Quantum Computation with Neutral Atoms

Tetsuya Mukai, Taro Eichler, Alexander Kasper, and Fujio Shimizu*
Physical Science Laboratory
*NTT Research Professor and University of Electro-Communications

 Neutral atoms are promising candidate to realize quantum gates. The advantage of using neutral atoms for quantum computation is as follows. 1) We can make arbitrary amplitude distribution among internal states with laser light. Therefore, single-qubit operations are easy and accurate. 2) Two qubits operations are possible with atom-atom interactions or indirectly via an auxiliary quantum system. 3) Disturbance from external fields to neutral atoms is relatively weak, and decoherence time is very long. 4) All atoms are identical and we can expand number of qubits without taking care of the characteristics of individual qubits. 5) We can fill qubit locations with few defects from Bose-Einstein Condensate (BEC). However, the progress of neutral-atom quantum computer is slow because implementation of all necessary functions in a system is not easy.
 Recent progress in laser cooling of atoms enables us to cool down atomic samples to a temperature below one micro-Kelvin and to make 2- or 3-dimensional array of neutral atoms. Employing high-quality micro-fabrication technology and our experiences in laser and efficient evaporative cooling for making BEC [1], we are trying to develop a model quantum computation system with ultra-cold neutral atoms.
 In the first project we plan to use 2-dimensional magnetic trap arrays. It consists of micro-fabricated Z-shaped wires on a silicon surface. To realize an array of single atoms in the ground vibrational state and to ensure atom-atom interaction we have to reduce the distance between atoms and surface. This short distance introduces extra potential noises and short decoherence time. We are trying to overcome this problem with super-conducting wires on silicon substrate (Atom Chips).
 In the second project we use an optical lattice. We have proposed a scheme to realize a scalable quantum computer with double optical lattices [2]. Employing this scheme we can compose an universal quantum computer with complete scalability and we hope to achieve a practical quantum computer with more than 1000 qubits operating together. We are trying to implement the double optical lattices scheme to a real system.

[1] T. Mukai and M. Yamashita, Physical Review A 70 (2004) 013615.
[2] F. Shimizu, Japanese Journal of Applied Physics 43 (2004) 8376.

Fig. 1. Sample Atom Chips
Fig. 2. Pattern of wire
Fig.3. Double Optical Lattice

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