A single trapped ion, immersed in a cloud of ultracold atoms, could become the core of a new kind of quantum computer. This project tackles a fundamental problem in quantum computing: qubits are fragile. Ions make excellent qubits but are hard to control, while neutral atoms are easier to manipulate but harder to entangle. By combining both in a "hybrid" system, the researchers aim to get the best of each. They will study how atoms and ions collide at ultracold temperatures, and use a Bose-Einstein condensate to cool a single ion to its quantum ground state—a necessary step for reliable quantum operations. If successful, the work could lead to a scalable hybrid quantum processor: a single ion linked to an array of neutral atoms in an optical lattice. The team also plans to build a "single-atom transistor" that transfers a qubit’s quantum state into a current of neutral atoms, enabling non-destructive measurements of individual qubits. This is fundamental science. There is no immediate practical application, but past work on ultracold atoms and ions has already led to advances in atomic clocks and quantum sensors—technologies that quietly underpin GPS and medical imaging.
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We propose to investigate hybrid quantum systems composed of ultracold atoms and ions. The mutual interaction of the cold neutral atoms and the trapped ion offers a wealth of interesting new physical problems. They span from ultracold quantum chemistry over new concepts for quantum information processing to genuine quantum many-body physics. We plan to explore aspects of quantum chemistry with ultracold atoms and ions to obtain a full understanding of the interactions in this hybrid system. We will investigate the regime of low energy collisions and search for Feshbach resonances to tune the interaction strength between atoms and ions. Moreover, we will study collective effects in chemical reactions between a Bose-Einstein condensate and a single ion. Taking advantage of the extraordinary properties of the atom-ion mixture we will perform quantum information processing with hybrid systems. In particular, we plan to realize sympathetic ground state cooling of the ion with a Bose-Einstein condensate to prepare it for experiments in quantum information processing. When the ion is immersed into the ultracold neutral atom environment the nature of the decoherence will be tailored by tuning properties of the environment: A dissipative quantum phase transition is predicted when the ion is coupled to a one-dimensional Bose gas. Moreover, we plan to realize a scalable hybrid quantum processor composed of a single ion and an array of neutral atoms in an optical lattice. The third direction we will pursue is the study of impurity effects in quantum many-body physics. We plan to study transport through a single impurity or atomic quantum dot with the goal of realizing a single atom transistor. A single atom transistor transfers the quantum state of the impurity coherently to a macroscopic neutral atom current and can thus realize quantum non-demolition measurements of individual atomic qubits.
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