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Entanglement for quantum information with ion strings

Final Report Summary - ENTANGLEMENT 40CAQIP (Entanglement for quantum information with ion strings)

The research in the Marie Curie project ENTANGLEMENT 40CaQIP involved fundamental investigations on quantum entanglement, novel quantum computing and simulation approaches as well as algorithms. All objectives were accomplished and the results were published in high impact journals (indicated in parenthesis below). Specifically, using trapped ions we
1 realized the largest entangled state to date, a 14-qubit entangled state (Phys. Rev. Lett.)
2 explored the dynamics of multiparticle entanglement induced by decoherence (Nature Physics)
3 demonstrated the elements of an open-system simulator (Nature)
4 implemented a repetitive quantum error correction algorithm (Science)
5 realized a universal digital quantum simulator (Science)
6 characterized quantum processes using a technique known as "Direct Characterization of Quantum Dynamics" (in preparation).

"14-qubit entanglement: creation and coherence"
We created Greenberger-Horne-Zeilinger states with up to 14 qubits. The creation of such large-scale multiparticle entangled quantum states and the investigation of their decay towards classicality may provide a better understanding of the quantum to classical transition. We investigated the decay of coherence of up to 8 ions over time and observed a decay which agreed with our proposed theoretical model. The majority of current experimental systems developed towards quantum computation and metrology are affected by this noise and they will benefit from our study.

"Experimental multiparticle entanglement dynamics induced by decoherence"
Multiparticle entanglement can give rise to striking contradiction with local realism, inequivalent classes of entanglement and applications such as one-way or topological quantum computing. Beyond the coherence decay mentioned above, when exposed to decohering or dissipative environments, multiparticle entanglement yields to subtle dynamical features and access to new classes of states and applications. We experimentally characterized the dynamics of entanglement under the influence of decoherence and observed a rich dynamics crossing distinctive domains. Further work done under this project did benefit from the environment engineering techniques demonstrated in this work.

"An open-system quantum simulator with trapped ions"
The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. We looked into the largely unexplored territory of engineering the dynamics of many particles by a controlled coupling to an environment (open quantum system). We combined multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. Our work opens the door to open-system quantum simulation and computation.

"Experimental repetitive quantum error correction"
Errors arising during a computation must be repetitively corrected to unleash the computational power of quantum processors. We implemented multiple quantum error correction cycles for phase-flip errors on our ion-trap based quantum computing architecture. A quantum-feedback algorithm used in combination with a reset technique for auxiliary qubits enabled the correction of error in up to three consecutive correction cycles. We also explored the algorithm behavior when exposed to different noise environments.

"Universal digital quantum simulation with trapped ions"
We demonstrated a digital quantum simulator, such a device can be programmed to efficiently simulate any other local system. A full-time dynamics digital simulation of a range of spin systems required sequences of up to 100 gates and 6 qubits. Remarkably, we reproduce with high accuracy the dynamics of interactions beyond those naturally present in our simulator. The key principles of digital quantum simulation here demonstrated show that the level of control for a full-scale simulator is within reach.