A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, would not only enable explorations of a large class of many-body physics phenomena, but could also be used for quantum information processing. We report on the creation of an ultracold dense gas of 40 K 87 Rb polar molecules. Using a single step of STIRAP (STImulated Raman Adiabatic Passage) via two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential. The polar molecular gas has a peak density of 10 12 cm −3 , and an expansion-determined translational temperature of 350 nK. The polar molecules have a permanent electric 1 arXiv:0808.2963v2 [quant-ph]

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Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range, and spatially anisotropic. This is in stark contrast to the dilute gases of ultracold atoms, which have isotropic and extremely short-range, or "contact", interactions. The large electric dipole moment of polar molecules can be tuned with an external electric field; this provides unique opportunities such as control of ultracold chemical reactions, quantum information processing, and the realization of novel quantum many-body systems. In spite of intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules, only recently have sufficiently high densities been achieved. Here, we report the observation of dipolar collisions in an ultracold molecular gas prepared close to quantum degeneracy. For modest values of an applied electric field, we observe a dramatic increase in the loss rate of fermionic KRb molecules due to ultrcold chemical reactions. We find that the loss rate has a steep power-law dependence on the induced electric dipole moment, and we show that this dependence can be understood with a relatively simple model based on quantum threshold laws for scattering of fermionic polar molecules. We directly observe the spatial anisotropy of the dipolar interaction as manifested in measurements of the thermodynamics of the dipolar gas. These results demonstrate how the long-range dipolar interaction can be used for electric-field control of chemical reaction rates in an ultracold polar molecule gas. The large loss rates in an applied electric field suggest that creating a long-lived ensemble of ultracold polar molecules may require confinement in a two-dimensional trap geometry to suppress the influence of the attractive dipolar interactions

Chemical reaction rates often depend strongly on stereodynamics, namely the orientation and movement of molecules in three-dimensional space [1][2][3]. An ultracold molecular gas, with a temperature below 1 µK, provides a highly unusual regime for chemistry, where polar molecules can easily be oriented using an external electric field and where, moreover, the motion of two colliding molecules is strictly quantized. Recently, atom-exchange reactions were observed in a trapped ultracold gas of KRb molecules [4]. In an external electric field, these exothermic and barrierless bimolecular reactions, KRb+KRb→ K 2 +Rb 2 , occur at a rate that rises steeply with increasing dipole moment [5]. Here we show that the quantum stereodynamics of the ultracold collisions can be exploited to suppress the bimolecular chemical reaction rate by nearly two orders of magnitude. We use an optical lattice trap to confine the fermionic polar molecules in a quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along the tight confinement direction [6,7]. With the combination of sufficiently tight confinement and Fermi statistics of the molecules, two polar molecules can approach each other only in a "side-by-side" collision, where the chemical reaction rate is suppressed by the repulsive dipole-dipole interaction. We show that the suppression of the bimolecular reaction rate requires quantum-state control of both the internal and external degrees of freedom of the molecules. The suppression of chemical reactions for polar molecules in a quasi-two-dimensional trap opens the way for investigation of a dipolar molecular quantum gas. Because of the strong, long-range character of the dipole-dipole interactions, such a gas brings fundamentally new abilities to quantum-gas-based studies of strongly correlated many-body physics, where quantum phase transitions and new states 2 of matter can emerge [8][9][10][11][12][13].Two colliding polar molecules interact via long-range dipole-dipole forces well before they reach the shorter distance scales where chemical forces become relevant. Therefore, the spatial anisotropy of the dipolar interaction can play an essential role in the stereochemistry of bimolecular reactions of polar molecules. In general, one expects the attraction between oriented dipoles in a "head-to-tail" collision to be favorable for chemical reactions, while the repulsion between two oriented polar molecules in a "side-by-side" collision presents an obstacle for reactions. Up to now, however, large center-of-mass collision energies have precluded the direct control of chemical reactions via dipolar interactions. In a cold collision regime, where tens of scattering partial waves contribute, one can begin to exert control of intermolecular dynamics through the dipolar effect [14]. An ultracold gas, however, provides an optimum environment in which to fully investigate the dipolar effects [5,15,16]. Here, the molecules can be prepared in identical internal quantum states, with the dipoles oriented using an external...

Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over kilometer-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1 × 10 –16 fractional level, surpassing the current best evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation–induced frequency shifts.

Ultracold molecules offer entirely new possibilities for the control of quantum processes due to their rich internal structure. Recently, near quantum degenerate gases of molecules have been prepared in their rovibronic ground state. For future experiments, it is crucial to also control their hyperfine state. Here, we report the preparation of a rovibronic ground state molecular quantum gas in a single hyperfine state and in particular in the absolute lowest quantum state. The demonstrated and presented scheme is general for bialkali polar molecules and allows the preparation of molecules in a single hyperfine state or in an arbitrary coherent superposition of hyperfine states. The scheme relies on electric-dipole, two-photon microwave transitions through rotationally excited states and makes use of electric nuclear quadrupole interactions to transfer molecular population between different hyperfine states.PACS numbers: 03.75. Kk, 03.75.Ss, 32.80.Pj, 34.20.Cf, The field of ultracold atomic quantum gases draws much of its success from the unprecedented ability to precisely control the external and internal degrees of freedom of the gas. Control over the external, or motional degree of freedom, comes from realizing ultracold gases in almost arbitrary confining potentials provided by magnetic or optical fields. The internal degrees of freedom, namely the quantum states of the atoms, can be manipulated by driving rf [1] or optical transitions. Because collisional interactions in the gas depend on the internal states, precise control of these quantum states is a prerequisite for creating trapped samples that are stable against inelastic collisions as well as for accessing scattering resonances in order to tune the interparticle interactions [2]. Manipulation of the internal degrees of freedom is also essential in the study of quantum gases, where samples of identical bosons or fermions in a single internal state can behave very differently from spin mixtures. Finally, the precise control of the atomic states is key to quantum information schemes where one seeks to initialize and manipulate atoms as quantum qubits with long coherence times [3].The precise control of external and internal degrees of freedom will be equally important for the emerging field of ultracold molecular quantum gases. This field has recently seen tremendous progress through the first preparation of near quantum degenerate gas of bialkali molecules in the rovibrational ground state of the electronic ground molecular potential [4,5]. These experiments have demonstrated a high degree of control over the electronic, vibrational and rotational degrees of freedom of ultracold molecules via two-photon optical Raman transitions [6,7]. However, most molecules will additionally have hyperfine structure within a single rotational and vibrational level [8], and, as is true for ultracold atomic gases, control of these quantum degrees of freedom is essential for future experiments. In particular, for experimental efforts to achieve a Bose-Einstein condensate...

Detailed information on probing behavior of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is critical for understanding the transmission process of phloem‐limited bacteria (Candidatus Liberibacter spp.) associated with citrus ‘huanglongbing’ by this vector. In this study, we investigated stylet penetration activities of D. citri on seedlings of Citrus sinensis (L.) Osbeck cv. Pêra (Rutaceae) by using the electrical penetration graph (EPG‐DC system) technique. EPG waveforms were described based on amplitude, frequency, voltage level, and electrical origin of the observed traces during stylet penetration into plant tissues. The main waveforms were correlated with histological observations of salivary sheath termini in plant tissues, to determine the putative location of stylet tips. The behavioral activities were also inferred based on waveform similarities in relation to other Sternorrhyncha, particularly aphids and whiteflies. In addition, we correlated the occurrence of specific waveforms with the acquisition of the phloem‐limited bacterium Ca. Liberibacter asiaticus by D. citri. The occurrence of a G‐like xylem sap ingestion waveform in starved and unstarved psyllids was also compared. By analyzing 8‐h EPGs of adult females, five waveforms were described: (C) salivary sheath secretion and other stylet pathway activities; (D) first contact with phloem (distinct from other waveforms reported for Sternorrhyncha); (E1) putative salivation in phloem sieve tubes; (E2) phloem sap ingestion; and (G) probably xylem sap ingestion. Diaphorina citri initiates a probe with stylet pathway through epidermis and parenchyma (C). Interestingly, no potential drops were observed during the stylet pathway phase, as are usually recorded in aphids and other Sternorrhyncha. Once in C, D. citri shows a higher propensity to return to non‐probing than to start a phloem or xylem phase. Several probes are usually observed before the phloem phase; waveform D is observed upon phloem contact, always immediately followed by E1. After E1, D. citri either returns to pathway activity (C) or starts phloem sap ingestion, which was the longest activity observed.

The absolute frequency of the 1 S 0 − 3 P 0 clock transition of 87 Sr has been measured to be 429 228 004 229 873.65 (37) Hz using lattice-confined atoms, where the fractional uncertainty of 8.6×10 −16 represents one of the most accurate measurements of an atomic transition frequency to date. After a detailed study of systematic effects, which reduced the total systematic uncertainty of the Sr lattice clock to 1.5×10 −16 , the clock frequency is measured against a hydrogen maser which is simultaneously calibrated to the US primary frequency standard, the NIST Cs fountain clock, NIST-F1. The comparison is made possible using a femtosecond laser based optical frequency comb to phase coherently connect the optical and microwave spectral regions and by a 3.5 km fiber transfer scheme to compare the remotely located clock signals.