A Feshbach resonance in collisions between triplet ground-state molecules.

Collisional resonances are important tools that have been used to modify interactions in ultracold gases, for realizing previously unknown Hamiltonians in quantum simulations1, for creating molecules from atomic gases2 and for controlling chemical reactions. So far, such resonances have been observed for atom-atom collisions, atom-molecule collisions3-7 and collisions between Feshbach molecules, which are very weakly bound8-10. Whether such resonances exist for ultracold ground-state molecules has been debated owing to the possibly high density of states and/or rapid decay of the resonant complex11-15. Here we report a very pronounced and narrow (25 mG) Feshbach resonance in collisions between two triplet ground-state NaLi molecules. This molecular Feshbach resonance has two special characteristics. First, the collisional loss rate is enhanced by more than two orders of magnitude above the background loss rate, which is saturated at the p-wave universal value, owing to strong chemical reactivity. Second, the resonance is located at a magnetic field where two open channels become nearly degenerate. This implies that the intermediate complex predominantly decays to the second open channel. We describe the resonant loss feature using a model with coupled modes that is analogous to a Fabry-Pérot cavity. Our observations provide strong evidence for the existence of long-lived coherent intermediate complexes even in systems without reaction barriers and open up the possibility of coherent control of chemical reactions.

Observation of the exceptional-point-enhanced Sagnac effect.

Exceptional points (EPs) are special spectral degeneracies of non-Hermitian Hamiltonians that govern the dynamics of open systems. At an EP, two or more eigenvalues, and the corresponding eigenstates, coalesce1-3. Recently, it was predicted that operation of an optical gyroscope near an EP results in improved response to rotations4,5. However, the performance of such a system has not been examined experimentally. Here we introduce a precisely controllable physical system for the study of non-Hermitian physics and nonlinear optics in high-quality-factor microresonators. Because this system dissipatively couples counter-propagating lightwaves within the resonator, it also functions as a sensitive gyroscope for the measurement of rotations. We use our system to investigate the predicted EP-enhanced Sagnac effect4,5 and observe a four-fold increase in the Sagnac scale factor by directly measuring rotations applied to the resonator. The level of enhancement can be controlled by adjusting the system bias relative to the EP, and modelling results confirm the observed enhancement. Moreover, we characterize the sensitivity of the gyroscope near the EP. Besides verifying EP physics, this work is important for the understanding of optical gyroscopes.

Reconfigurable Photon Sources Based on Quantum Plexcitonic Systems.

A single photon in a strongly nonlinear cavity is able to block the transmission of a second photon, thereby converting incident coherent light into antibunched light, which is known as the photon blockade effect. Photon antipairing, where only the entry of two photons is blocked and the emission of bunches of three or more photons is allowed, is based on an unconventional photon blockade mechanism due to destructive interference of two distinct excitation pathways. We propose quantum plexcitonic systems with moderate nonlinearity to generate both antibunched and antipaired photons. The proposed plexcitonic systems benefit from subwavelength field localizations that make quantum emitters spatially distinguishable, thus enabling a reconfigurable photon source between antibunched and antipaired states via tailoring the energy bands. For a realistic nanoprism plexcitonic system, chemical and optical schemes of reconfiguration are demonstrated. These results pave the way to realize reconfigurable nonclassical photon sources in a simple quantum plexcitonic platform.

Atomic physics on a 50-nm scale: Realization of a bilayer system of dipolar atoms.

Controlling ultracold atoms with laser light has greatly advanced quantum science. The wavelength of light sets a typical length scale for most experiments to the order of 500 nanometers (nm) or greater. In this work, we implemented a super-resolution technique that localizes and arranges atoms on a sub-50-nm scale, without any fundamental limit in resolution. We demonstrate this technique by creating a bilayer of dysprosium atoms and observing dipolar interactions between two physically separated layers through interlayer sympathetic cooling and coupled collective excitations. At 50-nm distance, dipolar interactions are 1000 times stronger than at 500 nm. For two atoms in optical tweezers, this should enable purely magnetic dipolar gates with kilohertz speed.

Control of reactive collisions by quantum interference.

In this study, we achieved magnetic control of reactive scattering in an ultracold mixture of 23Na atoms and 23Na6Li molecules. In most molecular collisions, particles react or are lost near short range with unity probability, leading to the so-called universal rate. By contrast, the Na + NaLi system was shown to have only ~4% loss probability in a fully spin-polarized state. By controlling the phase of the scattering wave function via a Feshbach resonance, we modified the loss rate by more than a factor of 100, from far below to far above the universal limit. The results are explained in analogy with an optical Fabry-Perot resonator by interference of reflections at short and long range. Our work demonstrates quantum control of chemistry by magnetic fields with the full dynamic range predicted by our models.

Poly[[(4,4'-bipyridine-κN)[µ(3)-(S)-2-hy-droxy-butane-dioato-κO,O:O:O]zinc] dihydrate].

In the title compound, {[Zn(C(4)H(4)O(5))(C(10)H(8)N(2))]·2H(2)O}(n), the Zn(II) ion displays a distorted tetra-gonal-pyramidal coordination environment with one hy-droxy O and three carboxyl-ate O atoms from three malate anions, and the one remaining position occupied by an N atom from a 4,4'-bipyridine ligand. The pyridine rings of the 4,4'-bipyridine ligand are twisted with respect to each other by a dihedral angle of 35.8 (2)°. The uncoordinated water mol-ecules are linked to the complex mol-ecules by O-H⋯O hydrogen bonds. Each malate anion forms four coordination bonds with three Zn atoms, establishing a layer structure parallel to the ac plane. Adjacent layers are further linked via O-H⋯N hydrogen bonding. π-π stacking between the pyridine rings [face-to-face distance = 3.651 (3) Å] occurs in the crystal structure.

Bis[tris-(ethyl-enediamine-κN,N')cobalt(III)] octa-kis-µ-(3)-oxido-hexa-deca-µ(2)-oxido-tetra-deca-oxido-µ(12)-tetra-oxo-silicato-octa-molybdenum(VI)hexa-vanadium(IV,V) hexa-hydrate.

The title compound, [Co(C(2)H(8)N(2))(3)](2)[SiMo(8)V(4)O(40)(VO)(2)]·6H(2)O, was prepared under hydro-thermal conditions. The asymmetric unit consists of a transition metal complex [Co(en)(3)](3+) cation (en is ethyl-enediamine), one half of an [SiMo(8)V(4)O(40)(VO)(2)](6-) heteropolyanion, two solvent water mol-ecules in general positions and two half-mol-ecules of water located on a mirror plane. In the complex cation, the Co(3+) ion is in a distorted octa-hedral coordination environment formed by six N atoms of the three chelating en ligands. One of the en ligands exhibits disorder of its aliphatic chain over two sets of sites of equal occupancy. The [SiMo(8)V(4)O(40)(VO)(2)](6-) heteropolyanion is a four-electron reduced bivanadyl-capped α-Keggin-type molybdenum-vanadium-oxide cluster. In the crystal, it is located on a mirror plane, which results in disorder of the central tetra-hedral SiO(4) group: the O atoms of this group occupy two sets of sites related by a mirror plane. Furthermore, all of the eight µ(2)-oxide groups are also disordered over two sets of sites with equal occupancy. There are extensive inter-molecular N-H⋯O hydrogen bonds between the complex cations and inorganic polyoxidoanions, leading to a three-dimensional supra-molecular network.

µ(3)-Dodeca-tungsto(V,VI)aluminato-κO:O':O''-tris-[aqua-bis-(ethyl-ene-diamine-κN,N')copper(II)].

The title compound, [AlCu(3)W(12)O(40)(C(2)H(8)N(2))(6)(H(2)O)(3)], was prepared under hydro-thermal conditions. The Cu(2+) ion displays an elongated octa-hedral geometry defined by one bridging O atom from the polyoxidoanion and a coordinated water mol-ecule in axial positions and four N atoms of the two chelating ethyl-enediamine (en) ligands in equatorial positions. The one-electron reduced [AlW(12)O(40)](6-) anion coordinates three [Cu(en)(H(2)O)](2+) fragments, generating a neutral tri-supported Keggin-type polyoxidometalate (POM). This tri-supported POM is located in a special position of [Formula: see text] symmetry and therefore O atoms from the central AlO(4) tetra-hedron are disordered over two sets of sites. Disorder is also observed for three other bridging O atoms of the POM. In the crystal, mol-ecules are connected via N-H⋯O and O-H⋯O hydrogen bonds, forming a three-dimensional framework.

Benzyl 5-phenyl-pyrazolo-[5,1-a]isoquino-line-1-carboxyl-ate.

In the title compound, C(25)H(18)N(2)O(2), the pyrazolo-[5,1-a]iso-quin-oline ring system is approximately planar [maximum deviation = 0.027 (2) Å] and is oriented at dihedral angles of 57.22 (6) and 71.36 (7)° with respect to the two phenyl rings. The phenyl rings are twisted to each other by a dihedral angle of 66.33 (8)°. A weak intra-molecular C-H⋯O hydrogen bond occurs. In the crystal, weak C-H⋯π inter-actions are present.

Pauli blocking of light scattering in degenerate fermions.

Pauli blocking of spontaneous emission is responsible for the stability of atoms. Electrons cannot decay to lower-lying internal states that are already occupied. Pauli blocking also occurs when free atoms scatter light elastically (Rayleigh scattering) and the final external momentum states are already populated. This was predicted more than 30 years ago but is challenging to realize experimentally. Here, we report on Pauli blocking of light scattering in a dense quantum-degenerate Fermi gas of ultracold lithium atoms. When the Fermi momentum is larger than the photon recoil, most final momentum states are within the Fermi surface. At low temperature, we find that light scattered even at large angles is suppressed by 37% compared with higher temperatures, where atoms scatter at the single-atom Rayleigh scattering rate.

Tris(piperazinediium) phosphatododeca-molybo(V,VI)phosphate.

The title compound, (C(4)H(12)N(2))(3)[PMo(12)O(40)] or (H(2)pip)(3)[PMo(12)O(40)] (pip is piperazine), was prepared under hydro-thermal conditions. The asymmetric unit contains one-sixth of a mixed-valent Mo(V,VI) pseudo-Keggin-type [PMo(12)O(40)](6-) anion and half a piperazinediium cation, (H(2)pip)(2+). The discrete Keggin-type [PMo(12)O(40)](6- )anion has site symmetry and the three (H(2)pip)(2+) cations each have site symmetry at the centres of the mol-ecules. The central P atom is on special position , which is a roto-inversion position and generates the disorder of the PO(4) tetra-hedron. Furthermore, six doubly bridging oxide groups are also disordered with an occupancy factor of 0.5 for each O atom. The anions and cations are linked by an extensive network of inter-molecular N-H⋯O and C-H⋯O hydrogen bonds.

Dirac solitons in optical microresonators.

Mode-coupling-induced dispersion has been used to engineer microresonators for soliton generation at the edge of the visible band. Here, we show that the optical soliton formed in this way is analogous to optical Bragg solitons and, more generally, to the Dirac soliton in quantum field theory. This optical Dirac soliton is studied theoretically, and a closed-form solution is derived in the corresponding conservative system. Both analytical and numerical solutions show unusual properties, such as polarization twisting and asymmetrical optical spectra. The closed-form solution is also used to study the repetition rate shift in the soliton. An observation of the asymmetrical spectrum is analysed using theory. The properties of Dirac optical solitons in microresonators are important at a fundamental level and provide a road map for soliton microcomb generation in the visible band.

Reconfigurable symmetry-broken laser in a symmetric microcavity.

The coherent light source is one of the most important foundations in both optical physics studies and applied photonic devices. However, the whispering gallery microcavity, as a prime platform for novel light sources, has the intrinsically chiral symmetry and severely rules out access to directional light output, all-optical flip-flops, efficient light extraction, etc. Here, we demonstrate a reconfigurable symmetry-broken microlaser in an ultrahigh-Q whispering gallery microcavity with the symmetric structure, in which a chirality of lasing field is empowered spontaneously by the optical nonlinear effect. Experimentally, the ratio of counter-propagating lasing intensities is found to exceed 160:1, and the chirality can be controlled dynamically and all-optically by the bias in the pump direction. This work not only presents a distinct recipe for coherent light sources with robust and reconfigurable performance, but also opens up an unexplored avenue to symmetry-broken physics in optical micro-structures.

Modified polyoxometalate: a novel monocapped bi-supporting and reduced α-Keggin structure {PMo12O40[Cu(2,2'-bpy)]}[Cu(2,2'-bpy)(en)(H2O)]2.

A novel modified polyoxometalate, {PMo12O40[Cu(2,2'-bpy)]}[Cu(2,2'-bpy)(en)(H2O)]2 [2,2'-bpy is 2,2'-bipyridyl (C10H8N2) and en is ethylenediamine (C2H8N2)], has been synthesized hydrothermally and structurally characterized by elemental analysis, TG, IR, XPS and single-crystal X-ray diffraction. The structural analysis reveals that the compound contains the reduced Keggin polyanion [PMo12O40]6- as the parent unit, which is monocapped by [Cu(2,2'-bpy)]2+ fragments via four bridging O atoms on an {Mo4O4} pit and bi-supported by two [Cu(2,2'-bpy)(en)(H2O)]2+ coordination cations simultaneously. There exist strong intramolecular π-π stacking between the capping and supporting units, which play a stabilizing role during the crystallization of the compound. Adjacent POM clusters are further aggregated to form a three-dimensional supramolecular network through noncovalent forces, hydrogen bonding and π-π stacking interactions. In addition, the photocatalytic properties were investigated in detail, and the results indicated that the compound can be used as a photocatalyst towards the decomposition of the organic pollutant methylene blue (MB).

Spontaneous T-symmetry breaking and exceptional points in cavity quantum electrodynamics systems.

Spontaneous symmetry breaking has revolutionized the understanding in numerous fields of modern physics. Here, we theoretically demonstrate the spontaneous time-reversal symmetry breaking in a cavity quantum electrodynamics system in which an atomic ensemble interacts coherently with a single resonant cavity mode. The interacting system can be effectively described by two coupled oscillators with positive and negative mass, when the two-level atoms are prepared in their excited states. The occurrence of symmetry breaking is controlled by the atomic detuning and the coupling to the cavity mode, which naturally divides the parameter space into the symmetry broken and symmetry unbroken phases. The two phases are separated by a spectral singularity, a so-called exceptional point, where the eigenstates of the Hamiltonian coalesce. When encircling the singularity in the parameter space, the quasi-adiabatic dynamics shows chiral mode switching which enables topological manipulation of quantum states.