Einstein-Podolsky-Rosen (EPR) steering is a form of quantum correlation and its intrinsic asymmetry makes it distinct from entanglement and Bell nonlocality. We propose here a scheme for realizing one-way Gaussian steering of two electromagnetic fields mediated by a mechanical oscillator. We reveal that the steady-state one-way steering of the intracavity and output fields is obtainable with different cavity losses or strong mechanical damping. The conditions for achieving this asymmetric steering are found, and it shows that the steering is robust against thermal mechanical fluctuations. The present scheme can realize hybrid microwave-optical asymmetric steering by optoelectromechanics. In addition, our results are generic and can also be applied to other three-mode parametrically coupled bosonic systems.

We present a theoretical study of the high-order-harmonic generation (HHG) from coherent superposition states of a three-level system driven by monochromatic cw laser fields. The HHG spectrum can be enhanced by about ${10}^5$ times via tuning the relative phase between the states which are initially prepared. Even if the ionization effects are taken into account, the enhancement factors are still up to about ${10}^2$ times. The enhancement is interpreted as the quantum constructive interference of different dipole transitions between quasienergy states. To realize the interference, the intensity of the driving laser field is chosen such that frequencies of the harmonics generated from the different dipole-transition paths, which are dependent on the ac Stark shifted energy separation of quasienergy states, are equal to each other. Moreover, the case studies of HHG spectra from a coherent superposition state of a three-level system show that the enhancement of harmonics is closely related to the harmonic strengths of the dipole transitions, as well as the parity of quasienergy states which are involved in the dipole-transition paths. The reduction of enhancement factors due to ionization effects is also qualitatively analyzed.

Two-photon emission of an electron in an electromagnetic plane wave of vanishing frequency is calculated. The unpolarized probability is split into a two-step process, which is shown to be exactly equal to an integration over polarized subprocesses, and a one-step process, which is found to be dominant over the formation length. The assumptions of neglecting spin and simultaneous emission, commonly used in numerical simulations, are discussed in light of these results.

A map from the charged Dirac oscillator on a noncommutative plane interacting with a homogeneous magnetic field onto a quantum optics model is proposed. We find that both anti-Jaynes-Cummings (AJC) and Jaynes-Cummings (JC) interactions are included in this mapped quantum optics model. A competition between AJC and JC interactions will happen when the intensity of the magnetic field changes. Contrary to previous studies, the competition is symmetrical since the phonons that take part in the competition are only associated with the excitation of the Dirac oscillator. Furthermore, this model will experience a quantum phase transition when the intensity of the magnetic field crosses the critical point. However, the critical point is different from the one in the model of the charged nonrelativistic harmonic oscillator interacting with a homogenous magnetic field on a noncommutative plane. It also deviates from its commutative counterpart because of spatial noncommutativity. Some characteristics of the quantum phase transition are studied. Then we investigate the nonrelativistic limit of this model. Our results show that its nonrelativistic counterpart inherits similar properties of the chirality quantum phase transition from the relativistic one.

The resonator-induced phase gate is a multiqubit controlled-phase gate for fixed-frequency superconducting qubits. Through off-resonant driving of a bus resonator, statically coupled qubits acquire a state-dependent phase. However, photon loss leads to dephasing during the gate, and any residual entanglement between the resonator and qubits after the gate leads to decoherence. Here we consider how to shape the drive pulse to minimize these unwanted effects. First, we review how the gate's entangling and dephasing rates depend on the system parameters and validate closed-form solutions against direct numerical solution of a master equation. Next, we propose spline pulse shapes that reduce residual qubit-bus entanglement, are robust to imprecise knowledge of the resonator shift, and can be shortened by using higher-degree polynomials. Finally, we present a procedure that optimizes over the subspace of pulses that leave the resonator unpopulated. This finds shaped drive pulses that further reduce the gate duration. Assuming realistic parameters, we exhibit shaped pulses that have the potential to realize $\sim 212$ ns spline pulse gates and $\sim 120$ ns optimized gates with $\sim 6\times {10}^{-4}$ average gate infidelity. These examples do not represent fundamental limits of the gate and, in principle, even shorter gates may be achievable.

The security of quantum communication using a weak coherent source requires an accurate knowledge of the source's mean photon number. Finite calibration precision or an active manipulation by an attacker may cause the actual emitted photon number to deviate from the known value. We model effects of this deviation on the security of three quantum communication protocols: the Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol without decoy states, Scarani-Acín-Ribordy-Gisin 2004 (SARG04) QKD protocol, and a coin-tossing protocol. For QKD we model both a strong attack using technology possible in principle and a realistic attack bounded by today's technology. To maintain the mean photon number in two-way systems, such as plug-and-play and relativistic quantum cryptography schemes, bright pulse energy incoming from the communication channel must be monitored. Implementation of a monitoring detector has largely been ignored so far, except for ID Quantique's commercial QKD system Clavis2. We scrutinize this implementation for security problems and show that designing a hack-proof pulse-energy-measuring detector is far from trivial. Indeed, the first implementation has three serious flaws confirmed experimentally, each of which may be exploited in a cleverly constructed Trojan-horse attack. We discuss requirements for a loophole-free implementation of the monitoring detector.

Based on spectroscopy of the $\left(3s3p\right){}^3{P}_0\text{−}\left(3s3d\right){}^3{D}_1$ Mg i transitions for the stable isotopes ${}^{24}\mathrm{Mg}(I=0),{}^{25}\mathrm{Mg}(I=5/2)$, and ${}^{26}\mathrm{Mg}(I=0)$ we report measurements of the ${}^{25}\mathrm{Mg}\left(3s3d\right){}^3{D}_J$ hyperfine coefficients $A\left({}^3{D}_1\right)=141\pm 7,A\left({}^3{D}_2\right)=-59\pm 6$, and $A\left({}^3{D}_3\right)=-97\pm 3\mathrm{MHz}$. We find the hyperfine coefficients in agreement with state-of-the-art theoretical predictions presented here giving $A\left({}^3{D}_1\right)=143.3\pm 1.4,A\left({}^3{D}_2\right)=-48.3\pm 0.5$, and $A\left({}^3{D}_3\right)=-96\pm 1\mathrm{MHz}$. We also report measurements of the isotope shifts for the investigated transitions ${\Delta }^{24–25}=6\pm 9$ and ${\Delta }^{24–26}=59.7\pm 0.5\mathrm{MHz}$, significantly reducing the uncertainty compared to previous measurements.

The current standard nuclear quadrupole moments (NQMs) of the ${}^{39}\mathrm{K}$, ${}^{40}\mathrm{K}$, and ${}^{41}\mathrm{K}$ isotopes have recently been contested by Singh and co-workers on the basis of their atomic computational data [Singh et al., Phys. Rev. A 86, 032509 (2012)]. Thus we performed relativistic calculations of electric field gradients at the potassium nuclei in three diatomic molecules (KF, KCl, and KBr) and combined these values with accurate experimental nuclear quadrupole coupling constants to provide an independent assessment of these NQMs. Our most accurate results, obtained by treating electron correlation with coupled cluster theory, employing a four-component Hamiltonian that includes the Gaunt two-electron correction, and with an incremented relativistic basis set of quadruple-ζ quality, yield $Q\left({}^{39}\mathrm{K}\right)=60.3\left(6\right)$, $Q\left({}^{40}\mathrm{K}\right)=–75.0\left(8\right)$, and $Q\left({}^{41}\mathrm{K}\right)=73.4\left(7\right)\mathrm{mb}$. These values are in better agreement with the results obtained by Singh et al. and indicate that the standard NQMs should be revised.

Magneto-optically trapped atoms enable the determination of lifetimes of metastable states and higher lying excited states like the $5d^2{}^3{F}_2$ state in barium. The state is efficiently populated by driving strong transitions from metastable states within the cooling cycle of the barium magneto-optical trap (MOT). The lifetime is inferred from the increase of MOT fluorescence after the transfer of up to $30\%$ of the trapped atoms to this state. The radiative decay of the $5d^2{}^3{F}_2$ state cascades to the cooling cycle of the MOT with a probability of $96.0\left(7\right)\%$ corresponding to a trap loss of $4.0\left(7\right)\%$ and its lifetime is determined to $160\left(10\right)\mu \mathrm{s}$. This is in good agreement with the theoretically calculated lifetime of $190\mu \mathrm{s}$ [V. A. Dzuba and V. V. Flambaum, J. Phys. B 40, 227 (2007)]. The determined loss of $4.0\left(7\right)\%$ from the cooling cycle is compared with the theoretically calculated branching ratios. This measurement extends the efficacy of trapped atoms to measure lifetimes of higher, long-lived states and validate the atomic structure calculations of heavy multielectron systems.

Using a relativistic adaptation of a three-step recollision model we calculate photoelectron energy spectra for ionization with elastic scattering in ultrastrong laser fields up to 24 a.u. $(2\times {10}^{19}$ ${\mathrm{W}/\mathrm{cm}}^2)$. Hydrogenlike and noble gas species with Hartree-Fock scattering potentials show a reduction in elastic rescattering beyond $6\times {10}^{16}$ ${\mathrm{W}/\mathrm{cm}}^2$ when the laser Lorentz deflection of the photoelectron exceeds its wave-function spread. A relativistic rescattering enhancement occurs at $2\times {10}^{18}$ ${\mathrm{W}/\mathrm{cm}}^2$, commensurate with the relativistic motion of a classical electron in a single field cycle. The noble gas results are compared with available experiments. The theory approach is well suited to modeling scattering in the ultrastrong intensity regime that lies between traditional strong fields and extreme relativistic interactions.

This combined experimental and theoretical study demonstrates that the surface plasmon resonance in ${\mathrm{C}}_{60}$ alters the valence photoemission quantum phase, resulting in strong effects in the photoelectron angular distribution and emission time delay. Electron momentum imaging spectroscopy is used to measure the photoelectron angular distribution asymmetry parameter that agrees well with our calculations from the time-dependent local density approximation (TDLDA). Significant structure in the valence photoemission time delay is simultaneously calculated by TDLDA over the plasmon active energies. Results reveal a unified spatial and temporal asymmetry pattern driven by the plasmon resonance and offer a sensitive probe of electron correlation. A semiclassical approach facilitates further insights into this link that can be generalized and applied to other molecular systems and nanometer-sized metallic materials exhibiting plasmon resonances.

We describe an apparatus designed to make nondemolition measurements on a Bose-Einstein condensate trapped in a double-well optical cavity. This apparatus contains, as well as the bosonic gas and the trap, an optical cavity. We show how the interaction between the light and the atoms, under appropriate conditions, can allow for a weakly disturbing yet highly precise measurement of the population imbalance between the two wells and its variance. We show that the setting is well suited for the implementation of quantum-limited estimation strategies for the inference of the key parameters defining the evolution of the atomic system and based on measurements performed on the cavity field. This would enable de facto Hamiltonian diagnosis via a highly controllable quantum probe.

Quantum systems can show qualitatively new forms of behavior when they are driven by fast time-periodic modulations. In the limit of large driving frequency, the long-time dynamics of such systems can often be described by a time-independent effective Hamiltonian, which is generally identified through a perturbative treatment. Here, we present a general formalism that describes time-modulated physical systems, in which the driving frequency is large, but resonant with respect to energy spacings inherent to the system at rest. Such a situation is currently exploited in optical-lattice setups, where superlattice (or Wannier-Stark-ladder) potentials are resonantly modulated so as to control the tunneling matrix elements between lattice sites, offering a powerful method to generate artificial fluxes for cold-atom systems. The formalism developed in this work identifies the basic ingredients needed to generate interesting flux patterns and band structures using resonant modulations. Additionally, our approach allows for a simple description of the micromotion underlying the dynamics; we illustrate its characteristics based on diverse dynamic-lattice configurations. It is shown that the impact of the micromotion on physical observables strongly depends on the implemented scheme, suggesting that a theoretical description in terms of the effective Hamiltonian alone is generally not sufficient to capture the full time evolution of the system.

Three-mode parametric interactions occur in triply resonant optomechanical systems: Photons from an optical pump mode are coherently scattered to a high-order mode by mechanical motion of the cavity mirrors, and these modes resonantly interact via radiation pressure force when some conditions are met. They can either pump energy into acoustic modes, leading to parametric instability, or extract mechanical energy, leading to optomechanical cooling. Such effects are predicted to occur in long baseline advanced gravitational-wave detectors, possibly jeopardizing their stable operation. We have demonstrated both three-mode cooling and amplification in two different three-mode optomechanical systems. We report an observation of the three-mode parametric instability in a free-space Fabry-Perot cavity, with ring-up amplitude saturation.

We study the light propagation in the twisted anisotropic optical fibers endowed with torsional mechanical stress by obtaining the analytical solution of the vector wave equation. We show that at certain interplay between fiber parameters optical vortex beams of topological charge $\ell =0,\pm 1,\pm 2,...$ become the modes of the fibers in question. To explain the splitting of the optical vortex propagation constants we introduce the notions of orbital birefringence and optical Zeeman effect. Moreover, we unveil that induced by torsional stress circular birefringence makes the vortex beams with the well-defined orbital angular momentum robust against small perturbations characterized by both constant and spatially varying orientation of a director. We believe that such fibers can be successfully utilized for the long-range robust transmission of information encoded in the light's orbital degrees of freedom.

A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of electromagnetically induced transparency, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for coherently transferring a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of nitrogen-vacancy centers and approaches 99% for cold atomic ensemble, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime in order to efficiently combine superconducting and optical components in quantum networks.

Strong-field ionization provides fundamental insight into light-matter interactions, encoding the structure of atoms and molecules on the subångström and subfemtosecond scales. In this Rapid Communication, we explore an important regime: strong-field ionization by two-color circularly polarized laser fields. In contrast to past work using linearly polarized drivers, we probe electron trajectories that are driven in a two-dimensional plane, thus separating the tunneling angle from the rescattering angle. This allows us to make several findings. First, we observe a single-lobed electron distribution for co-rotating fields, and a three-lobed distribution for counter-rotating fields, providing experimental validation of the theoretical model explaining the generation of circularly polarized high harmonic light. Second, we discover that there is significant electron-ion rescattering using counter-rotating fields, but not with co-rotating fields. Finally, we show that the rescattered electrons are well separated from the directly ionized electrons, in striking contrast to similar low-energy structures seen with linearly polarized fields. These findings help overcome the long-standing problem of how to decouple the tunneling and rescattering steps in strong-field ionization, which will enable new dynamic probes of atomic and molecular structure.