Abstract. -We present new techniques in cooling 39 K atoms using laser light close to the D1 transition. First, a new compressed-MOT configuration is taking advantage of gray molasses type cooling induced by blue-detuned D1 light. It yields an optimized density of atoms. Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6 µK. The resulting phase-space density is 2 × 10 −4 and will ease future experiments with ultracold potassium. As an example, we use it to directly load up to 3 × 10 7 atoms in a far detuned optical trap, a result that opens the way to the all-optical production of potassium degenerate gases.

Long-term inertial navigation is currently limited by the bias drifts of gyroscopes and accelerometers. Ultra-stable cold-atom interferometers offer a promising alternative for the next generation of high-end navigation systems. Here, we present an experimental setup and an algorithm hybridizing a stable matter-wave interferometer with a classical accelerometer. We use correlations between the quantum and classical devices to track the bias drift of the latter and form a hybrid sensor. We apply the Kalman filter formalism to obtain an optimal estimate of the bias and simulate experimentally a harsh environment representative of that encountered in mobile sensing applications. We show that our method is more precise and robust than traditional sine-fitting methods. The resulting sensor exhibits a 400 Hz bandwidth and reaches a stability of 10 ng after 11 h of integration.Inertial navigation systems determine the position of a moving vehicle by continuously measuring its acceleration and rotation rate, and subsequently integrating the equations of motion [1]. These systems are limited by slow drifts of the biases inherent to their inertial sensors, which ultimately lead to large speed and position errors after integration. Currently, the long-term bias stability of navigation-grade accelerometers is on the order of 10 µg-which, in the absence of aiding sensors such as satellite navigation systems, leads to horizontal position oscillations of 60 m at the characteristic Schuler period of 84.4 minutes [1,2].Since their first demonstration in the early 1990s, atom interferometers (AIs) have proven to be excellent absolute inertial sensors-having been exploited as ultra-high sensitivity instruments for fundamental tests of physics [3][4][5][6][7][8], and as state-of-the-art gravimeters with accuracies in the range of 1 − 10 ng achieved both in laboratories [9][10][11][12][13][14] and with compact transportable systems [15][16][17][18][19]. As a result, they have been proposed for the next generation of inertial navigation systems [20][21][22][23]. However, cold-atom-based sensors generally possess a small bandwidth, and suffer from low repetition rates (with the exceptions of Refs. [24,25]) and dead times during which no inertial measurements can be made. In comparison, mechanical accelerometers exhibit broad bandwidths compatible with navigation applications [26], but are afflicted by long-term bias and scale factor drifts. These two types of sensors can thus be hybridized [27] in order to benefit from the best of both worlds-in strong analogy with the strategy employed in atomic clocks [28].Here, we use correlations between an AI and a classical accelerometer to track the bias of the latter, and we present an approach based on a non-linear Kalman *

We report on the production of 39 K matter-wave bright solitons, i.e., 1D matter-waves that propagate without dispersion thanks to attractive interactions. The volume of the soliton is studied as a function of the scattering length through three-body losses, revealing peak densities as high as ∼ 5 × 10 20 m −3 . Our solitons, close to the collapse threshold, are strongly bound and will find applications in fundamental physics and atom interferometry.PACS numbers: 03.75. Lm, Solitons are one-dimensional wave-packets that propagate with neither change of shape nor loss of energy. They are a consequence of non-linearities that balance wave-packet spreading due to dispersion. They appear in numerous physical systems such as water waves, optical fibers, plasmas, acoustic waves or even in energy propagation along proteins [1]. Solitons are also observed in ultracold quantum gases [2][3][4][5][6]. In this context, matterwave bright solitons are Bose-Einstein condensates that remain bound thanks to mean-field attractive interactions in a one dimensional geometry [2,3].Matter-wave bright solitons are predicted to be a great tool to locally probe rapidly varying forces for example close to a surface [7,8], or probe (surface) bound states [7,9] which do not appear in linear scattering. For example, the small size of bright solitons has been used in the measurement of quantum reflection from a barrier [10,11]. Because of their dispersion-free propagation, bright solitons are also believed to be good candidates for performing very long time atom interferometry measurements [12] although interactions may cause additional phase shifts [13][14][15][16]. Recently, an experiment demonstrated an increased visibility for a soliton atomic interferometer as compared to its non interacting counterpart [17]. The interactions in solitons can also lead to squeezed or entangled states, which could improve the sensitivity of interferometric measurements beyond the shot noise limit [18][19][20][21][22][23][24]. In some cases, the formation of mesoscopic Schrödinger cat states or NOON states is predicted [25][26][27]. A problem in using these states is losses, such as three-body collisions, which are an intrinsic source of decoherence. They can also induce unusual soliton center of mass dynamics [28].Experiments producing and studying matter-wave bright solitons, despite their interest in both applied and fundamental physics, have remained scarce. In fact, only two elements have been turned into bright solitons, 7 Li [2, 3, 29, 30] and 85 Rb [10,31]. In this paper, we describe the production of 39 K solitons in the |F = 1, m F = −1 state using the Feshbach resonance at 561 G [32] and its associated zero-crossing of the scattering length at 504.4 G (see figure 1). We have optimized the setup in order to produce strongly bound solitons, i.e., solitons with a large negative interaction energy. We thus pro- The evaporation to Bose-Einstein condensation takes place at 550 G (red bullet). The magnetic field is then ramped in two steps to 507 G (vi...