Spatial entanglement patterns and Einstein-Podolsky-Rosen steering in a Bose-Einstein condensate

Many-particle entanglement is a fundamental concept of quantum physics that still presents conceptual challenges. While spin-squeezed and other nonclassical states of atomic ensembles were used to enhance measurement precision in quantum metrology, the notion of entanglement in these systems remained controversial because the correlations between the indistinguishable atoms were witnessed by collective measurements only. Here we use highresolution imaging to directly measure the spin correlations between spatially separated parts of a spin-squeezed Bose-Einstein condensate. We observe entanglement that is strong enough for Einstein-Podolsky-Rosen steering: we can predict measurement outcomes for non-commuting observables in one spatial region based on a corresponding measurement in another region with an inferred uncertainty product below the Heisenberg relation. This could be exploited for entanglement-enhanced imaging of electromagnetic field distributions and quantum information tasks beyond metrology

A*Magazine: *Art *Africa *Analysis

A* Magazine – A* standing for Art, Africa and Analysis – is the official publication of the ECAS (7th European Conference on African Studies) arts and culture programme. It takes the form of a once-off newspaper that, in addition to providing a programme overview, also features a broad spectrum of articles, analyses, features, interviews, letters and poems related to the many questions and issues raised during these events

Imaging of microwave fields using ultracold atoms

We report a technique that uses clouds of ultracold atoms as sensitive, tunable, and non-invasive probes for microwave field imaging with micrometer spatial resolution. The microwave magnetic field components drive Rabi oscillations on atomic hyperfine transitions whose frequency can be tuned with a static magnetic field. Readout is accomplished using state-selective absorption imaging. Quantitative data extraction is simple and it is possible to reconstruct the distribution of microwave magnetic field amplitudes and phases. While we demonstrate 2d imaging, an extension to 3d imaging is straightforward. We use the method to determine the microwave near-field distribution around a coplanar waveguide integrated on an atom chip.Comment: 11 pages, 4 figure

Coherent manipulation of ultracold atoms with microwave near-fields

The spectacular progress in the field of ultracold quantum gases is intimately connected with the availability of sophisticated techniques for quantum-level control of internal states, motional states, and collisional interactions. Atom chips provide such control in compact, robust, and scalable setups, which makes them attractive for both applications and fundamental studies. In this thesis I report on experiments that use a new method for coherent manipulation of ultracold atoms. The method is based on microwave near-fields, provided by a waveguide structure that is fully integrated on an atom chip. We generate microwave near-field potentials that combine the versatility of optical traps with the robustness and tailorability of static magnetic microtraps. These potentials depend on the internal atomic state, and we use them for state-selective splitting of Rb Bose-Einstein condensates. We show for the first time combined coherent manipulation of internal and motional states on an atom chip, realizing a trapped-atom interferometer with internal state labeling of the interferometer paths. Moreover, we use microwave near-field potentials for the preparation of spin-squeezed states for quantum-enhanced metrology through controlling state-dependent collisional interactions. In addition, very promising proposals exist for the implementation of a quantum phase gate using these potentials. Measuring microwave fields is important for engineering of microwave devices as well as in science, e.g. to characterize the field homogeneity in the interaction regions of an atomic clock. We develop a novel technique that uses clouds of uncondensed ultracold atoms as sensitive, tunable and non-invasive probes for microwave field imaging with micrometer spatial resolution. The microwave magnetic field components drive Rabi oscillations on atomic hyperfine transitions whose frequency can be tuned with a static magnetic field. Readout is accomplished using state-selective absorption imaging. Quantitative data extraction is simple and it is possible to reconstruct the amplitudes and phases of the different microwave magnetic field components. While we demonstrate 2D imaging, an extension to 3D imaging is straightforward. We use the method to determine the microwave near-field distribution around the on-chip waveguide and reconstruct the corresponding current distribution. For our experimental parameters, the method provides a microwave magnetic field sensitivity of 0.2 mG, which can even be improved further with variants discussed. The experiments presented in this thesis open the path for the realization of portable quantum-enhanced interferometer devices, the implementation of a quantum phase gate as well as for a new generation of microwave field sensors.Der spektakuläre Fortschritt auf dem Gebiet der ultrakalten Quantengase beruht auf der Kontrolle von internen- und Bewegungs-Quantenzuständen sowie Kollisions-Wechselwirkungen. Atomchips ermöglichen solche Kontrolle von Quantensystemen in kompakten, robusten und skalierbaren Aufbauten. In dieser Dissertation berichte ich über Experimente auf einem Atomchip mit integrierten Wellenleitern, deren Mikrowellen-Nahfelder als eine neue Methode zur kohärenten Manipulation von ultrakaltem Rubidium verwendet werden. Mikrowellen-Nahfeldpotentiale vereinen die Flexibilität und Vielseitigkeit von optischen Fallen mit der Robustheit und Konfigurierbarkeit von statischen Mikrofallen. Die Mikrowellenpotentiale hängen vom internen atomaren Hyperfeinzustand ab, was wir für die zustandsselektive Aufspaltung von Bose-Einstein Kondensaten verwenden. Wir demonstrieren erstmalig die kombinierte kohärente Manipulation von internen- und Bewegungs-Zuständen in einem Atominterferometer auf einem Atomchip, mit Kennzeichnung der Interferometerarme durch interne Hyperfeinzustände. Weiter verwenden wir die Nahfeld-Potentiale um via zustandsabhängiger Kollisions-Wechselwirkungen gequetschte Spin-Zustände für die Quanten-Metrologie herzustellen. Ausserdem existieren sehr vielversprechende Vorschläge, mittels dieser Potentiale ein Quanten-Phasengatter zu realisieren. Das Vermessen von Mikrowellen-Feldern ist bedeutsam für die Entwicklung von Mikrowellen-Komponenten sowie in der Wissenschaft, z.B. zur Vermessung der Feld-Homogenität in den Wechselwirkungs-Regionen einer Atomuhr. Wir haben eine Technik entwickelt, die Wolken von ultrakalten Atomen als empfindliche, abstimmbare, und nichtinvasive Sonden für das Abbilden von Mikrowellen-Feldverteilungen mit einer räumlichen Auflösung im Mikrometerbereich benutzt. Die Mikrowellenmagnetfeld-Komponenten treiben Rabi-Oszillationen zwischen atomaren Hyperfeinzuständen, deren Resonanzbedingung mittels eines statischen Magnetfelds abgestimmt werden kann. Das Auslesen geschieht mit zustandsselektiver Absorptionsabbildung. Eine quantitative Auswertung ist einfach und es ist möglich die Verteilung der verschiedenen Polarisationskomponenten sowie Phasen des Mikrowellenmagnetfelds zu rekonstruieren. Die Mikrowellen-Nahfeldverteilung um einen der Wellenleiter auf dem Atomchip wird vermessen und die damit korrespondierende Stromverteilung auf dem Wellenleiter wird rekonstruiert. Für unsere experimentellen Parameter können wir Amplituden des Mikrowellenmagnetfelds von bis zu 0.2 mG messen. Die vorgestellten Experimente sind Basis für die Realisierung transportabler, auf Verschränkung basierender Quanten-Interferometer, Quanten-Phasengatter und eines neuen Verfahrens zur Charakterisierung von Mikrowellen-Feldverteilungen

Simple microwave field imaging technique using hot atomic vapor cells

We demonstrate a simple technique for microwave field imaging using alkali atoms in a vapor cell. The microwave field to be measured drives Rabi oscillations on atomic hyperfine transitions, which are detected in a spatially resolved way using a laser beam and a camera. Our vapor cell geometry enables single-shot recording of two-dimensional microwave field images with 350 {\mu}m spatial resolution. Using microfabricated vapor cell arrays, a resolution of a few micrometers seems feasible. All vector components of the microwave magnetic field can be imaged. Our apparatus is simple and compact and does not require cryogenics or ultra-high vacuum

Atom chip based generation of entanglement for quantum metrology

Atom chips provide a versatile `quantum laboratory on a microchip' for experiments with ultracold atomic gases. They have been used in experiments on diverse topics such as low-dimensional quantum gases, cavity quantum electrodynamics, atom-surface interactions, and chip-based atomic clocks and interferometers. A severe limitation of atom chips, however, is that techniques to control atomic interactions and to generate entanglement have not been experimentally available so far. Such techniques enable chip-based studies of entangled many-body systems and are a key prerequisite for atom chip applications in quantum simulations, quantum information processing, and quantum metrology. Here we report experiments where we generate multi-particle entanglement on an atom chip by controlling elastic collisional interactions with a state-dependent potential. We employ this technique to generate spin-squeezed states of a two-component Bose-Einstein condensate and show that they are useful for quantum metrology. The observed 3.7 dB reduction in spin noise combined with the spin coherence imply four-partite entanglement between the condensate atoms and could be used to improve an interferometric measurement by 2.5 dB over the standard quantum limit. Our data show good agreement with a dynamical multi-mode simulation and allow us to reconstruct the Wigner function of the spin-squeezed condensate. The techniques demonstrated here could be directly applied in chip-based atomic clocks which are currently being set up

Inductively guided circuits for ultracold dressed atoms

Recent progress in optics, atomic physics and material science has paved the way to study quantum effects in ultracold atomic alkali gases confined to non-trivial geometries. Multiply connected traps for cold atoms can be prepared by combining inhomogeneous distributions of DC and radio-frequency electromagnetic fields with optical fields that require complex systems for frequency control and stabilization. Here we propose a flexible and robust scheme that creates closed quasi-one-dimensional guides for ultracold atoms through the ‘dressing’ of hyperfine sublevels of the atomic ground state, where the dressing field is spatially modulated by inductive effects over a micro-engineered conducting loop. Remarkably, for commonly used atomic species (for example, 7Li and 87Rb), the guide operation relies entirely on controlling static and low-frequency fields in the regimes of radio-frequency and microwave frequencies. This novel trapping scheme can be implemented with current technology for micro-fabrication and electronic control

A high-speed tunable beam splitter for feed-forward photonic quantum information processing

We realize quantum gates for path qubits with a high-speed, polarization-independent and tunable beam splitter. Two electro-optical modulators act in a Mach-Zehnder interferometer as high-speed phase shifters and rapidly tune its splitting ratio. We test its performance with heralded single photons, observing a polarization-independent interference contrast above 95%. The switching time is about 5.6 ns, and a maximal repetition rate is 2.5 MHz. We demonstrate tunable feed-forward operations of a single-qubit gate of path-encoded qubits and a two-qubit gate via measurement-induced interaction between two photons