Prof. Dr. Tommaso Calarco
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Research Highlights
 | Quantum Optimal ControlOptimal control theory (OCT) is a set of methods developed to design systems that can achieve a desired behavior with limited resources and the biggest possible probability of success. Its applications cover such diverse fields as aerodynamics and ultrafast laser-assisted chemical reactions.The basic idea of OCT is illustrated in the picture to the left: when you want to perform a non-trivial process, you can't get it right the first time - but retrying helps improve. Once you know your stuff, you may be able to do pretty amazing things, though. The first goal of our research is to extend the applicability of OCT to a broader range of quantum systems than already demonstrated. Several open questions concern the robustness of the approach in the presence of noise, dissipation and other imperfections. Thus a second goal is the development of techniques to deal with those issues that limit the applicability of the method in several of the physical scenarios currently considered as candidate implementations for quantum information processing (see below). |
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 | Implementations of Quantum Information ProcessingQuantum information processing (QIP) is a rapidly expanding field that builds on experimental capabilities to manipulate physical systems at the quantum level. It holds the promise of immense computing power and of absolutely secure communications. Building a working quantum computer requires ultra-precise control of quantum dynamics, with errors below the so-called fault-tolerance threshold (a few per mille at most). In a real-world environment, this amounts to overcoming enormous practical challenges, because laboratory systems only allow for imperfect implementation of theoretical schemes.Over the last few years, we have pioneered the application of quantum optimal control theory (see above) to quantum information processing, in particular to the implementation of scalable quantum gates with real physical systems. The figure of merit to be optimized in this case is the fidelity F, defined as the projection of the physical state obtained by actually manipulating the chosen system onto the logical state that the gate aims at obtaining. Several examples (reported in the works referenced above), from atoms in optical lattices and atom chips to trapped ions and superconducting charge qubits, have indicated systematic improvements in fidelity beyond the fault-tolerance threshold, taking into account experimentally available configurations and known sources of imperfection. |
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 | Atomic InteractionsIn this line of research, we investigate the interaction between ultracold ions and neutral atoms in traps. The guiding concept is to exploit the available control of the geometry and strength of trapping potentials and of external fields in order to generate interesting forms of interaction that lead to a time dependent tuning of quantum correlations in combined systems of ions and atoms.When a neutral atom and an ion are brought together, the ion charge induces in the atom an electric dipole moment, which attracts it with an internal-state-independent r-4 dependence at large distances. The short-range part of the interaction potential, conversely, presents an intricate dependence on the diatomic molecular state, exhibiting many features known from atomic collision physics, for instance Feshbach resonances. In confined geometries, the bigger interaction strength of the induced-dipole potential, compared to neutral collisions, gives rise to novel phenomena like trap-induced resonances. Understanding ion-atom interactions over a broad range of distances allows to achieve control over their quantum states and to engineer and investigate interesting quantum correlations in experimentally realizable systems involving from a few up to many such particles, for fundamental as well as for applied aims, e.g. in the context of quantum engineering. |