Lasing and suppressed cavity-pulling effect of Cesium active optical clock

We experimentally demonstrate the collective emission behavior and suppressed cavity-pulling effect of four-level active optical clock with Cesium atoms. Thermal Cesium atoms in a glass cell velocity selective pumped with a 455.5 nm laser operating at 6S$_{1/2}$ to 7P$_{3/2}$ transition are used as lasing medium. Population inverted Cesium atoms between 7S$_{1/2}$ and 6P$_{3/2}$ levels are optical weakly coupled by a pair cavity mirrors working at deep bad-cavity regime with a finesse of 4.3, and the ratio between cavity bandwidth and gain bandwidth is approximately 45. With increased 455.5 nm pumping laser intensity, the output power of cesium active optical clock at 1469.9 nm from 7S$_{1/2}$ level to 6P$_{3/2}$ level shows a threshold and reach a power of 13 $\mu$W. Active optical clock would dramatically improve the optical clock stability since the lasing frequency does not follow the cavity length variation exactly, but in a form of suppressed cavity pulling effect. In this letter the cavity pulling effect is measured using a Fabry-Perot interferometer (FPI) to be reduced by a factor of 38.2 and 41.4 as the detuning between the 1469.9 nm cavity length of the Cs active optical clock and the Cs 1469.9 nm transition is set to be 140.8 MHz and 281.6 MHz respectively. The mechanism demonstrated here is of great significance for new generation optical clocks and can be applied to improve the stability of best optical clocks by at least two orders of magnitude.Comment: 12 pages, 8 figure

Transverse Momentum Distribution as a Probe of J/psi Production Mechanism in Heavy Ion Collisions

We investigate J/psi transverse momentum distribution in a transport approach. While the nuclear modification factor R_{AA}(N_p) at RHIC is almost the same as at SPS, the averaged transverse momentum square and R_{AA}(p_t) are very different at SPS, RHIC and LHC and can be used to differentiate from the J/psi production mechanisms in high energy nuclear collisions.Comment: 4pages,3figures.proceeding for NN200

Stable Gapless Bose Liquid Phases without any Symmetry

It is well-known that a stable algebraic spin liquid state (or equivalently an algebraic Bose liquid (ABL) state) with emergent gapless photon excitations can exist in quantum spin ice systems, or in a quantum dimer model on a bipartite $3d$ lattice. This photon phase is stable against any weak perturbation without assuming any symmetry. Further works concluded that certain lattice models give rise to more exotic stable algebraic Bose liquid phases with graviton-like excitations. In this paper we will show how these algebraic Bose liquid states can be generalized to stable phases with even more exotic types of gapless excitations and then argue that these new phases are stable against weak perturbations. We also explicitly show that these theories have an (algebraic) topological ground state degeneracy on a torus, and construct the corresponding topological invariants.Comment: 8 pages, 1 figur

"Self-Dual" Quantum Critical Point on the surface of 3d3d Topological Insulator

In the last few years a lot of exotic and anomalous topological phases were constructed by proliferating the vortex like topological defects on the surface of the $3d$ topological insulator (TI). In this work, rather than considering topological phases at the boundary, we will study quantum critical points driven by vortex like topological defects. In general we will discuss a $(2+1)d$ quantum phase transition described by the following field theory: $\mathcal{L} = \bar{\psi}\gamma_\mu (\partial_\mu - i a_\mu) \psi + |(\partial_\mu - i k a_\mu)\phi|^2 + r |\phi|^2 + g |\phi|^4$, with tuning parameter $r$, arbitrary integer $k$, Dirac fermion $\psi$ and complex scalar bosonic field $\phi$ which both couple to the same $(2+1)d$ dynamical noncompact U(1) gauge field $a_\mu$. The physical meaning of these quantities/fields will be explained in the text. We demonstrate that this quantum critical point has a quasi self-dual nature. And at this quantum critical point, various universal quantities such as the electrical conductivity, and scaling dimension of gauge invariant operators can be calculated systematically through a $1/k^2$ expansion, based on the observation that the limit $k \rightarrow + \infty$ corresponds to an ordinary $3d$ XY transition.Comment: 5 pages, 2 figure

Interacting Topological Superconductors and possible Origin of 16n16n Chiral Fermions in the Standard Model

Motivated by the observation that the Standard Model of particle physics (plus a right-handed neutrino) has precisely 16 Weyl fermions per generation, we search for $(3+1)$-dimensional chiral fermionic theories and chiral gauge theories that can be regularized on a 3 dimensional spatial lattice when and only when the number of flavors is an integral multiple of 16. All these results are based on the observation that local interactions reduce the classification of certain $(4+1)$-dimensional topological superconductors from $\mathbb{Z}$ to $\mathbb{Z}_{8}$, which means that one of their $(3+1)$-dimensional boundaries can be gapped out by interactions without breaking any symmetry when and only when the number of boundary chiral fermions is an integral multiple of $16$.Comment: 5 pages, 2 figure

Topological Orders with Global Gauge Anomalies

By definition, the physics of the $d-$dimensional (dim) boundary of a $(d+1)-$dim symmetry protected topological (SPT) state cannot be realized as itself on a $d-$dim lattice. If the symmetry of the system is unitary, then a formal way to determine whether a $d-$dim theory must be a boundary or not, is to couple this theory to a gauge field (or to "gauge" its symmetry), and check if there is a gauge anomaly. In this paper we discuss the following question: can the boundary of a SPT state be driven into a fully gapped topological order which preserves all the symmetries? We argue that if the gauge anomaly of the boundary is "perturbative", then the boundary must remain gapless; while if the boundary only has global gauge anomaly but no perturbative anomaly, then it is possible to gap out the boundary by driving it into a topological state, when $d \geq 2$. We will demonstrate this conclusion with two examples: (1) the $3d$ spin-1/2 chiral fermion with the well-known Witten's global anomaly, which is the boundary of a $4d$ topological superconductor with SU(2) or U(1)$\rtimes Z_2$ symmetry; and (2) the $4d$ boundary of a $5d$ topological superconductor with the same symmetry. We show that these boundary systems can be driven into a fully gapped $\mathbb{Z}_{2N}$ topological order with topological degeneracy, but this $\mathbb{Z}_{2N}$ topological order cannot be future driven into a trivial confined phase that preserves all the symmetries due to some special properties of its topological defects.Comment: 12 page

Heavy Quark and Quarkonium Transport in High Energy Nuclear Collisions

The strong interaction between heavy quarks and the quark gluon plasma makes the open and hidden charm hadrons be sensitive probes of the deconfinement phase transition in high energy nuclear collisions. Both the cold and hot nuclear matter effects change with the colliding energy and significantly influence the heavy quark and charmonium yield and their transverse momentum distributions. The ratio of averaged quarkonium transverse momentum square and the elliptic flow reveal the nature of the QCD medium created in heavy ion collisions at SPS, RHIC and LHC energies.Comment: proceeding for Quark Matter 2015, Kobe, Japan; 8 pages, 1 figure

Medium Effects on Charmonium Production at ultrarelativistic energies available at the CERN Large Haron Collider

We investigate with a transport approach the cold and hot nuclear matter effects on the charmonium transverse momentum distributions in relativistic heavy ion collisions. The newly defined nuclear modification factor $r_{AA}=\langle p_{T}^2\rangle_{AA}/\langle p_{T}^2\rangle_{pp}$ and elliptic flow $v_2$ for $J/\psi$ are sensitive to the nature of the hot medium and the thermalization of heavy quarks. From SPS through RHIC to LHC colliding energies, we observe dramatic changes in the centrality dependence of $r_{AA}$. We find that at LHC energy, the finally observed charmonia are dominated by the regeneration from thermalized heavy quarks.Comment: 11 pages,10 figure

Dual-wavelength active optical clock

We experimentally realize the dual-wavelength active optical clock for the first time. As the Cs cell temperature is kept between 118 $^{\circ }C$ and 144 $^{\circ }C$, both the 1359 nm and the 1470 nm stimulated emission output of Cs four-level active optical clock are detected. The 1470 nm output linewidth of each experimental setup of Cs four-level active optical clock is measured to be 590 Hz with the main cavity length unstabilized. To stabilize the cavity length of active optical clock, the experimental scheme of 633 nm and 1359 nm good-bad cavity dual-wavelength active optical clock is proposed, where 633 nm and 1359 nm stimulated emission is working at good-cavity and bad-cavity regime respectively. The cavity length is stabilized by locking the 633 nm output frequency to a super-cavity with the Pound-Drever-Hall (PDH) technique. The frequency stability of 1359 nm bad-cavity stimulated emission output is then expected to be further improved by at least 1 order of magnitude than the 633 nm PDH system due to the suppressed cavity pulling effect of active optical clock, and the quantum limited linewidth of 1359 nm output is estimated to be 77.6 mHz.Comment: 12 pages, 5 figure

Symmetry Protected Topological States of Interacting Fermions and Bosons

We study the classification of interacting fermionic and bosonic symmetry protected topological (SPT) states. We define a SPT state as whether or not it is separated from the trivial state through a bulk phase transition, which is a general definition applicable to SPT states with or without spatial symmetries. We show that in all dimensions short range interactions can reduce the classification of free fermion SPT states, and we demonstrate these results by making connection between fermionic and bosonic SPT states. We first demonstrate that our formalism gives the correct classification for all the known SPT states, with or without interaction, then we will generalize our method to SPT states that involve the spatial inversion symmetry.Comment: 19 pages, 3 figure