The recently-developed notion of 'parity-time (PT) symmetry' in optical systems with a controlled gain-loss interplay has spawned an intriguing way of achieving optical behaviors that are presently unattainable with standard arrangements. In most experimental studies so far, however, the implementations rely highly on the advances of nanotechnologies and sophisticated fabrication techniques to synthesize solid-state materials. Here, we report the first experimental demonstration of optical anti-PT symmetry, a counterpart of conventional PT symmetry, in a warm atomic-vapor cell. By exploiting rapid coherence transport via flying atoms, our scheme illustrates essential features of anti-PT symmetry with an unprecedented precision on phase-transition threshold, and substantially reduces experimental complexity and cost. This result represents a significant advance in non-Hermitian optics by bridging a firm connection with the field of atomic, molecular and optical physics, where novel phenomena and applications in quantum and nonlinear optics aided by (anti-)PT symmetry could be anticipated.Canonical quantum mechanics postulates Hermitian Hamiltonians to describe closed physical systems so that the system energy is conserved with real eigenvalues and the orthogonality between eigenstates with different eigen-energy is ensured. For systems with open boundaries, non-Hermitian Hamiltonians with complex eigenvalues and non-orthogonal eigen-functions are commonly expected. However, a counterintuitive discovery by Bender and Boettcher in 1998 1 has radically challenged this cognition and evoked considerable efforts on extending canonical quantum theory to non-Hermitian Hamiltonian systems 2,3,4 . In their pioneering work 1 , they showed that below some phase-transition point, a wide class of non-Hermitian Hamiltonians (̂) display entirely real spectra if they are invariant under the anti-linear parity-time (PT) operator (̂̂), [̂,̂̂] = 0 . In non-relativistic quantum mechanics, governed by Schrӧdinger equation, a necessary but not sufficient condition 1 for PT symmetry to hold mandates the complex potential involved to satisfy ( ) = * (− ), which implies its real (imaginary) part is an even (odd) function of position. Even more exciting is the occurrence of a sharp, symmetry-breaking transition once a non-Hermitian parameter crosses an exceptional point. As such, the Hamiltonian and PT operator no longer share the same set of eigen-functions and the real eigen-spectra of the system start to become complex.Despite the ramifications of these theoretical developments 1,2,3,4 , unfortunately, quantum mechanics is by nature an Hermitian theory and thus any attempt to observe PT symmetry in such systems is out of reach. On the other hand, due to the presence of gain and loss, optics has been recognized as a fertile ground for experimental investigations of PT symmetry. Given such optical settings, they can be fully realized without introducing any conflict with the Hermiticity of standard quantum mechanics. Thanks to the mathemat...

We report enhanced optical Faraday rotation in gold-coated maghemite (gamma-Fe(2)O(3)) nanoparticles. The Faraday rotation spectrum measured from 480-690 nm shows a peak at about 530 nm, not present in either uncoated maghemite nanoparticles or solid gold nanoparticles. This peak corresponds to an intrinsic electronic transition in the maghemite nanoparticles and is consistent with a near-field enhancement of Faraday rotation resulting from the spectral overlap of the surface plasmon resonance in the gold with the electronic transition in maghemite. This demonstration of surface plasmon resonance-enhanced magneto-optics (SuPREMO) in a composite magnetic/plasmonic nanosystem may enable design of nanostructures for remote sensing and imaging of magnetic fields and for miniaturized magneto-optical devices.

This paper reviews recent efforts to realize a highefficiency memory for optical pulses using slow and stored light based on electromagnetically induced transparency (EIT) in ensembles of warm atoms in vapor cells. After a brief summary of basic continuous-wave and dynamic EIT properties, studies using weak classical signal pulses in optically dense coherent media are discussed, including optimization strategies for stored light efficiency and pulse-shape control, and modification of EIT and slow/stored light spectral properties due to atomic motion. Quantum memory demonstrations using both single photons and pulses of squeezed light are then reviewed. Finally a brief comparison with other approaches is presented.

Room-temperature ionic liquids (ILs) have been demonstrated to absorb SO(2) efficiently. However, after absorbing a large amount of SO(2), the viscosity, the conductivity, and the density of the ILs have not been studied systematically, and the mechanism of the interaction between SO(2) and ILs is still being disputed. In this work, two kinds of ILs (task-specific ILs and normal ILs) have been studied to absorb pure SO(2) at atmospheric pressure. It is found that the viscosity, the conductivity, and the density show different behaviors between task-specific ILs and normal ILs. For the task-specific ILs to absorb SO(2), before a 0.5 mol ratio of SO(2) to IL, the viscosity and density increase, and the conductivity decreases with an increase of the mole ratio of SO(2) to IL. After that, the conductivity and density increase, and the viscosity decreases with further increasing the mole ratio of SO(2) to IL. However, for the normal ILs, the conductivity and density increase and the viscosity decreases with an increase of the mole ratio of SO(2) to IL. A new mechanism of ILs absorbing SO(2) has been proposed. Task-specific ILs can chemically absorb SO(2) when the mole ratio of SO(2) to IL is not more than 0.5, and they can physically absorb SO(2) when the mole ratio is more than 0.5. The normal ILs can only physically absorb SO(2).