In the Stern-Gerlach effect, a magnetic field gradient splits particles into spatially separated paths according to their spin projection. The idea of exploiting this effect for creating coherent momentum superpositions for matter-wave interferometry appeared shortly after its discovery, almost a century ago, but was judged to be far beyond practical reach. Here we demonstrate a viable version of this idea. Our scheme uses pulsed magnetic field gradients, generated by currents in an atom chip wire, and radio-frequency Rabi transitions between Zeeman sublevels. We transform an atomic Bose-Einstein condensate into a superposition of spatially separated propagating wavepackets and observe spatial interference fringes with a measurable phase repeatability. The method is versatile in its range of momentum transfer and the different available splitting geometries. These features make our method a good candidate for supporting a variety of future applications and fundamental studies.

Here we review the field of atom chips in the context of Bose–Einstein Condensates (BEC) as well as cold matter in general. Twenty years after the first realization of the BEC and 15 years after the realization of the atom chip, the latter has been found to enable extraordinary feats: from producing BECs at a rate of several per second, through the realization of matter-wave interferometry, and all the way to novel probing of surfaces and new forces. In addition, technological applications are also being intensively pursued. This review will describe these developments and more, including new ideas which have not yet been realized.

We experimentally demonstrate a new interferometry paradigm: a self-interfering clock. We split a clock into two spatially separated wave packets, and observe an interference pattern with a stable phase showing that the splitting was coherent, i.e., the clock was in two places simultaneously. We then make the clock wave packets "tick" at different rates to simulate a proper time lag. The entanglement between the clock's time and its path yields "which path" information, which affects the visibility of the clock's self-interference. By contrast, in standard interferometry, time cannot yield "which path" information. As a clock we use an atom prepared in a superposition of two spin states. This first proof-of-principle experiment may have far-reaching implications for the study of time and general relativity and their impact on fundamental quantum effects such as decoherence and wave packet collapse. Two-slit interferometry of quanta, such as photons and electrons, figured prominently in the Bohr-Einstein debates on the consistency of quantum theory [1, 2]. A fundamental principle emerging from those debates-intimately related to the uncertainty principle-is that "which path" information about the quanta passing through slits blocks their interference. At the climax of the debates, Einstein claimed that a clock, emitting a photon at a precise time while being weighed on a spring scale to measure the change in its massenergy, could evade the uncertainty principle. Yet Bohr showed that the clock's gravitational redshift introduced enough uncertainty in the emission time to satisfy the uncertainty principle. Inspired by the subtle role time may play in quantum mechanics, we have now sent a clock through a spatial interferometer. The proof-of-principle experiment described below presents clock interferometry as a new tool for studying the interplay of general relativity[3] and quantum mechanics [4].Quantum mechanics cannot fully describe a self-interfering clock in a gravitational field.If the paths of a clock through an interferometer have different heights, then general relativity predicts that the clock must "tick" slower along the lower path. However, time in quantum mechanics is a global parameter, which cannot differ between paths. In standard interferometry (e.g.[5]), a difference in height between two paths affects their relative phase and shifts their interference pattern; but in clock interferometry, a time differential between paths yields "which path" information, degrading the visibility of the interference pattern [6]. It follows that, while standard interferometry may probe general relativity [7][8][9] In principle, any system evolving with a well defined period can be a clock. In our experiment, we utilize a quantum two-level system. Specifically, each clock is a 87 Rb atom in a superposition of two Zeeman sublevels, the m F = 1 and m F = 2 sublevels of the F = 2 hyperfine state.The general scheme of the clock interferometer is shown in Fig. 1 atoms 90 µm below the chip surface). Initially, af...

Ultracold atom magnetic field microscopy enables the probing of current flow patterns in planar structures with unprecedented sensitivity. In polycrystalline metal (gold) films we observe longrange correlations forming organized patterns oriented at ±45• relative to the mean current flow, even at room temperature and at length scales orders of magnitude larger than the diffusion length or the grain size. The preference to form patterns at these angles is a direct consequence of universal scattering properties at defects. The observed amplitude of the current direction fluctuations scales inversely to that expected from the relative thickness variations, the grain size and the defect concentration, all determined independently by standard methods. This indicates that ultracold atom magnetometry enables new insight into the interplay between disorder and transport.Thin metal films are the classic environment for studying the effect of geometric constraints [1,2] and crystal defects [3,4] on the transport of electrons. In a perfectly straight long wire that is free from structural defects, a direct current (DC) strictly follows the wire direction and creates a magnetic field in the plane perpendicular to the wire. An obstacle may locally change the direction of the current and consequently locally rotate the magnetic field close to the wire by an angle β in a plane parallel to the plane of the thin film wire.Ultracold atom magnetometry [5,6] on atom chips [7,8,9] allows for the sensitive probing of this angle β (and its spatial variation) with µrad (µm) resolution. Compared to scanning probes having a µm scale spatial resolution and 10 −5 T sensitivity, or superconducting quantum interference devices (SQUIDs) having 10 −13 T sensitivity but a resolution of tens of µm, ultracold atom magnetometry has both high sensitivity (10 −10 T) and high resolution (several µm) [6]. In addition, ultracold atoms enable high resolution over a large length scale (mm) in a single shot. This enables the simultaneous observations of microscopic and macroscopic phenomena, as described in this work.Using cold atoms just above the transition to BoseEinstein Condensation (BEC), we apply ultracold atom magnetometry to study the current deflection in three different precision-fabricated polycrystalline gold wires with a rectangular cross section of 200µm width and different thicknesses and crystalline grain sizes, as summarized in Table I [10]. Choosing the wire length along x, its width along y and thickness along z, Fig. 1 shows the maps of the angular variations β(x, y, z 0 ) = δB x (x, y, z 0 )/B y of the magnetic field created by a current of 180 mA flowing along the wire, measured at z 0 =3.5µm above its center (far from the edges).Even though at ambient temperature scattering by lattice vibrations (phonons) quickly diffuses the electronic motion, long-range correlations (tens of µm) in the current flow patterns can be seen. This is surprising as effects of static defects are usually observed only on a Table I. These fluctuations are...