Session C2: DAMOP Thesis Prize

Ultrafast Control of Spin and Motion in Trapped Ions

Trapped atomic ions are a promising medium for quantum computing, due to their long coherence times and potential for scalability. Current methods of entangling ions rely on addressing individual modes of motion within the trap and applying qubit state dependent forces with external fields. This approach can limit the speed of entangling gates and make them vulnerable to decoherence due to coupling to unwanted modes or ion heating. This research is directed towards demonstrating novel entanglement schemes which are not limited by the trap frequency, and can be made almost arbitrarily fast. Towards this goal, I will present results from the first experiments using ultrafast laser pulses to control the internal and external states of a single trapped ion. I will begin with experiments in ultrafast spin control, showing how a single laser pulse can be used to completely control both spin degrees of freedom of the ion qubit in tens of picoseconds. Second, I will discuss experiments using pulses to rapidly entangle the spin with the motion, and how careful spectral redistribution allows a single pulse to execute a spin-dependent momentum kick. Finally, I will explain how these spin-dependent momentum kicks can be used in the future to create an ultrafast entangling gate, and will present experimentally realizable pulse sequences. Such a gate would create a maximally entangled state of two ions in a time faster than the period of motion in the trap.   

Quantum simulations with ultracold atoms: Beyond standard optical lattices

Many prominent problems of quantum many-body physics (such as high-Tc superconductivity or quark confinement) remain unsolved, because the exponential growth of Hilbert space prevents numerical treatment of more than a few particles. To solve such models, Feynman proposed thirty years ago to design quantum devices that are governed by the same equations as the original, abstract model. Ultracold atoms in optical lattices are -- thanks to their unprecedented cleanness and control -- ideal candidates for such ``quantum simulators,'' and experiments that exceed the capabilities of classical computers are already being performed. In this talk, I present various new avenues that become open by going beyond standard setups, e.g., via exotic geometries, higher orbitals, or spin-dependent lattices. In particular, I discuss the exciting possibilities given by a periodical lattice driving, which allows us to explore frustrated quantum magnetism and synthetic gauge fields. First experiments using this technique have already been performed, opening prospects for the realizion of topological phases, anomalous quantum-Hall states, or spin liquids, thus promising insight into some of the most important problems of condensed-matter and high energy physics.   

Strongly interacting photons in a quantum nonlinear medium

Photons are fast and robust carriers of information but their lack of mutual interactions hinders their use in quantum information protocols. Interactions can be mediated by nonlinear media, and optical nonlinearities at the single photon level are a long-standing goal of quantum optical science. By coherently coupling slowly propagating photons to Rydberg states in a dense cold atomic gas, we create a single-pass medium with large photon-photon interactions. We first demonstrate that combining electromagnetically induced transparency techniques with the Rydberg blockade effect leads to strong dissipative interactions between individual photons. As a result, the simultaneous propagation of photons is suppressed in an otherwise transparent medium, and coherent laser pulses are converted into single photons. We subsequently explore the regime of coherent interactions, where simultaneously propagating photons acquire a large conditional phase-shift and become entangled. In this regime, the photons behave as massive particles exerting an attractive force onto each other and their evolution is governed by the existence of a photonic bound-state. This work paves the way for cavity-free deterministic optical quantum gates and quantum many-body physics with light.   

Quantum Magnetism with Ultracold Fermions in an Optical Lattice

In my thesis, I present the observation of quantum magnetism in an ultracold fermionic quantum gas confined to a 3D optical lattice. Ultracold fermionic atoms in optical lattices have long been proposed as a general platform for studying various model systems in condensed matter physics, ranging from geometries that give rise to Dirac points, to magnetically ordered phases. Of particular interest are models for quantum magnetism, which originates from the exchange coupling between quantum-mechanical spins. Yet, reaching the low temperatures required for entering the quantum magnetism regime has proven to be challenging, and has hindered progress for systems based on ultracold fermions in optical lattices. We have addressed and overcome this challenge. We designed an original scheme that enabled us to locally redistribute entropy, such that a subset of lattice bonds reaches temperatures below the exchange energy. The key to this scheme has been a novel type of optical lattice with tunable geometry. Using this lattice, we successfully observed quantum magnetism emerging in the many-body state of a thermalized Fermi gas. Beyond that, the same lattice was the enabling tool for the realization of a tunable artificial graphene system, highlighting the versatility of our approach. This work was performed at ETH Zurich under the supervision of Prof. Tilman Esslinger.