Temperature controlled Ru and RuO2 growth via O* radical-enhanced atomic layer deposition with Ru(EtCp)2.

This work demonstrates by in vacuo X-ray photoelectron spectroscopy and grazing-incidence X-ray diffraction that Ru(EtCp)2 and O* radical-enhanced atomic layer deposition, where EtCp means the ethylcyclopentadienyl group, provides the growth of either RuO2 or Ru thin films depending on the deposition temperature (Tdep), while different mechanisms are responsible for the growth of RuO2 and Ru. The thin films deposited at temperatures ranging from 200 to 260 °C consisted of polycrystalline rutile RuO2 phase revealing, according to atomic force microscopy and the four-point probe method, a low roughness (∼1.7 nm at 15 nm film thickness) and a resistivity of ≈83 µΩ cm. This low-temperature RuO2 growth was based on Ru(EtCp)2 adsorption, subsequent ligand removal, and Ru oxidation by active oxygen. The clear saturative behavior with regard to the precursor and reactant doses and each purge time, as well as the good step coverage of the film growth onto 3D structures, inherent to genuine surface-controlled atomic layer deposition, were confirmed for the lowest Tdep of 200 °C. However, at Tdep = 260 °C, a competition between film growth and etching was found, resulted in not-saturative growth. At higher deposition temperatures (300-340 °C), the growth of metallic Ru thin films with a resistivity down to ≈12 µΩ cm was demonstrated, where the film growth was proved to follow a combustion mechanism known for molecular oxygen-based Ru growth processes. However, this process lacked the truly saturative growth with regard to the precursor and reactant doses due to the etching predominance.

Higher-Order Quantum Ghost Imaging with Ultracold Atoms.

Ghost imaging is a quantum optics technique that uses correlations between two beams to reconstruct an image from photons that do not interact with the object being imaged. While pairwise (second-order) correlations are usually used to create the ghost image, higher-order correlations can be utilized to improve the performance. In this Letter, we demonstrate higher-order atomic ghost imaging, using entangled ultracold metastable helium atoms from an s-wave collision halo. We construct higher-order ghost images up to fifth order and show that using higher-order correlations can improve the visibility of the images without impacting the resolution. This is the first demonstration of higher-order ghost imaging with massive particles and the first higher-order ghost imaging protocol of any type using a quantum source.

Solving the Quantum Many-Body Problem via Correlations Measured with a Momentum Microscope.

In quantum many-body theory, all physical observables are described in terms of correlation functions between particle creation or annihilation operators. Measurement of such correlation functions can therefore be regarded as an operational solution to the quantum many-body problem. Here, we demonstrate this paradigm by measuring multiparticle momentum correlations up to third order between ultracold helium atoms in an s-wave scattering halo of colliding Bose-Einstein condensates, using a quantum many-body momentum microscope. Our measurements allow us to extract a key building block of all higher-order correlations in this system-the pairing field amplitude. In addition, we demonstrate a record violation of the classical Cauchy-Schwarz inequality for correlated atom pairs and triples. Measuring multiparticle momentum correlations could provide new insights into effects such as unconventional superconductivity and many-body localization.

Ghost imaging with atoms.

Ghost imaging is a counter-intuitive phenomenon-first realized in quantum optics-that enables the image of a two-dimensional object (mask) to be reconstructed using the spatio-temporal properties of a beam of particles with which it never interacts. Typically, two beams of correlated photons are used: one passes through the mask to a single-pixel (bucket) detector while the spatial profile of the other is measured by a high-resolution (multi-pixel) detector. The second beam never interacts with the mask. Neither detector can reconstruct the mask independently, but temporal cross-correlation between the two beams can be used to recover a 'ghost' image. Here we report the realization of ghost imaging using massive particles instead of photons. In our experiment, the two beams are formed by correlated pairs of ultracold, metastable helium atoms, which originate from s-wave scattering of two colliding Bose-Einstein condensates. We use higher-order Kapitza-Dirac scattering to generate a large number of correlated atom pairs, enabling the creation of a clear ghost image with submillimetre resolution. Future extensions of our technique could lead to the realization of ghost interference, and enable tests of Einstein-Podolsky-Rosen entanglement and Bell's inequalities with atoms.

Precision Measurement for Metastable Helium Atoms of the 413 nm Tune-Out Wavelength at Which the Atomic Polarizability Vanishes.

We present the first measurement for helium atoms of the tune-out wavelength at which the atomic polarizability vanishes. We utilize a novel, highly sensitive technique for precisely measuring the effect of variations in the trapping potential of confined metastable (2^{3}S_{1}) helium atoms illuminated by a perturbing laser light field. The measured tune-out wavelength of 413.0938(9_{stat})(20_{syst}) nm compares well with the value predicted by a theoretical calculation [413.02(9) nm] which is sensitive to finite nuclear mass, relativistic, and quantum electrodynamic effects. This provides motivation for more detailed theoretical investigations to test quantum electrodynamics.

Single-atom source in the picokelvin regime.

An important aspect of the rapidly growing field of quantum atom optics is exploring the behavior of ultracold atoms at a deeper level than the mean field approximation, where the quantum properties of individual atoms becomes important. Major recent advances have been achieved with the creation and detection of reliable single-atom sources, which is a crucial tool for testing fundamental quantum processes. Here, we create a source comprised of a single ultracold metastable helium atom, which enables novel free-space quantum atom optics experiments to be performed with single massive particles with large de Broglie wavelengths.