We experimentally investigate an optical frequency standard based on the (2)S1/2(F=0)→(2)F7/2(F=3) electric octupole (E3) transition of a single trapped (171)Yb+ ion. For the spectroscopy of this strongly forbidden transition, we utilize a Ramsey-type excitation scheme that provides immunity to probe-induced frequency shifts. The cancellation of these shifts is controlled by interleaved single-pulse Rabi spectroscopy, which reduces the related relative frequency uncertainty to 1.1×10(-18). To determine the frequency shift due to thermal radiation emitted by the ion's environment, we measure the static scalar differential polarizability of the E3 transition as 0.888(16)×10(-40)  J m(2)/V(2) and a dynamic correction η(300  K)=-0.0015(7). This reduces the uncertainty due to thermal radiation to 1.8×10(-18). The residual motion of the ion yields the largest contribution (2.1×10(-18)) to the total systematic relative uncertainty of the clock of 3.2×10(-18).

The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behaviour. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings.

Doubly quantized vortices were topologically imprinted in |F = 1 23 Na condensates, and their time evolution was observed using a tomographic imaging technique. The decay into two singly quantized vortices was characterized and attributed to dynamical instability. The time scale of the splitting process was found to be longer at higher atom density.PACS numbers: 03.75. Kk, 03.75.Lm, 67.90.+z Quantum fluids, like superfluid He, electrons in a superconductor or a Bose-Einstein condensate of atoms, are described by a macroscopic wavefunction. This requires the flow field to be irrotational, and gives rise to superfluidity and quantized circulation [1]. Atoms in a BoseEinstein condensate, for example, can only circulate with angular momentum equal to integer multiple ofh, in the form of a quantized vortex [2].Vortices are excited states of motion and therefore energetically unstable towards relaxation into the motional ground state, where the condensate is at rest. However, quantization constrains the decay: a vortex in BoseEinstein condensates cannot simply fade away or disappear, it is only allowed to move out of the condensate or annihilate with another vortex of opposite circulation. Vortex decay and metastability, due to inhibition of decay, have been a central issue in the study of superfluidity [3,4,5,6,7,8]. In almost pure Bose-Einstein condensates, vortices with lifetimes up to tens of seconds have been observed [9,10,11].Giving a Bose-Einstein condensate angular momentum per particle larger thanh can result in one multiplyquantized vortex with large circulation or, alternatively, in many singly-quantized vortices each with angular momentumh. The kinetic energy of atoms circulating around the vortex is proportional to the square of the angular momentum; therefore the kinetic energy associated with the presence of a multiply-quantized vortex is larger than the kinetic energy of a collection of singlyquantized vortices carrying the same angular momentum. A multiply-quantized vortex can decay coherently by splitting into singly-quantized vortices and transferring the kinetic energy to coherent excitation modes, a phenomenon called dynamical instability which is driven by atomic interactions [5,12,13,14], and not caused by dissipation in an external bath. Observations of arrays of singly-quantized vortices in rapidly rotating condensates [10,11] indirectly suggests that the dynamical instability leads to fast decay of multiply-quantized vortices. However, the existence of stable multiply-quantized vortices in trapped Bose-Einstein condensates has been predicted with a localized pinning potential [12] or in a quartic potential [15]. Stable doubly-quantized vortices were observed in superconductors in presence of pinning forces [16] and in superfluid 3 He-A which has a multicomponent order parameter [17]. Recently, formation of a multiply-quantized vortex in a Bose-Einstein condensate has been demonstrated using topological phases [18,19], and surprisingly long lifetime of a "giant" vortex core has been report...

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We observed quantum reflection of ultracold atoms from the attractive potential of a solid surface. Extremely dilute Bose-Einstein condensates of 23Na, with peak density 10(11)-10(12) atoms/cm(3), confined in a weak gravitomagnetic trap were normally incident on a silicon surface. Reflection probabilities of up to 20% were observed for incident velocities of 1-8 mm/s. The velocity dependence agrees qualitatively with the prediction for quantum reflection from the attractive Casimir-Polder potential. Atoms confined in a harmonic trap divided in half by a solid surface exhibited extended lifetime due to quantum reflection from the surface, implying a reflection probability above 50%.

A degenerate Fermi gas is rapidly quenched into the regime of strong effective repulsion near a Feshbach resonance. The spin fluctuations are monitored using speckle imaging and, contrary to several theoretical predictions, the samples remain in the paramagnetic phase for an arbitrarily large scattering length. Over a wide range of interaction strengths a rapid decay into bound pairs is observed over times on the order of 10ℏ/E(F), preventing the study of equilibrium phases of strongly repulsive fermions. Our work suggests that a Fermi gas with strong short-range repulsive interactions does not undergo a ferromagnetic phase transition.

Questioning the presumably most basic assumptions about the structure of space and time has revolutionized our understanding of Nature. State-of-theart atomic clocks make it possible to precisely test fundamental symmetry properties of spacetime, and search for physics beyond the standard model at low energy scales of just a few electron volts. Here, we experimentally demonstrate for the first time agreement of two single-ion clocks at the 10 −18 level and directly confirm the validity of their uncertainty budgets over a halfyear long comparison period. The two clock ions are confined in separate ion traps with quantization axes aligned along nonparallel directions. Hypothetical Lorentz symmetry violations would lead to sidereal modulations of the frequency offset. From the absence of such modulations at the 10 −19 level we deduce stringent limits on Lorentz symmetry violation parameters for electrons in the range of 10 −21 , improving previous limits by two orders of magnitude. 1 arXiv:1809.10742v1 [physics.atom-ph] 27 Sep 2018 operator using the Wigner-Eckart theorem (cf. Ref. [15]) Jm J |T (2) 0 |Jm J = (−1) J−m J J J J −m J 0 m J J||T (2) ||J . Explicitly we find Jm J |p 2 − 3p 2 z |Jm J = −J (J + 1) + 3m 2 J

Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is most robust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of dissipation when the velocity of a moving one dimensional optical lattice was varied. The dependence of the critical velocity on lattice depth and on the inhomogeneous density profile was studied.PACS numbers: 03.75. Kk, 03.75.Lm, 03.75.Ss The recent realization of the BEC-BCS crossover in ultracold atomic gases [1] allows one to study how bosonic superfluidity transforms into fermionic superfluidity. The critical velocity for superfluid flow is determined by the low-lying excitations of the superfluid. For weakly bound fermions, the (Landau) critical velocity is proportional to the binding energy of the pairs, which increases monotonically along the crossover into the BEC regime. However, the speed of sound, which sets the critical velocity for phonon excitations, is almost constant in the BCS regime, but then decreases monotonically on the BEC side, since the strongly bound molecules are weakly interacting. At the BEC-BCS crossover, one expects a rather narrow transition from a region where excitation of sound limits superfluid flow, to a region where pair breaking dominates. In this transition region, the critical velocity is predicted to reach a maximum [2,3,4]. This makes the critical velocity one of the few quantities which show a pronounced peak across the BEC-BCS crossover in contrast to the chemical potential, the transition temperature [5], the speed of sound [6,7] and the frequencies of shape oscillations [8], which all vary monotonically.In this paper, we report the first study of critical velocities across the BEC-BCS crossover, where a Feshbach resonance allows the magnetic tuning of the atomic interactions, and find that superfluid flow is most robust near the resonance. Our observation of a pronounced maximum of the critical velocity is in agreement with the predicted crossover between the two different mechanisms for dissipation.Critical velocities have been determined before in atomic BECs perturbed by a stirring beam [9,10,11] as well as by a 1D moving optical lattice [12]. In both cases, the inhomogeneous density of the harmonically trapped sample had to be carefully accounted for in order to make quantitative comparisons to theory. Here * Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138 † Website: cua.mit.edu/ketterle group we mitigate this problem by probing only the central region of our sample with a tightly focused moving lattice formed from two intersecting laser beams. For decreasing lattice depths, the critical velocity increases and, at very small depths, approaches a value which is in agreement with theoretical predictions.In our experiments, we first create a superfluid of 6 Li pairs according to the procedure de...