# Quantum information processing with ultracold atoms

The quantum world is difficult to be described with a classical theory. Scientists, a representative of whom is Richard Feynman [1,2], suggest to build a quantum system with similar structure to simulate the one whose mechanical or dynamical properties are yet unknown. Though the two systems, the artificial one and the real one are not exactly the same, most aspects of the real one could be revealed from studying the artificial one. This is the basic idea for quantum simulation, a way of solving a quantum problem in its own nature, the “QUANTUM” routine.

Quantum entanglement is the essential resource for various algorithms on quantum information processing. Typical entangled systems include photonic entanglement of 8 photons[3,4] and a large entangled array of 14 ions [5]. One may ask, what is the limit of the number of entangled particles? As it is known to us, multi-photon entanglement is normally obtained per connecting photon pairs. Well, photon pairs are generated through parametric down conversion and this process is probabilistic, say with a probability of around 1%. Therefore, it is extremely hard to scale up photonic entanglement of more than ten photons since one needs 5 pairs of photons and the probability of success is now 0.01^{5}=10^{-10}. Though in principle entangled system of ions is scalable, the largest entangled state is up to 14 ions while the researchers are now struggling with the decoherece induced by the noisy environment.

A promising system, optical lattices [6], might have bright future for creating and manipulating multipartite entanglement of neutral atoms. This assumption is based on the experiments that a huge amount of atoms can be coherently controlled in definite quantum states. One can prepare massive qubits deterministically via a phase transition from superfluid to Mott insulator (macroscopically).With the state-of-the-art technique of manipulating single atomic spins [7,8], one may implement scalable entangled state, the essential resource for quantum computation.

- R. Feynman, Simulating physics with computers, http://link.springer.com/article/10.1007%2FBF02650179.
- S. Lloyd,
*Universal Quantum Simulators*, Science, http://www.sciencemag.org/content/273/5278/1073.abstract. - Yao et al,
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*Experimental generation of an eight-photon Greenberger-Horne-Zeilinger state*, Nature Communications 2011, http://www.nature.com/ncomms/journal/v2/n11/pdf/ncomms1556.pdf?WT.ec_id=NCOMMS-20111122. - T. Monz et al,
*14-Qubit Entanglement: Creation and Coherence*, Phys. Rev. Lett. 2011, http://prl.aps.org/abstract/PRL/v106/i13/e130506. - I. Bloch et al,
*Many-body physics with ultracold gases*, Rev. Mod. Phys. 80, 885(2008) - W. S. Bakr et al,
*A Quantum Gas Microscope for detecting single atoms in a Hubbard regime optical lattice*Nature 462, 74-77 (2009). - J. F. Sherson et al,
*Single-atom-resolved fluorescence imaging of an atomic Mott insulator*, Nature 467, 68 (2010).