We report on the coupling of an electric quadrupole transition in atomic vapor with plasmonic excitation in a nanostructured metallic metamaterial. The quadrupole transition at 685 nm in the gas of cesium atoms is optically pumped, while the induced ground-state population depletion is probed with light tuned on the strong electric dipole transition at 852 nm. We use selective reflection to resolve the Doppler-free hyperfine structure of cesium atoms. We observe a strong modification of the reflection spectra at the presence of metamaterial and discuss the role of the spatial variation of the surface plasmon polariton on the quadrupole coupling.

We study statistics of work done by two classical electric-field pumps (two-photon and one-photon resonant pumps) on a quantum optical oscillator. We compute the moment generating function for the energy change of the oscillator, interpreted as work done by the classical drives on the quantum oscillator starting out in a thermalized Boltzmann state. The moment generating function is inverted, analytically when only one of the pumps is turned on and numerically when both the pumps are turned on, to get the probability function for the work. The resulting probability function for the work done by the classical drive is shown to satisfy transient detailed and integral work fluctuation theorems. Interestingly, we find that, in order for the work distribution function to satisfy the fluctuation theorem in the presence of both the drivings, the relative phases of drivings need to be shifted by $\pi$, which is related to the broken time-reversal symmetry of the Hamiltonian.

We present a scheme for coherently manipulating quantum states of photons by incorporating multiple photonic modes in a system with long-range interactions. The presence of nonlocal photon-photon interactions destroys the energy- or momentum-matching conditions between distinct propagating polaritons and consequently gives rise to blockaded effective coupling between the corresponding polaritons. Such a blockade mechanism protects the system from interaction-induced dissipations and enables highly tunable few-photon nonlinearities. Taking the Rydberg atomic ensemble as an example, we illustrate several intriguing phenomena based on the proposed scheme, e.g., the deterministic generation of entangled photon pairs, the nonlinear beam splitting, as well as the establishment of a tunable dressed interaction between individual photons.

While the scaling of entanglement in a quantum system can be used to distinguish many-body quantum phases, it is usually hard to quantify the amount of entanglement in mixed states of open quantum systems, while measuring entanglement experimentally, even for the closed systems, requires in general quantum state tomography. In this work we show how to remedy this situation in system with a fixed or conserved charge, e.g., density or magnetization, due to an emerging relation between quantum correlations and coherence. First, we show how, in these cases, the presence of multipartite entanglement or quantumness can be faithfully witnessed simply by detecting coherence in the quantum system, while bipartite entanglement or bipartite quantum discord are implied by asymmetry (block coherence) in the system. Second, we prove that the relation between quantum correlations and coherence is also quantitative. Namely, we establish upper and lower bounds on the amount of multipartite and bipartite entanglement in a many-body system with a fixed local charge, in terms of the amount of coherence and asymmetry present in the system. Importantly, both for pure and mixed quantum states, these bounds are expressed as closed formulas, and furthermore, for bipartite entanglement, are experimentally accessible by means of the multiple quantum coherence spectra. In particular, in one-dimensional systems, our bounds may detect breaking of the area law of entanglement entropy. We illustrate our results on the example of a many-body localized system, also in the presence of dephasing.

Differential equations are ubiquitous in models of physical phenomena. Applications like steady-state analysis of heat flow and deflection in elastic bars often admit to a second-order differential equation. In this paper, we discuss the use of a quantum annealer to solve such differential equations by recasting a finite element model in the form of an Ising Hamiltonian. The discrete variables involved in the Ising model introduce complications when defining differential quantities, for instance, gradients involved in scientific computations of solid and fluid mechanics. To address this issue, a graph-coloring-based methodology is proposed which searches iteratively for solutions in a subspace of weak solutions defined over a graph, hereafter called the “box algorithm.” The box algorithm is demonstrated by solving a truss mechanics problem on a D-Wave quantum computer.

Accurate evaluation of the $\mathcal{P},\mathcal{T}$-odd Faraday effect (rotation of the polarization plane for the light propagating through a medium in presence of an external electric field) is presented. This effect can arise only due to the $\mathcal{P},\mathcal{T}$-odd ($\mathcal{P}$—space parity, $\mathcal{T}$—time reflection) interactions and is different from the ordinary Faraday effect, i.e., the light polarization plane rotation in an external magnetic field. The rotation angle is evaluated for the intracavity absorption spectroscopy- type experiments with Xe and Hg atoms. The results show that the Hg atom may become a good candidate for a search for the $\mathcal{P},\mathcal{T}$-odd effects in atomic physics.

We investigate the effect of nuclear spins on the phase shift and polarization rotation of photons scattered off a quantum-dot–cavity system. We show that as the phase shift depends strongly on the resonance energy of an electronic transition in the quantum dot, it can provide a sensitive probe of the quantum state of nuclear spins that broaden this transition energy. By including the electron-nuclear spin coupling at a Hamiltonian level within an extended input-output formalism, we show how a photon-scattering event acts as a nuclear spin measurement, which when rapidly applied leads to an inhibition of the nuclear dynamics via the quantum Zeno effect, and a corresponding stabilization of the optical resonance. We show how such an effect manifests in the intensity autocorrelation $g^{\left(2\right)}\left(\tau \right)$ of scattered photons, whose long-time bunching behavior changes from quadratic decay for low photon-scattering rates (weak laser intensities) to ever slower exponential decay for increasing laser intensities as optical measurements impede the nuclear spin evolution.

The search for supersolidlike phases has attracted great attention in the fields of condensed matter and ultracold atom physics. Here, we experimentally demonstrate a route for realizing a superfluid stripe phase in a spin-orbit coupled Bose-Einstein condensate by employing a weak optical lattice to induce momentum-space hopping between two spin-orbit band minima. We characterize the striped ground state as a function of lattice coupling strength and spin-orbit detuning and find good agreement with mean-field simulations. We observe coherent Rabi oscillations in momentum space between two band minima and demonstrate a long lifetime of the ground state. Our work offers an exciting and stable experimental platform for exploring superfluid stripe phases and their exotic excitations, which may shed light on the properties of supersolidlike states.

We investigate quantum sensing of rotation with a multiatom Sagnac interferometer and present multipartite entangled states to enhance the sensitivity of rotation frequency. For studying the sensitivity, we first present a Hermitian generator with respect to the rotation frequency. The generator, which contains the Sagnac phase, is a linear superposition of a $z$ component of the collective spin and a quadrature operator of collective bosons depicting the trapping modes, which enables us to conveniently study the quantum Fisher information (QFI) for any initial states. With the generator, we derive the general QFI which can be of square dependence on the particle number, leading to the Heisenberg limit. We further find that the QFI may be of biquadratic dependence on the radius of the ring which confines the atoms, indicating that larger QFI is achieved by enlarging the radius. In order to obtain the square and biquadratic dependence, we propose to use partially and globally entangled states as inputs to enhance the sensitivity of rotation.

Finding optimal correction of errors in generic stabilizer codes is a computationally hard problem, even for simple noise models. While this task can be simplified for codes with some structure, such as topological stabilizer codes, developing good and efficient decoders still remains a challenge. In our paper, we systematically study a very versatile class of decoders based on feedforward neural networks. To demonstrate adaptability, we apply neural decoders to the triangular color and toric codes under various noise models with realistic features, such as spatially correlated errors. We report that neural decoders provide a significant improvement over leading efficient decoders in terms of the error-correction threshold. In particular, the neural decoder threshold for the two-dimensional color code is very close to the toric code threshold. Using neural networks simplifies the design of decoders and does not require prior knowledge of the underlying noise.

Valid transformations between quantum states are necessarily described by completely positive maps, instead of just positive maps. Positive but not completely positive maps such as the transposition map cannot be implemented due to the existence of entanglement in composite quantum systems, but there are classes of states for which the positivity is guaranteed, e.g., states not correlated to other systems. this paper, we report on the concept of $N$-copy extension of maps to quantitatively analyze the difference between positive maps and completely positive maps. We consider implementations of the action of positive but not completely positive maps on uncorrelated states by allowing an extra resource of consuming multiple copies of the input state and characterize the positive maps in terms of implementability with multiple copies. We show that by consuming multiple copies, the set of implementable positive maps becomes larger, and almost all positive maps are implementable with finite copies of an input state. The number of copies of the input state required to implement a positive map quantifies the degree by which a positive map violates complete positivity. We then analyze the optimal $N$-copy implementability of a noisy version of the transposition map.

We construct a correspondence between quantum states and the observable input-output correlations they are compatible with. The problem is framed as a game involving an experimenter, claiming to be able to prepare some family of states, and a theoretician, whose aim is to falsify such a claim based on observed correlations only. For any such claim, the optimal strategy consists of providing (i) to the experimenter, all the measurements that generate extremal input-output correlations, and (ii) to the theoretician, the full characterization of such correlations. Comparing the correlations observed in (i) with those predicted by (ii) corresponds to device-independently testing the states. While no assumption is made about the actual states and measurements, we derive the optimal strategy in closed form for the case when the claim consists of qubit states and the performed measurements are tests, and as applications we specify our results to the case of any pair of pure states and to the case of pure states uniformly distributed on the Bloch equatorial plane.

Molecular orientation methods based on nonresonant two-color laser pulses having parallel polarizations have been reported theoretically and experimentally. In this work, we demonstrate that perpendicularly polarized two-color laser fields can be used to achieve stronger molecular orientation when nanosecond laser pulses are used. The two-color fields align the molecules to the two-dimensional plane parallel to the field polarization; at the same time, they orient the molecules in the direction of the $2\omega$ polarization. We show that the interplay between the interactions due to the $\omega$- and $2\omega$-laser fields provides stronger molecular orientation than the parallel field configuration. This is due to temporally synchronized generations of alignment and orientation, which reduce the nonadiabatic effects.

Exchange correlation plays an important role in double ionization of complex atoms by ultrashort laser pulse. In this work we investigate two-photon double inner-shell electron ionization of neon induced by an attosecond extreme ultraviolet pulse in the framework of the quantum master equation. Our simulations reveal a distinct nonsequential effect via broadened double peaks, as a result of energy sharing between the two ionized electrons. When dissipation is included to show the interplay of coherence and decoherence, the two-photon double-ionization scaling law breaks down. We further study the total cross section of $2s^2$ double ionization as a function of photon energy in both nonsequential and sequential regions.

We study Rydberg electromagnetically induced transparency (EIT) of cesium atoms in a magnetic field. The ladder level scheme consists of ground ($6S_{1/2}$), excited ($6P_{3/2}$), and Rydberg ($48D_{5/2}$) levels. The relevant 96 relevant magnetic sublevels are coupled to each other via coherent coupling and decay. A quantum Monte Carlo wave-function (QMCWF) approach is employed to solve the quantum master equation. The simulated EIT probe-absorption spectra and their magnetic-field dependence are compared with results of a cold-atom experiment, in which we perform an in situ, atom-based measurement of a rapidly decaying eddy-current magnetic field. The EIT spectrum in the magnetic field has two dominant lines with a Zeeman splitting of 5.6 MHz per gauss, which are employed to measure the magnetic field. The QMCWF results show good agreement with the experiment, exhibit additional spectroscopic features, and provide insights into the optical-pumping dynamics, radiation-pressure effects, and the relation of these phenomena with the EIT behavior.

In a momentum coincidence measurement experiment, we studied the three-body fragmentation dynamics of carbon dioxide dimers in intense femtosecond laser fields. The three-dimensional momentum vectors and kinetic energies of the fragments were recorded in a cold-target recoil-ion momentum spectrometer. An analysis shows that ${\left({\mathrm{CO}}_2\right)}_2{}^{3+}$ breaks up into ${\mathrm{CO}}_2{}^{+}+{\mathrm{CO}}^{+}+{\mathrm{O}}^{+}$ ions through both concerted and sequential fragmentation channels. These two fragmentation channels are consistent with the instantaneous and metastable dissociation channels in the dissociation dynamics of ${\mathrm{CO}}_2{}^{2+}$ ion breakup into ${\mathrm{CO}}^{+}$ and ${\mathrm{O}}^{+}$ ions. From Coulomb explosion imaging, our results show that, for the parallel sliding structure of the carbon dioxide dimer molecules, the angle between the C=O bond and the van der Waals bond is $48{}^{\circ }$, and the intermolecular nuclear distance $R\left({\mathrm{CO}}_2\text{−}{\mathrm{CO}}_2\right)$ is 4.0 Å. By tracing the direction of the fragmentation ions, the ${\mathrm{O}}^{+}$ was found to be produced from the C=O bond that is closer to the center of the C-C bond during direct dissociation. These results indicate that the dynamic characteristics of the monomer is retained and gives rise to new dynamics in the molecular cluster complexes. Finally, we have measured angular distributions of ion fragments produced by different dissociation channels of ${\left({\mathrm{CO}}_2\right)}_2$, and the results reveal that the structure of the dimer and its relative orientation in the laser field govern the ionization dynamics.

We investigate theoretically the rovibrational cooling of the ${}^{85}\mathrm{Rb}_2$ molecule resulting from the photoassociation of ${}^{85}\mathrm{Rb}_2$ atoms at millikelvin temperature. At several millikelvin temperature, the populations of the initial state and the photoassociated ${}^{85}\mathrm{Rb}_2$ molecule are distributed on hundreds of rovibrational levels. In the laser rovibrational cooling process, the population of the photoassociated molecule is first transferred to the deep vibrational state $|{\psi }_{v^{″}=39}\rangle$ and then further to the rovibrational ground state $|{\psi }_{v^{″}=0,l=0}\rangle$ via the intermediate state $|{\psi }_{v^{\prime }=42,l_e=1}\rangle$. After the cooling process is over, the population transferred from the intermediate state $|{\psi }_{v^{\prime }=42,l_e=1}\rangle$ is fully distributed in the vibrational ground state $|v^{″}=0\rangle$. Of the population in the vibrational ground state, $51.59\%$ is concentrated in the rotational ground state $|l=0\rangle$, and the rest of population is distributed in the low rotational states $|l=1-3\rangle$.

We have developed a semianalytical theory to study a quantum plasmonic system composed of a quantum particle and a metallic particle with finite size. The quantum particle is composed of molecules driven by three laser fields and produces a coherent anti-Stokes Raman scattering (CARS) signal which is enhanced by plasmonic resonance of the metallic particle. The enhanced CARS field and the control laser form a $\mathrm{\Lambda }$ scheme that gives highly nonlinear and spatially nonlocal effects. We obtain absorption spectra that show periodic oscillations with radius of the spherical quantum particle, the whispering gallery effect. Plasmonic resonance gives very large and narrow absorption peaks as well as vanishing absorption or Fano dip at specific values of frequency and interparticle distance. The space between the two particles provides a resonant cavity effect which explains the oscillations of absorption power spectra with interparticle distance.

We present a study of manipulating and improving the multiphoton blockade phenomenon in a single-mode cavity with two cascade three-level atoms. Using an external control field, we show that the two-photon blockade can be changed to the three-photon blockade by just increasing the control field Rabi frequency when two atoms radiate in-phase. In the case of out-of-phase radiations, we show that the three-photon blockade phenomenon can be significantly improved with an increased mean photon number when the control field is present. We also show that the optical switching from the three-photon blockade $[g^{\left(2\right)}\left(0\right)>1$ and $g^{\left(3\right)}\left(0\right)<1]$ to the super-Poissonian field $[g^{\left(2\right)}\left(0\right)>1$ and $g^{\left(3\right)}\left(0\right)>1]$ can be accomplished by increasing the control field Rabi frequency. The results presented in this work provide potential applications in quantum communication and quantum networking.

Laser-driven cold atoms near a plane retroreflecting mirror exhibit self-organization above a pump threshold. We analyze the properties of self-organized spin patterns in the ground state of cold rubidium atoms. Antiferromagnetic patterns in zero magnetic field give way to ferrimagnetic patterns if a small longitudinal field is applied. We demonstrate how the experimental system can be modeled as spin-1 atoms diffractively coupled by the light reflected by the mirror. The roles of both dipolar and quadrupolar magnetization components in determining the threshold and symmetry variations with a weak longitudinal magnetic field are examined. Although the magnetic structures correspond dominantly to a lattice of magnetic dipoles, the symmetry breaking to ferrimagnetic structures in a finite field is mediated by the coupling to a homogenous quadrupole (alignment), which is not possible in a spin-1/2 system. Our study provides a basis for further exploration of instabilities in driven multilevel systems with feedback.