We report on the first direct observation of fast spin-exchange coherent oscillations between different long-lived electronic orbitals of ultracold 173 Yb fermions. We measure, in a model-independent way, the strength of the exchange interaction driving this coherent process. This observation allows us to retrieve important information on the interorbital collisional properties of 173 Yb atoms and paves the way to novel quantum simulations of paradigmatic models of two-orbital quantum magnetism. DOI: 10.1103/PhysRevLett.113.120402 PACS numbers: 03.75.Ss, 34.50.Cx, 37.10.Jk, 67.85.Lm Alkaline-earth-like (AEL) atoms are providing a new valuable experimental platform for advancing the possibilities of quantum simulation with ultracold gases [1]. For instance, the purely nuclear spin of ground-state AEL fermionic isotopes results in the independence of the atom-atom scattering properties from the nuclear spin projection. This feature has enabled the investigation of multicomponent 173 Yb fermions with SUðNÞ interaction symmetry both in optical lattices [2] and in onedimensional quantum wires [3]. In addition to their nuclear spin, AEL atoms offer experimental access to supplementary degrees of freedom, in particular, to a long-lived electronically excited state jei ¼ j 3 P 0 i which can be coherently populated from the ground state jgi ¼ j 1 S 0 i by optical excitation on an ultranarrow clock transition. The possibility of coherently manipulating both the orbital and the spin degree of freedom has recently been envisioned to grant the realization of paradigmatic models of two-orbital magnetism, like the Kondo model [4]. In this context, the two electronic states jgi and jei play the roles of two different orbitals.Recent experiments have investigated the SUðNÞ symmetry in jgi-jei ultracold collisions of two-electron atoms [5] and reported on first signatures of spin-exchange interactions between atoms in the two electronic states [6]. Spin-exchange interactions arise from the difference in the spin-singlet and spin-triplet potential curves in the scattering of one jgi and one jei atom. Let us assume that the two interacting atoms are in different nuclear spin states j↑i and j↓i (where the arrows are placeholders for two arbitrary nuclear spin states) and that they share the same spatial wave function. At zero magnetic field, the degeneracy of the configurations jg↑; e↓i and jg↓; e↑i, which are associated with a well-defined spin in each orbital [7], is lifted by the atom-atom interaction and the eigenstates are the orbital-symmetric (spin-singlet) jeg þ i and the orbitalantisymmetric (spin-triplet) jegFIG. 1 (color online). Two-orbital spin-exchange interaction in AEL atoms. (a) One atom in the ground state jgi and one atom in the long-lived electronic state jei periodically "exchange" their nuclear spins because of the different interaction energy in the spin-singlet jeg þ i and spin-triplet jeg − i two-particle states (note that in the graphical notation, the two-particle exchange symmetry is implicit [7]). (b...

Abstract. The international research project RECONCILE has addressed central questions regarding polar ozone depletion, with the objective to quantify some of the most relevant yet still uncertain physical and chemical processes and thereby improve prognostic modelling capabilities to realistically predict the response of the ozone layer to climate change. This overview paper outlines the scope and the general approach of RECONCILE, and it provides a summary of observations and modelling in 2010 and 2011 that have generated an in many respects unprecedented dataset to study processes in the Arctic winter stratosphere. Principally, it summarises important outcomes of RECONCILE including (i) better constraints and enhanced consistency on the set of parameters governing catalytic ozone destruction cycles, (ii) a better understanding of the role of cold binary aerosols in heterogeneous chlorine activation, (iii) an improved scheme of polar stratospheric cloud (PSC) processes that includes heterogeneous nucleation of nitric acid trihydrate (NAT) and ice on non-volatile background aerosol leading to better model parameterisations with respect to denitrification, and (iv) long transient simulations with a chemistry-climate model (CCM) updated based on the results of RECONCILE that better reproduce past ozone trends in Antarctica and are deemed to produce more reliable predictions of future ozone trends. The process studies and the global simulations conducted in RECONCILE show that in the Arctic, ozone depletion uncertainties in the chemical and microphysical processes are now clearly smaller than the sensitivity to dynamic variability.

The major development recently undergone by quantum cascade lasers has effectively extended frequency comb emission to longer-wavelength spectral regions, i.e. the mid and far infrared. Unlike classical pulsed frequency combs, their mode-locking mechanism relies on four-wave mixing nonlinear processes, with a temporal intensity profile different from conventional short-pulses trains. Measuring the absolute phase pattern of the modes in these combs enables a thorough characterization of the onset of mode-locking in absence of shortpulses emission, as well as of the coherence properties.Here, by combining dual-comb multi-heterodyne detection with Fourier-transform analysis, we show how to simultaneously acquire and monitor over a wide range of timescales the phase pattern of a generic frequency comb. The technique is applied to characterize a mid-infrared and a terahertz quantum cascade laser frequency comb, conclusively proving the high degree of coherence and the remarkable long-term stability of these sources. Moreover, the technique allows also the reconstruction of electric field, intensity profile and instantaneous frequency of the emission. IntroductionThe optical frequency comb (FC) is a peculiar multi-frequency coherent photonic state made of a series of evenly-spaced modes in the frequency domain, typically generated by frequency-stabilized and controlled femtoseconds mode-locked lasers [1][2][3][4]. In the visible and near infrared (IR) such technology is nowadays well established [5]. The miniaturization of these sources, together with the expansion of their operation range towards other spectral regions (e.g. mid and far IR), is crucial for broadening their application range.In this direction, the most interesting results have recently been achieved with quantum cascade lasers (QCLs), current-driven semiconductor lasers based on intersubband transitions in quantum wells, emitting high-power coherent radiation in the mid IR and terahertz (THz) [6][7][8][9][10][11]. In

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We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250 kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1 kHz, and its frequency is stabilized with an absolute traceability of 2×10−12. This source has allowed the measurement of the absolute frequency of a CO2 molecular transition with an uncertainty of about 1 kHz.