SummaryCells chemically isolate molecules in compartments to both facilitate and regulate their interactions. In addition to membrane-encapsulated compartments, cells can form proteinaceous and membraneless organelles, including nucleoli, Cajal and PML bodies, and stress granules. The principles that determine when and why these structures form have remained elusive. Here, we demonstrate that the disordered tails of Ddx4, a primary constituent of nuage or germ granules, form phase-separated organelles both in live cells and in vitro. These bodies are stabilized by patterned electrostatic interactions that are highly sensitive to temperature, ionic strength, arginine methylation, and splicing. Sequence determinants are used to identify proteins found in both membraneless organelles and cell adhesion. Moreover, the bodies provide an alternative solvent environment that can concentrate single-stranded DNA but largely exclude double-stranded DNA. We propose that phase separation of disordered proteins containing weakly interacting blocks is a general mechanism for forming regulated, membraneless organelles.

We demonstrate fluorescence microscopy of individual fermionic potassium atoms in a 527-nmperiod optical lattice. Using electromagnetically induced transparency (EIT) cooling on the 770.1-nm D1 transition of 40 K, we find that atoms remain at individual sites of a 0.3-mK-deep lattice, with a 1/e pinning lifetime of 67(9) s, while scattering ∼ 10 3 photons per second. The plane to be imaged is isolated using microwave spectroscopy in a magnetic field gradient, and can be chosen at any depth within the three-dimensional lattice. With a similar protocol, we also demonstrate patterned selection within a single lattice plane. High resolution images are acquired using a microscope objective with 0.8 numerical aperture, from which we determine the occupation of lattice sites in the imaging plane with 94(2)% fidelity per atom. Imaging with single-atom sensitivity and addressing with single-site accuracy are key steps towards the search for unconventional superfluidity of fermions in optical lattices, the initialization and characterization of transport and non-equilibrium dynamics, and the observation of magnetic domains.Ultracold fermionic atoms in an optical lattice realize an impurity-free analog of electrons in crystalline materials, with full control of parameters such as interaction strength, dimensionality, and tunneling [1,2]. Furthermore, ultracold systems can study many-body physics in scenarios currently inaccessible to materials, such as gauge fields equivalent to thousands of Tesla [3][4][5], interactions at the unitary limit [6], and quantum manybody physics far from equilibrium [7]. With sufficient control and probes, these experiments can be considered analog quantum simulations [8,9]. However, two important tools have been lacking: imaging and addressing fermionic atoms at the single-site and single-atom level [9]. When applied to bosonic atoms, these tools have already been dramatically successful [10][11][12][13][14][15][16][17][18][19][20].High-resolution imaging and manipulation of ultracold fermions solves several outstanding problems at once. First, in-situ spatial probes directly reveal the order parameter of insulating phases, magnetic domain formation, and other correlations inaccessible in time-of-flight imaging [13,14,19]. Second, an ensemble of density distributions provides a direct measure of entropy [13,14], extending thermometry of lattice fermions [21]. Third, manipulation of atoms with single-site precision can initiate dynamics [15,16], project or remove disorder [14], and selectively remove high entropy atoms to perform in-situ cooling [22,23].This year, five research groups have succeeded in imaging single fermions in an optical lattice: three using Raman sideband cooling [24][25][26] and two using EIT cooling [27], including the results reported in this Article. Our approach is distinguished by a unique imaging configuration, and takes a further step by implementing threedimensional spatial addressing, which is used here for selective removal of atoms from the lattice. Figure 1 ill...

We study the population dynamics of a Bose-Einstein condensate in a double-well potential throughout the crossover from Josephson dynamics to hydrodynamics. At barriers higher than the chemical potential, we observe slow oscillations well described by a Josephson model. In the limit of low barriers, the fundamental frequency agrees with a simple hydrodynamic model, but we also observe a second, higher frequency. A full numerical simulation of the Gross-Pitaevskii equation giving the frequencies and amplitudes of the observed modes between these two limits is compared to the data and is used to understand the origin of the higher mode. Implications for trapped matter-wave interferometers are discussed.

We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic 40 K is captured using a magneto-optical trap (MOT) on the closed 4S 1/2 → 4P 3/2 transition at 767 nm and then transferred, with unit efficiency, to a MOT on the open 4S 1/2 → 5P 3/2 transition at 405 nm. Because the 5P 3/2 state has a smaller line width than the 4P 3/2 state, the Doppler limit is reduced. We observe temperatures as low as 63(6) µK, the coldest potassium MOT reported to date. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of β = 2 × 10 −10 cm 3 s −1 , and estimate an upper bound of 8 × 10 −18 cm 2 for the ionization cross section of the 5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase space density over our 767 nm MOT, showing enhanced pre-cooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic 41

We present a dual chamber atom chip apparatus for generating ultracold (87)Rb and (39)K atomic gases. The apparatus produces quasi-pure Bose-Einstein condensates of 10(4) (87)Rb atoms in an atom chip trap that features a dimple and good optical access. We have also demonstrated production of ultracold (39)K and subsequent loading into the chip trap. We describe the details of the dual chamber vacuum system, the cooling lasers, the magnetic trap, the multicoil magnetic transport system, the atom chip, and two optical dipole traps. Due in part to the use of light-induced atom desorption, the laser cooling chamber features a sufficiently good vacuum to also support optical dipole trap-based experiments. The apparatus is well suited for studies of atom-surface forces, quantum pumping and transport experiments, atom interferometry, novel chip-based traps, and studies of one-dimensional many-body systems.