We introduce topological phases in curved-space photonic lattices. In such systems, the interplay between the curvature of space and the topology of the system, as manifested in the topology of the band structure, gives rise to a wealth of new phenomena. We demonstrate the topological curved-space concepts in an experimentally realizable setting of a waveguiding layer covering the surface of a three-dimensional body, and show that the curvature of space can induce topological edge states, topological phase transitions, Thouless pumping, and localization effects. We also describe the analogy between our system and topological phases in dynamical curved space-time settings known from general relativity.

Most studies in multiparameter estimation assume the dynamics is fixed and focus on identifying the optimal probe state and the optimal measurements. In practice, however, controls are usually available to alter the dynamics, which provides another degree of freedom. In this paper we employ optimal control methods, particularly the gradient ascent pulse engineering (GRAPE), to design optimal controls for the improvement of the precision limit in multiparameter estimation. We show that the controlled schemes are not only capable to provide a higher precision limit, but also have a higher stability to the inaccuracy of the time point performing the measurements. This high time stability will benefit the practical metrology, where it is hard to perform the measurement at a very accurate time point due to the response time of the measurement apparatus.

An ensemble of nitrogen-vacancy (NV) centers in diamond is an attractive device to detect small magnetic fields. In particular, by exploiting the fact that the NV center can be aligned along one of four different axes due to $C_{3\nu }$ symmetry, it is possible to extract information concerning vector magnetic fields. However, in the conventional scheme, low readout contrasts of the NV centers significantly decrease the sensitivity of the vector-magnetic-field sensing. Here, we propose a way to improve the sensitivity of the vector-magnetic-field sensing of the NV centers using multifrequency control. Since the Zeeman energy of the NV centers depends on the direction of the axis, we can independently control the four types of NV centers using microwave pulses with different frequencies. This allows us to use every NV center for the vector field detection in parallel, which effectively increases the readout contrast. Our results pave the way to realize a practical diamond-based vector field sensor.

We study quantum states generated by a sequence of nearest neighbor bipartite entangling operations along a one-dimensional chain of spin qubits. After a single sweep of such a set of operations, the system is effectively described by a matrix product state (MPS) with the same virtual dimension as the spin qubits. We employ the explicit form of the MPS to calculate expectation values and two-site correlation functions of local observables, and we use the results to study fluctuations of collective observables. Through the so-called macroscopicity and the squeezing properties of the collective spin variables they witness the quantum correlations and multiparticle entanglement within the chain. Macroscopicity only occurs over the entire chain if the nearest neighbor interaction is maximally entangling, while a finite, sequential interaction between nearest neighbor particles leads to squeezing of the collective spin.

We present systematic calculations of energy levels of the $1s^{2}2l$ and $1s2l2l^{\prime }$ states of ions along the lithium isoelectronic sequence from carbon to chlorine. The calculations are performed by using the relativistic configuration-interaction method adapted to the treatment of autoionizing core-excited states. The relativistic energies are supplemented with the QED energy shifts calculated within the model QED operator approach. A systematic estimation of the theoretical uncertainties is performed for every electronic state and every nuclear charge. The results are in agreement with existing high-precision theoretical and experimental data for the ground and first excited states. For the core-excited states, our theory is much more accurate than the presently available measurements.

In this work we investigate elastic collisions of low-energy positrons with silane (${\mathrm{SiH}}_4$). We employed the Schwinger multichannel method to calculate integral and differential cross sections for impact energies up to 10 eV. The calculations were performed within the static plus polarization approximation. We carried out a systematic study employing different schemes to account for the polarization effects of the target due the presence of the incoming positron. We investigate how the inclusion of extra functions in different extra (chargeless) centers affects the calculated cross sections and the physical phenomena such as the Ramsauer–Townsend minimum and the virtual state formation. Our results are compared with available experimental total cross sections and with integral and differential cross sections computed with a model correlation-polarization potential. In particular, our integral cross section agrees well with the experiment at low energies and our differential cross sections agree well with the results from previous calculations.

We present calculations of the electron angular distributions in the single ionization of helium by 1-MeV proton impact at momentum transfer of 0.75 a.u. and ejected-electron energy of 6.5 eV. The results using the first and second Born approximations and the 3C model with different trial helium functions are compared to the experimental data. A good agreement between theory and experiment is found in the case of the 3C final state and a strongly correlated helium wave function. The electron-electron correlations in the He atom are found to influence the ratio of the binary and recoil peak intensities.

The results of both quantum-mechanical numerical calculations beyond the electric dipole approximation and relativistic classical Monte Carlo simulations are presented for a ground-state hydrogen atom exposed to a high-frequency circularly polarized laser field in a wide intensity range. The persistence of the light-induced metastable bound states well into the relativistic regime of laser-atom interaction is demonstrated. The feasibility of high-efficiency electron trapping into these metastable states is examined in the frame of a simple two-stage scenario for a laser field turning on. The optimal parameters of the laser pulse front are found, which provide an optimal balance between the needs to achieve as quickly as possible the higher intensities, for which the decay rate of the metastable states is lower, and to ensure sufficient adiabaticity of the field turning on to avoid the unwanted “shake-off” processes. As a result, more than 60% probability of electron trapping into the metastable states in a relativistically intense high-frequency laser field is demonstrated.

This paper is devoted to clarifying the implications of hyperfine (HF) interaction in the formation of adiabatic (i.e., “laser-dressed”) states and their expression in the Autler-Townes (AT) spectra. We first use the Morris-Shore model [J. R. Morris and B. W. Shore, Phys. Rev. A 27, 906 (1983)] to illustrate how bright and dark states are formed in a simple reference system where closely spaced energy levels are coupled to a single state with a strong laser field with the respective Rabi frequency ${\mathrm{\Omega }}_S$. We then expand the simulations to realistic hyperfine level systems in Na atoms for a more general case when non-negligible HF interaction can be treated as a perturbation in the total system Hamiltonian. A numerical analysis of the adiabatic states that are formed by coupling of the $3p_{3/2}$ and $4d_{5/2}$ states by the strong laser field and probed by a weak laser field on the $3s_{1/2}-3p_{3/2}$ transition yielded two important conclusions. Firstly, the perturbation introduced by the HF interaction leads to the observation of what we term “chameleon” states—states that change their appearance in the AT spectrum, behaving as bright states at small to moderate ${\mathrm{\Omega }}_S$, and fading from the spectrum similarly to dark states when ${\mathrm{\Omega }}_S$ is much larger than the HF splitting of the $3p_{3/2}$ state. Secondly, excitation by the probe field from two different HF levels of the ground state allows one to address orthogonal sets of adiabatic states; this enables, with appropriate choice of ${\mathrm{\Omega }}_S$ and the involved quantum states, a selective excitation of otherwise unresolved hyperfine levels in excited electronic states.

Nitrogen molecules are ionized when an intense 800-nm femtosecond laser propagates through a nitrogen gas cell. We experimentally probe the relative population in different vibrational states and electronic states of ionized nitrogen molecules by using an external seed with various wavelengths. It is found that the seed is amplified or attenuated depending on its wavelength when the seed passes through the ionized nitrogen gas. Through measuring the temporal profile, the amplified emission is attributed to the seed-triggered superradiance. Combining with theoretical calculation, our results demonstrate that both vibrational and electronic excitation occurs for ionized nitrogen molecules in 800-nm femtosecond laser fields. The population inversion is built between some quantum states of ionized nitrogen molecules, through which the seed-triggered superradiance is achieved.

We investigate the photoionization of atoms by attosecond $X$ waves carrying orbital angular momentum in the presence of a strong, linearly polarized, near infrared (NIR) laser pulse. In the plane-wave case, the streaking of photoelectrons by the NIR pulse has been used to characterize the ionizing pulse. In contrast to plane-wave pulses, $X$ waves have a spatially dependent temporal profile, which modifies the ionization process. Here we explore theoretically the influence of this complex pulse structure on the streaking of photoelectrons for both localized and macroscopically extended targets. On the basis of the strong-field approximation, we find that the streaking spectra of localized targets sensitively depend on the opening angle of the $X$ wave and the position of the atomic target relative to the beam axis. For macroscopically extended targets, we find that the streaking spectra do not depend on the parameters characterizing the twist of the $X$ wave.

We study spontaneous emission from a rubidium atom into the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We show that the spontaneous emission rate depends on the magnetic sublevel, the type of modes, the orientation of the quantization axis, the position of the atom, and the fiber radius. We find that the rate of spontaneous emission into the TE modes is always symmetric with respect to the propagation directions. Directional asymmetry of spontaneous emission into other modes may appear when the quantization axis does not lie in the meridional plane containing the position of the atom. When the fiber radius is in the range from 330 to 450 nm, the spontaneous emission from an atom on the fiber surface into the ${\mathrm{HE}}_{21}$ modes is stronger than into the ${\mathrm{HE}}_{11}$, ${\mathrm{TE}}_{01}$, and ${\mathrm{TM}}_{01}$ modes. At the cutoff for higher-order modes, the rates of spontaneous emission into guided and radiation modes undergo steep variations, which are caused by the changes in the mode structure. We show that the spontaneous emission from the upper level of the cyclic transition into the TM modes is unidirectional when the quantization axis lies at an appropriate azimuthal angle in the fiber transverse plane.

We investigate the dynamics of a generic interacting many-body system under conditions of electromagnetically induced transparency (EIT). This problem is of current relevance due to its connection to nonlinear optical media realized by Rydberg atoms. In an interacting system the structure of the dynamics and the approach to the stationary state becomes far more complex than in the case of conventional EIT. In particular, we discuss the emergence of a metastable decoherence-free subspace, whose dimension for a single Rydberg excitation grows linearly in the number of atoms. On approach to stationarity this leads to a slow dynamics, which renders the typical assumption of fast relaxation invalid. We derive analytically the effective nonequilibrium dynamics in the decoherence-free subspace, which features coherent and dissipative two-body interactions. We discuss the use of this scenario for the preparation of collective entangled dark states and the realization of general unitary dynamics within the spin-wave subspace.

Entanglement is the basic building block of linear optical quantum computation, and as such understanding how to generate it in detail is of great importance for optical architectures. We prove that Bell states cannot be generated using only three photons in the dual-rail encoding, and give strong numerical evidence for the optimality of the existing four photon schemes. In a setup with a single photon in each input mode, we find a fundamental limit on the possible entanglement between a single mode Alice and arbitrary Bob. We investigate and compare other setups aimed at characterizing entanglement in settings more general than dual-rail encoding. The results draw attention to the trade-off between the entanglement a state has and the probability of postselecting that state, which can give surprising constant bounds on entanglement even with increasing numbers of photons.

The realization of a geometric phase optical device operating over a broad spectral range is usually confronted with intrinsic limitations depending on the physical process at play. Here we propose to use chiral nematic liquid-crystal slabs with helical ordering that varies in three dimensions, namely, gradient-pitch cholesterics endowed with in-plane space-variant angular positioning of the supramolecular helix. By doing so, we show that the recently introduced Bragg-Berry mirrors [M. Rafayelyan and E. Brasselet, Opt. Lett. 41, 3972 (2016).] can be endowed with an ultrabroadband spectral range. Experimental demonstration is made in the case of ultrabroadband optical vortex generation in the visible domain. These results offer a practical solution to the polychromatic management of the orbital angular momentum of light combining the circular Bragg reflection of chiral media with the Berry phase.

We analyze the breathing-mode oscillations of a harmonically quenched Tonks-Giradeau (TG) gas using an exact finite-temperature dynamical theory. We predict a striking collective manifestation of impenetrability—a collective many-body bounce effect. The effect, although being invisible in the evolution of the in situ density profile of the gas, can be revealed through a nontrivial periodic narrowing of its momentum distribution, taking place at twice the rate of the fundamental breathing-mode frequency. We identify physical regimes for observing the many-body bounce and construct the respective nonequilibrium phase diagram as a function of the quench strength and the initial temperature of the gas. We also develop a finite-temperature hydrodynamic theory of the TG gas wherein the many-body bounce is explained by an increased thermodynamic pressure during the isentropic compression cycle, which acts as a potential barrier for the particles to bounce off.

Balanced homodyning, heterodyning, and unbalanced homodyning are three well-known sampling techniques used in quantum optics to characterize photonic sources in the continuous-variable regime. We show that for all quantum states and all observable-parameter tomography schemes, which includes reconstructions of arbitrary operator moments and phase-space quasidistributions, localized sampling with unbalanced homodyning is always tomographically more powerful (gives more accurate estimators) than delocalized sampling with heterodyning. The latter is recently known to often give more accurate parameter reconstructions than conventional marginalized sampling with balanced homodyning. This result also holds for realistic photodetectors with subunit efficiency. With examples from first- through fourth-moment tomography, we demonstrate that unbalanced homodyning can outperform balanced homodyning when heterodyning fails to do so. This new benchmark takes us one step towards optimal continuous-variable tomography with conventional photodetectors and minimal experimental components.

We study the impact of the finite-size effect on the continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) protocol, mainly considering the finite-size effect on the parameter estimation procedure. The central-limit theorem and maximum likelihood estimation theorem are used to estimate the parameters. We also analyze the relationship between the number of exchanged signals and the optimal modulation variance in the protocol. It is proved that when Charlie's position is close to Bob, the CV-MDI QKD protocol has the farthest transmission distance in the finite-size scenario. Finally, we discuss the impact of finite-size effects related to the practical detection in the CV-MDI QKD protocol. The overall results indicate that the finite-size effect has a great influence on the secret-key rate of the CV-MDI QKD protocol and should not be ignored.

Through analyzing the effective dynamics of two Rydberg atoms under the dispersive coupling process, we give a method to modify the traditional antiblockade regime with simultaneous driving without adding any extra controls and resources. The modified regime can be used to construct controlled-PHASE and controlled-NOT gates in one step without manipulating the shape and amplitude, and tailoring sequences of the driving pulses. For the controlled-PHASE gate, only one pumping process from the ground state to the Rydberg state of each atom is required thus the resource is minimal. And the atomic addressability is not necessary. The scheme can be improved a lot after adding single-qubit operations since arbitrary phases can be achieved accurately with a short period of evolution time. For one-step construction of controlled-NOT gate, two schemes with high fidelities are given and discussed. Besides, the modified regime can also be used to improve the performance of the dissipation-based quantum entanglement preparation scheme. The optimal value of the modified condition for entanglement preparation is discussed for a wide range of parameters. Our study enriches the physics and applications of the simultaneous-driving-based Rydberg antiblockade regime without adding any operation complexity.

We introduce a discrete coherence monotone named the coherence number, which is a generalization of the coherence rank to mixed states. After defining the coherence number in a manner similar to that of the Schmidt number in entanglement theory, we present a necessary and sufficient condition of the coherence number for a coherent state to be converted to an entangled state of nonzero $k$ concurrence (a member of the generalized concurrence family with $2\le k\le d$). As an application of the coherence number to a practical quantum system, Grover's search algorithm of $N$ items is considered. We show that the coherence number remains $N$ and falls abruptly when the success probability of a searching process becomes maximal. This phenomenon motivates us to analyze the depletion pattern of $C_c^{\left(N\right)}$ (the last member of the generalized coherence concurrence, nonzero when the coherence number is $N$), which turns out to be an optimal resource for the process since it is completely consumed to finish the searching task. The generalization of the original Grover algorithm with arbitrary (mixed) initial states is also discussed, which reveals the boundary condition for the coherence to be monotonically decreasing under the process.