The strong demand for quantum memory, a key building block of quantum network, has inspired new methodologies and led to experimental progress for quantum storage. The use of quantum memory for spatial multimode or image storage could dramatically increase the channel bit rate. Furthermore, quantum memory that can store multiple optical modes would lead to higher efficiencies in quantum communication and computation. Here, by using resonant tripod electromagnetically induced transparency in a cold atomic ensemble, we experimentally demonstrate the storage of two probes with different frequencies and images in the frequency domain. In addition, by using different read light, we realize the frequency conversion of retrieved images with high efficiency. Besides, our method may be used to create a superposition of the images by realizing the function of a beam splitter. All advantages make our method useful in many fields, including quantum information, detection, imaging, sensing, and so on.

In this article, the processes of energy absorption and coherent transfer in a dimer are studied. The dimer includes two two-level pigments, donor and acceptor, where the donor is assumed to be excited by a control pulse in the time domain. We investigate the dynamics of probability that the acceptor is in the excited state and the total efficiency of energy absorption and transfer under different temporal shapes of the control pulse. Quantum concurrence of the dimer is also discussed.

An analytical approach is used to describe the dispersion of phase resonances occurring between waveguide cavity modes in compound transmission gratings. The strongly enhanced evanescent fields associated with the phase resonances are used to derive an approximate closed-form equation for the dispersion of phase resonances. The equation that is derived predicts the dependence of the frequencies and dispersion of the phase resonances on structural feature sizes, material parameters, and the momentum of the phase resonance. This analytical description is compared to results obtained by using a rigorous coupled wave algorithm. This equation allows one to design a compound transmission grating that supports phase resonances with particular properties, such as resonant frequencies and momenta. Applications of the phase resonances to high-finesse electromagnetic filters and corrugated surface antennas are discussed.

We propose an interferometric method for the direct measurement of the moments of the transverse spatial variables of an optical field. The key elements are spatial light modulators, used to imprint a transverse phase on the field. We show that our measurement scheme can be used to measure the spatial correlation functions of a two-photon state directly, which allows for the identification of spatial entanglement without the need for partial state reconstruction. Our method can also be extended to $N$-photon states. We show that the method is more efficient than the measurement of marginal probability distributions, and that it can be made resilient to small alignment errors.

We theoretically propose a method to enhance Raman scattering by injecting a seeded light field which is correlated with the initially prepared atomic spin wave. Such a light-atom correlation leads to an interference in the Raman scattering. The interference is sensitive to the relative phase between the seeded light field and initially prepared atomic spin wave. For constructive interference, the Raman scattering is greatly enhanced. Such an enhanced Raman scattering may find applications in quantum information, nonlinear optics, and optical metrology due to its simplicity.

Ground-state phase diagram of two-component Bose gases with Rashba spin-orbit coupling is determined via a variational approach. A phase in which the fully polarized condensate occupies zero momentum is identified. This zero-momentum phase competes with the spin density wave phase when interspecies interaction is stronger than intraspecies interaction, and the former one is always the ground state for weak spin-orbit coupling. When the energies of these two phases are close, there is a phase separation between them. At finite temperature, such a zero-momentum condensation can be induced by a ferromagnetic phase transition in normal state. The spontaneous spin polarization removes the degeneracy of quasiparticles’ energy minima, and consequently the modified density of state accommodates a Bose condensation to appear below a critical temperature.

The framework of generalized probabilistic theories is a popular approach for studying the physical foundations of quantum theory. The standard framework assumes the no-restriction hypothesis, in which the state space of a physical theory determines the set of measurements. However, this assumption is not physically motivated. We generalize the framework to account for systems that do not obey the no-restriction hypothesis. We then show how our framework can be used to describe certain classes of probabilistic theories, for example, those which include intrinsic noise. Relaxing the restriction hypothesis also allows us to introduce a “self-dualization” procedure, which yields a class of theories that share many features of quantum theory. We then characterize joint states, generalizing the maximal tensor product. We show how this tensor product can be used to describe the convex closure of the Spekkens toy theory, and in doing so we obtain an analysis of why it is local in terms of the geometry of its state space. We show that the unrestricted version of the Spekkens toy theory is the theory known as “boxworld” that allows maximal nonlocal correlations.

We introduce a method to witness the quantumness of a system. The method relies on the fact that the anticommutator of two classical states is always positive. By contrast, we show that there is always a nonpositive anticommutator due to any two quantum states. We notice that interference depends on the trace of the anticommutator of two states, and it is therefore operationally more suitable to detect quantumness by looking at anticommutators of states rather than their commutators.

Laser light is widely used for communication and sensing applications, so the optimal discrimination of coherent states—the quantum states of light emitted by an ideal laser—has immense practical importance. Due to fundamental limits imposed by quantum mechanics, such discrimination has a finite minimum probability of error. While concrete optical circuits for the optimal discrimination between two coherent states are well known, the generalization to larger sets of coherent states has been challenging. In this paper, we show how to achieve optimal discrimination of any set of coherent states using a resource-efficient quantum computer. Our construction leverages a recent result on discriminating multicopy quantum hypotheses [Blume-Kohout, Croke, and Zwolak, arXiv:1201.6625]. As illustrative examples, we analyze the performance of discriminating a ternary alphabet and show how the quantum circuit of a receiver designed to discriminate a binary alphabet can be reused in discriminating multimode hypotheses. Finally, we show that our result can be used to achieve the quantum limit on the rate of classical information transmission on a lossy optical channel, which is known to exceed the Shannon rate of all conventional optical receivers.

Multiphoton states can be produced in multiple parametric down-conversion (PDC) processes. The nonlinear crystal in such a case is pumped with high power. In theory, the more populated these states are, the deeper is the conflict with local realistic description. However, the interference contrast in multiphoton PDC experiments can be quite low for high pumping. We show how the contrast can be improved. The idea employs currently accessible optical devices, the multiport beam splitters. They are capable of splitting the incoming light in one input mode to $M$ output modes. Our scheme works as a positive operator-valued measure filter. It may provide a feasible Clauser-Horne-Shimony-Holt-Bell inequality test, and thus can be useful in, e.g., schemes reducing communication complexity.

We attempt to evaluate the applicability of the concept of the critical power for self-focusing, originally developed for intense quasi-continuous-wave (cw) beams, to the case of ultra-intense and ultrashort laser pulses propagating in air. Our results show that, unlike in the cw case, no particular value of peak pulse power can be viewed as a sharp demarcation line between linear and nonlinear propagation regimes. Our analysis further reveals the important role played by chromatic dispersion in the propagation dynamics of the laser pulse.

We present a formalism which allows one to define probabilities for expressions that involve properties at different times for classical and quantum systems and we study its lattice structure. The formalism is based on the notion of time translation of properties. In the quantum case, the properties involved should satisfy compatibility conditions in order to obtain well-defined probabilities. The formalism is applied to describe the double-slit experiment.

We investigate the conditions under which the trace distance between two different states of a given open system increases in time due to the interaction with an environment, therefore signaling non-Markovianity. We find that the finite-time difference in trace distance is bounded by two sharply defined quantities that are strictly linked to the occurrence of system-environment correlations created throughout their interaction and affecting the subsequent evolution of the system. This allows us to shed light on the origin of non-Markovian behaviors in quantum dynamics. We best illustrate our findings by tackling two physically relevant examples: a non-Markovian dephasing mechanism that has been the focus of a recent experimental endeavor and the open-system dynamics experienced by a spin connected to a finite-size quantum spin chain.

We present results of an experiment to probe for the electric dipole moment (EDM) of the electron using an $\Omega$-doublet state in a polar molecule. If the molecule is both massive and has a large molecular-fixed frame dipole moment, then the $\Omega$-doublet states have the potential to greatly increase the sensitivity of experiments searching for the EDM while also allowing for new methods of systematic error rejection. Here, we use the metastable $a{\left(1\right)}^3{\Sigma }^{+}$ state of lead monoxide (PbO) to probe for the electron EDM. Our best fit for the electron EDM of $d_e=(-4.4\pm 9.5_{\mathrm{stat}}\pm 1.8_{\mathrm{syst}})\times {10}^{-27}e\mathrm{cm}$ allows us to place an upper limit on the magnitude of the EDM of $|d_e|<1.7\times {10}^{-26}e\mathrm{cm}$ (90$\%$ confidence). While this is less stringent than limits from other, previous experiments, our work emphasizes the systematic error rejection properties associated with the $\Omega$-doublet level structure. The results should inform the work of other, ongoing experiments that use molecules with analogous level structure.

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for the treatment of double-ionization processes of many-electron systems. The method is based on the extension of the Krieger-Li-Iafrate (KLI) treatment of the optimized effective potential (OEP) theory and the incorporation of an explicit self-interaction correction (SIC) term. In the framework of the time-dependent density functional theory, we have performed three-dimensional (3D) calculations of double ionization of He and Be atoms by intense near-infrared laser fields. We make use of the exchange-correlation potential with the integer discontinuity which improves the description of the double-ionization process. We found that a proper description of the double ionization requires the TDDFT exchange-correlation potential with the discontinuity with respect to the variation of the total particle number (TPN). The results for the intensity-dependent rates of double ionization of He and Be atoms are presented.

We report calculations for the single- and multiphoton ionization of the H${}_2$${}^{+}$ molecular ion irradiated by ultrashort elliptically polarized laser pulses for central photon energies ranging from close to the ionization threshold up to 300 eV. Using the fixed-nuclei approximation, the electronic response of the system is obtained through an ab initio time-dependent grid-based approach implemented in two-center prolate spheroidal coordinates. The predicted cross sections for single-photon absorption are analyzed in a time-independent framework. A concise decomposition of the cross-section patterns exists in terms of the degrees of linear and circular portions of the elliptically polarized radiation. An asymmetric rotational effect in the photoelectron angular distribution is predicted and discussed according to the above decomposition. The confinement effect, previously seen in angle-resolved cross sections for linearly polarized light, persists for circularly polarized radiation. The dependence of the calculated angular distributions on the pulse length in intense laser fields is also analyzed.

Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching ${10}^{15}\text{W}/{\text{cm}}^2$ with an accuracy of 1$\%$. This accuracy is achieved by comparing experimental photoelectron yields from atomic hydrogen with predictions from exact numerical solutions of the three-dimensional time-dependent Schrödinger equation. Our method can be extended to relativistic intensities and to the use of other atomic species.

For a weakly interacting Bose gas rotating in a harmonic trap we relate the yrast states of small systems (that can be treated exactly) to the thermodynamic limit (derived within the mean-field approximation). For a few dozens of atoms, the yrast line shows distinct quasiperiodic oscillations with increasing angular momentum that originate from the internal structure of the exact many-body states. These finite-size effects disappear in the thermodynamic limit, where the Gross-Pitaevskii approximation provides the exact energy to leading order in the number of particles $N$. However, the exact yrast states reveal significant structure not captured by the mean-field approximation: Even in the limit of large $N$, the corresponding mean-field solution accounts for only a fraction of the total weight of the exact quantum state.

By adopting a $T$-matrix-based method within the $G_{0}G$ approximation for the pair susceptibility, we study the effects of the pairing fluctuation on the three-dimensional spin-orbit-coupled Fermi gases at finite temperature. The critical temperatures of the superfluid to normal phase transition are determined for three different types of spin-orbit coupling (SOC): (1) the extreme oblate (EO) or Rashba SOC, (2) the extreme prolate or equal Rashba-Dresselhaus SOC, and (3) the spherical (S) SOC. For EO- and S-type SOC, the SOC dependence of the critical temperature signals a crossover from BCS to BEC state; at strong SOC limit, the critical temperature recovers those of ideal BEC of rashbons. The pairing fluctuation induces a pseudogap in the fermionic excitation spectrum in both superfluid and normal phases. We find that, for EO- and S-type SOC, even at weak coupling, sufficiently strong SOC can induce sizable pseudogap. Our research suggests that the spin-orbit-coupled Fermi gases may open new means to the study of the pseudogap formation in fermionic systems.

Multiple phases occurring in a Bose gas with finite-range interaction are investigated. In the vicinity of the onset of Bose-Einstein condensation (BEC), the chemical potential and the pressure show a van der Waals–like behavior indicating a first-order phase transition although there is no long-range attraction. Furthermore, the equation of state becomes multivalued near the BEC transition. For a Hartree-Fock or Popov (Hartree-Fock-Bogoliubov) approximation, such a multivalued region can be avoided by the Maxwell construction. For sufficiently weak interaction, the multivalued region can also be removed using a many-body $T$-matrix approximation. However, for strong interactions there remains a multivalued region even for the $T$-matrix approximation and after the Maxwell construction, which is interpreted as a density hysteresis. This unified treatment of normal and condensed phases becomes possible due to the recently found scheme to eliminate self-interaction in the $T$-matrix approximation, which allows one to calculate properties below and above the critical temperature.