A collisional quantum phase gate on an atom chip

Microwave near-fields are a key ingredient for quantum information processing with single atoms on atom chips. A long-term goal of our experiment is to realize a quantum gate with the following features: The qubit is encoded in two hyperfine states of ⁸⁷Rb which are both magnetically trappable and allow for very long coherence lifetimes (see our experiments on coherence near chip surfaces). Microwave near-fields guided on the atom chip are used to drive single qubit rotations and provide state-selectivity to the magnetic trapping potential. The quantum phase gate is implemented by state-selective collisions of two qubit atoms in this potential. Optimal control of the trapping potential is used to improve gate performance in realistic potentials.

In a detailed theoretical investigation (P. Treutlein et al., Phys. Rev. A 74, 022311 (2006), see our list of publications), we have shown that for a realistic atom chip geometry the gate operation time is 1 ms, three orders of magnitude shorter than the experimentally demonstrated coherence lifetime of the qubit. Taking a large number of error sources into account, such as loss and decoherence effects due to the proximity of the surface, we find an overall infidelity of the order of a few 10^-3, compatible with requirements for fault-tolerant quantum computation.

Experimentally, this gate can be realized by combining the state selective microwave potentials demonstrated in our current experiment with the on-chip fiber cavities for single atom detection and preparation developed by our collaborators from Prof. J. Reichel's group at ENS Paris.