Collective Oscillations of an Imbalanced Fermi Gas: Axial Compression Modes and Polaron Effective Mass

By S. Nascimbene, ..., & C. Salomon

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasi-particle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find $m^*/m=1.17(10)$, in agreement with the most recent theoretical predictions.

**Groupmeeting by Jason McKeever**

Entangled Mechanical Oscillators

By J.D. Jost, ..., & D. Wineland

Superposition and entanglement are hallmarks of quantum mechanics. One system ubiquitous to nature where entanglement has not previously been shown is distinct mechanical oscillators, such as springs or pendula. Here, deterministic entanglement of separated mechanical oscillators—consisting of the vibrational states of two pairs of atomic ions held in different locations—is demonstrated.

**Groupmeeting by Chris Paul**

Quantum Walk in Position Space with Single Optically Trapped Atoms

By Michal Karski, ..., & Dieter Meschede

The quantum walk is the quantum analog of the well-known random walk, which forms the basis for models and applications in many realms of science. Its properties are markedly different from the classical counterpart and might lead to extensive applications in quantum information science. In our experiment, we implemented a quantum walk on the line with single neutral atoms by deterministically delocalizing them over the sites of a one-dimensional spin-dependent optical lattice. With the use of site-resolved fluorescence imaging, the final wave function is characterized by local quantum state tomography, and its spatial coherence is demonstrated. Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata.

**Groupmeeting by Alma Bardon**

Attosecond Ionization and Tunneling Delay Time Measurements in Helium

By P. Eckle, ... , & U. Keller

It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 x 10¹⁴ to 3.5 x 10¹⁴ watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector..

**Groupmeeting by Rockson Chang**