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A New Magic Optical Trap is Constructed in WIPM and the Coherence Time of

Single Atom Qubits is Increased Over One Hundred Times

Very recently, a research group led by Prof. Mingsheng Zhan in Wuhan Institute of Physics and Mathematics (WIPM), CAS realized a new type of optical dipole trap (ODT) to manipulate the single qubits, that is the magic-intensity ODT. In this trap, the coherence time of single atom qubits is prolonged to 225 ms, more than 100 times longer than the previous results. This novel work has been published in Physical Review Letters (see Phys.Rev.Lett.117, 123201, 2016). 

A quantum computer or a quantum simulator is a scalable physical system with coherently controllable and well characterized qubits. As an important candidate for quantum information processing and quantum simulation, a microscopic array of single atoms confined in ODTs has attracted a great deal of interest in recent years. In such architectures, an important requirement is the ability to controllably transport a remote qubit, acting as a mobile qubit, into the interaction range with other register atoms for performing two-qubit gates, see Fig.1 for example. This transfer must be carried out without influencing other qubits of the large-scale quantum register. Typically, an atomic qubit is encoded into a superposition of two hyperfine Zeeman levels of the ground states of an alkali-metal atom. Generically different hyperfine states experience mismatched light shifts induced by the trapping laser field, leading to the so-called differential light shift (DLS). The DLS has been found to be the leading source of the coherence loss during the transfer process of mobile qubits. The dynamical decoupling methods are found to be efficient for qubits in static ODTs but inefficient for mobile qubits. This loss greatly prevents the application of single atom arrays in the quantum information processing.

To solve this problem,Dr. Xiaodong HE and student Mr. Jiaheng Yang apply a magic-intensity trapping technique which mitigates the detrimental effects of DLS. They measured the previously neglected hyperpolarizability for the first time, which makes the light shift dependence on the trapping laser intensity parabolic. Because of the parabolic dependence, at a certain “magic” intensity, the first order sensitivity to trapping light-intensity variations over ODT volume is eliminated. In this trap, the coherence time of qubits is substantially increased to 225 ms. Furthermore, they demonstrated that the coherence of a single mobile atomic qubit can be well preserved during a transfer process among different ODTs. The results pave the way for constructing scalable quantum-computing architectures with single atoms trapped in an array of magic ODTs.

This work was supported by the National Basic Research Program of China, the National Natural Science Foundation of China, and the Strategic Priority Research Program of the Chinese Academy of Sciences.

Fig.1 Transfer of mobile qubit in an optical ring lattice



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