational self-testing protocol for Bell states that certifies generation of Bell states and measurements on them. Since their protocol relies on the fact that the target states are stabilizer states, it is highly nontrivial to reveal whether the other class of quantum states, nonstabilizer states, can be self-tested. Among nonstabilizer states, magic states are indispensable resources for universal quantum computation. Here, we show that a magic state for the $CCZ$ gate can be self-tested while that for the $T$ gate cannot. Our result is applicable to a proof of quantumness, where we can classically verify whether a quantum device generates a quantum state having nonzero magic.

We relate observables in open quantum systems with the topology of non-Hermitian models using the Keldysh path-integral method. This allows to extract an effective Hamiltonian from the Green's function which contains all the relevant topological information and produces $\omega$-dependent topological invariants, linked to the response functions at a given frequency. Then, we show how to detect a transition between different topological phases by measuring the response to local perturbations. Our formalism is exemplified in a one-dimensional Hatano-Nelson model, highlighting the difference between the bosonic and the fermionic case.

Virtual states are a central concept in quantum mechanics. By definition, the probability of finding a quantum system in a virtual state should be vanishingly small at all times. In contrast to this notion, we report a phenomenon occurring in open quantum systems by which virtual states can acquire a sizable population in the long-time limit, even if they are not directly coupled to any dissipative channel. This means that the situation in which the virtual state remains unpopulated can be metastable. We describe this effect by introducing a two-step adiabatic elimination method, which we termed hierarchical adiabatic elimination, that allows one to obtain analytical expressions of the timescale of metastability in general open quantum systems. We show how these results can be relevant for practical questions such as the generation of stable and metastable entangled states in dissipative systems of interacting qubits.

We investigate the classification of Riemann surface topology generated by multiple arbitrary-order exceptional points of quasistationary states based on the permutation group. Our studies reveal all possible product permutations of holonomy matrices that describe a stroboscopic encircling of second-order exceptional points. The permutations turn out to be categorized into a finite number of classes according to the topological structures of the Riemann surfaces. Our results are verified by an effective non-Hermitian Hamiltonian founded on generic Jordan forms and then examined in physical systems of desymmetrized optical microcavities.