We present a family of non-Calderbank-Shor-Steane quantum error-correcting code consisting of geometrically local stabilizer generators on a 3D lattice. We study the Hamiltonian constructed from ferromagnetic interaction of overcomplete set of local stabilizer generators. The degenerate ground state of the system is characterized by a quantum error-correcting code whose number of encoded qubits are equal to the second Betti number of the manifold. These models (i) have solely local interactions; (ii) admit a strong-weak duality relation with an Ising model on a dual lattice; (iii) have topological order in the ground state, some of which survive at finite temperature; and (iv) behave as classical memory at finite temperature.

The production mechanisms of protons in dissociation of methane by 54 eV electron impact is investigated using the reaction microscope. By measuring all three charged particles in the final state in triple coincidence, the energy deposited in the target is determined. It is found that the (2$a_1$)${}^{-1}$(${\mathit{npt}}_2$)${}^1$, (2$a_1$)${}^{-1}$, (1$t_2$)${}^{-2}$(3$a_1$)${}^1$, and (2$a_1$)${}^{-2}$(3$a_1$)${}^1$ states of the intermediate CH${}_4$${}^{+}$ make the major contributions to the formation of protons at this incident energy. The decay of each state results in a different kinetic energy distribution of protons. Possible decay mechanisms of these states are analyzed.

We investigated ionization and fragmentation of ${\mathrm{C}}_{60}$ in the charge-changing collisions of 2.0 keV/amu Au${}^{+}$ ions with ${\mathrm{C}}_{60}$ molecules. We performed simultaneous measurements of product ions, the number of secondary electrons, and a charge-selected outgoing Au projectile. The production of ${\mathrm{C}}_{60}^{+}$ ions was predominant in the single-electron capture (1-capture) process. In contrast, multifragmentation of ${\mathrm{C}}_{60}$ was predominant in the single-electron loss (1-loss) process. The multifragmentation was enhanced in the 1-loss process compared with the 1-capture process when the same number of electrons was emitted from ${\mathrm{C}}_{60}$. This enhancement was also observed when a fast Si ion traveled close to a carbon nucleus [T. Majima et al., Phys. Rev. A 74, 033201 (2006)]. The 1-loss process of slow Au${}^{+}$ ions is considered to occur closer to a carbon nucleus than does the 1-capture process. Multifragmentation of ${\mathrm{C}}_{60}$ is caused by internal electronic excitation although it is probably assisted by nuclear stopping when, at least, the electronic excitation of ${\mathrm{C}}_{60}$ is small.

There are several effective nonadiabatic alignment control schemes that use multilaser pulse excitation. To explain the control mechanisms in a unified manner through a case study, we apply nonresonant optimal control simulation to a rovibrational model of ${\mathrm{N}}_2$. For experimental feasibility, we introduce a penalty and/or constraint to the simulation to impose restrictions on the number of optically accessible rotational states. When the control time is set to one rotational period, optimal pulses with linear polarization and a wavelength of around 800 nm are composed of three subpulses. From the power spectra of the intensities of the optimal pulses, we see that the optimal pulses construct a frequency network that connects the lowest three rotational Raman transitions. The frequency-network mechanism is confirmed by calculating the degree of alignment using sets of three Gaussian pulses. It shows several combinations of pulses that (approximately) satisfy the frequency-network condition, which can explain why there exist several control schemes that achieve a high degree of alignment.

We present a theoretical and experimental study on the possibility of spectral manipulation of weak probe-laser pulses in the presence of dynamical electromagnetically induced transparency. We predict a spectral enlargement or narrowing process depending on whether the probe-laser pulse is overlapped by the rising or the falling edge of the coupling pulse, respectively. The results of an experiment in sodium atomic vapors confirm the theoretical predictions.

A method for the generation of subwavelength optical lattices based on multilevel dark states is proposed. The dark state is formed by a suitable combination of standing wave light fields, leading to position-dependent populations of the ground states. An additional field coupling dispersively to one of the ground states translates this position dependence into a subwavelength optical potential. We provide two semiclassical approaches to understand the involved physics, and demonstrate that they lead to identical results in a certain meaningful limit. Then we apply a Monte Carlo simulation technique to study the full quantum dynamics of the subwavelength trapping. Finally, we discuss the relevant time scales for the trapping, optimum conditions, and possible implementations.

We propose and analyze a scheme for sympathetic cooling of the translational motion of polar molecules confined in the wells of a deep optical lattice and interacting one by one with laser-cooled ions in a radio-frequency trap. The energy gap between the excitation spectra of the particles in their respective trapping potentials is bridged by means of a parametric resonance, provided by the additional modulation of the rf field. We analyze two scenarios: simultaneous laser cooling and energy exchange between the ion and the molecule, and a scheme where these two processes take place separately. We calculate the lowest final energy of the molecule and the cooling rate depending on the amplitude of the parametric modulation. For small parametric modulation, the dynamics can be solved analytically within the rotating wave approximation.

Anomalous photoelectron angular distributions are observed at certain wavelengths in strong-field ionization of ${\mathrm{H}}_2$. We relate this feature to ac Stark shifts from bound-bound transitions in the Rydberg manifold of principal quantum number $n=3$ and 4. A model of the multistate interaction supports this interpretation.

We investigate theoretically and experimentally the propagation dynamics of a nonlinear cross-talk effect between two probe channels in a double-ladder system and show that an interplay between ${\chi }^{(1)}$ and ${\chi }^{(3)}$ processes leads to the control of cross-talk. We derive analytical solutions to describe the propagation dynamics of the probe fields with the cross-talk effect built in. From the analytical results we identify and examine the regimes of interest where contributions of either ${\chi }^{(1)}$ or ${\chi }^{(3)}$ or both are significant. The control of cross-talk is demonstrated experimentally, and good quantitative agreement is found between the analytical solutions and the experiment.

We present a solution to the problem of reflection and refraction of a polarized Gaussian beam at the interface between the transparent medium and the chiral metamaterials. Some unusual spin Hall effects of reflected and transmitted light have been found. It is shown that the spin-dependent displacements of the reflected beam centroid can not only reach several tens of wavelengths at certain incident angles; the reversed effect for the transmitted beams can also be realized by tuning the chiral parameters. These findings provide an alternative pathway for controlling the spin Hall effects of light and thereby open up the possibility for developing new nanophotonic devices.

Charge transfer with carbon ions has been identified as important in both magnetic fusion plasma devices and, more recently, in solar wind interactions with comets, planets, or neutrals in the heliosphere. A merged-beams technique is used to measure the absolute total charge-transfer cross section for ${\mathrm{C}}^{5+}$ and atomic H over four orders of magnitude in collision energy, from 12 000 to 0.64 eV/u. The present measurements are compared with previous measurements using an atomic hydrogen target and benchmark available classical trajectory Monte Carlo and molecular-orbital close-coupling calculations. An observed increasing cross section below 10 eV/u is attributed to trajectory effects due to the ion-induced dipole attraction between reactants.

In this Rapid Communication, we propose, following the spirit of quantum imaging, to generalize the theory of multipartite entanglement for the continuous-variable Gaussian states by considering, instead of the global covariance matrix, the local correlation matrix at two different spatiotemporal points $(\mathrm{\rho ⃗},t)$ and $(\mathrm{\rho ⃗}\hspace{0.5em}{}^{\text{'}},t^{\text{'}})$, with $\mathrm{\rho ⃗}$ being the transverse coordinate. Our approach makes it possible to introduce the characteristic spatial length and the characteristic time of the multipartite entanglement, which in general depend on the number of “parties” in the system. As an example, we consider tripartite entanglement in spontaneous parametric down-conversion with a spatially structured pump. We investigate spatiotemporal properties of such entanglement and calculate its characteristic spatial length and time.

We address the problem of a single “spin-down” impurity atom interacting attractively with a spin-up Fermi gas in two dimensions (2D). We consider the case where the mass of the impurity is greater than or equal to the mass of a spin-up fermion. Using a variational approach, we resolve the questions raised by previous studies and show that there is, in fact, a transition between polaron and molecule (dimer) ground states in 2D. For the molecule state, we use a variational wave function with a single particle-hole excitation on the Fermi sea and we find that its energy matches that of the exact solution in the limit of infinite impurity mass. Thus, we expect the variational approach to provide a reliable tool for investigating 2D systems.

Generalized quantum measurements [also known as positive operator-valued measures (POVMs)] are of great importance in quantum information and quantum foundations but are often difficult to perform. We present an experimental approach which can in principle be used to perform arbitrary POVMs in a linear-optical context. One of the most interesting POVMs, the symmetric informationally complete (SIC) POVM, is the most compact set of measurements that can be used to fully describe a quantum state. We use our technique to carry out the first experimental characterization of the state of a qutrit using SIC POVMs. Because of the highly symmetric nature of this measurement, such a representation has the unique property that it permits all other measurement outcomes to be predicted by a simple extension of the classical Bayesian sum rule, making no use of complex amplitudes or Hilbert-space operators. We demonstrate this approach on several qutrit states encoded in single photons.

We experimentally demonstrate phase-insensitive linear amplification of a continuous variable system in the optical regime, preserving the ancilla system at the output. Since our amplification operation is unitary up to small excess noise, it is reversible beyond the classical limit. Here, entanglement between the amplified output system and the ancilla system is the resource for the reversibility, and the amplification gain is $G=2.0$. In addition, combining this amplifier with a beamsplitter, we also demonstrate approximate cloning of coherent states where an anticlone is present. We investigate the reversibility by reconstructing the initial state from the output correlations, and the results are slightly beyond the cloning limit. Furthermore, full characterization of the amplifier and cloner is given by using coherent states with several different mean values as inputs. Our amplifier is based on linear optics, offline-prepared additional ancillas in nonclassical states, and homodyne measurements followed by feedforward. Squeezed states are used as the additional ancillas, and nonlinear optical effects are exploited only for their generation. They introduce nonclassicality into the amplifying operation, making entanglement at the output.

Anomalously dispersive cavities, particularly white-light cavities, may have larger bandwidth to finesse ratios than their normally dispersive counterparts. Partly for this reason, they have been proposed for use in laser interferometer gravitational-wave observatory (LIGO)-like gravity-wave detectors and in ring-laser gyroscopes. In this paper we analyze the quantum noise associated with anomalously dispersive cavity modes. The vacuum field energy associated with a particular cavity mode is proportional to the cavity-averaged group velocity of that mode. For anomalously dispersive cavities with group index values between 1 and 0, this means that the total vacuum field energy associated with a particular cavity mode must exceed $\hslash \omega /2$. For white-light cavities in particular, the group index approaches zero and the vacuum field energy of a particular spatial mode may be significantly enhanced. We predict enhanced spontaneous emission rates into anomalously dispersive cavity modes and broadened laser linewidths when the linewidth of intracavity emitters is broader than the cavity linewidth.

Explicit expressions for optical tomograms of photon-added coherent states, even and odd photon-added coherent states, and photon-added thermal states are given in terms of Hermite polynomials. Suggestions for experimental homodyne detection of the considered photon states are presented.

I show that an optical amplifier, when combined with photon subtraction, can be used for quantum state amplification, adding noise at a level below the standard minimum. The device could be used to significantly decrease the probability of incorrectly identifying coherent states chosen from a finite set.

Although information gain and disturbance in quantum systems are always antagonistic in quantum theory, a recently proposed optimal feedback control system [A. M. Brańczyk, P. E. M. F. Mendonca, A. Gilchrist, A. C. Doherty, and S. D. Bartlett, Phys. Rev. A 75, 012329 (2007)] could reach a trade-off by using weak measurements in the case of pure dephasing noise. We reexamine this approach for stabilizing quantum states in the presence of different noise sources and with respect to different system states. We find that the approach is sensitive to the system state but, for some suitable states, it works well for all typical types of noise sources.

We study analytically the quantum thermalization of two coupled two-level systems (TLSs), which are connected with either two independent heat baths (IHBs) or a common heat bath (CHB). We understand the quantum thermalization in eigenstate and bare-state representations when the coupling between the two TLSs is stronger and weaker than the TLS-bath couplings, respectively. In the IHB case, we find that, when the two IHBs have the same temperatures, the two coupled TLSs in eigenstate representation can be thermalized with the same temperature as those of the IHBs. However, in the case of two IHBs at different temperatures, just when the energy detuning between the two TLSs satisfies a special condition, the two coupled TLSs in eigenstate representation can be thermalized with an immediate temperature between those of the two IHBs. In bare-state representation, we find a counterintuitive phenomenon that, under some conditions, the temperature of the TLS connected with the high-temperature bath is lower than that of the other TLS, which is connected with the low-temperature bath. In the CHB case, the coupled TLSs in eigenstate representation can be thermalized with the same temperature as that of the CHB in nonresonant cases. In bare-state representation, the TLS with a larger energy separation can be thermalized to a thermal equilibrium with a lower temperature. In the resonant case, we find a phenomenon of antithermalization. We also study the steady-state entanglement between the two TLSs in both the IHB and CHB cases.