Coherence Properties and Quantum State Transportation in an Optical Conveyor Belt

Abstract

We have prepared and detected quantum coherences with long dephasing times at the level of single trapped cesium atoms. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The limiting dephasing effects are experimentally identified and are of technical rather than fundamental nature. We present an analytical model of the reversible and irreversible dephasing mechanisms. Coherent quantum bit operations along with quantum state transport open the route towards a "quantum shift register" of individual neutral atoms.PACS numbers: 32.80. Lg, 32.80.Pj, 42.50.Vk Obtaining full control of all internal and external degrees of freedom of individual microscopic particles is the goal of intense experimental efforts. The long lived internal states of ions and neutral atoms are excellent candidates for quantum bits (qubits), in which information is stored in a coherent superposition of two quantum states. Once sufficient control of individual particles is established, one of the most attractive perspectives is the engineered construction of quantum systems of two or more particles. Their coherent interaction is a key element for the realization of quantum gates and can be implemented by transporting selected qubits into an interaction zone [1]. In this context, ions have successfully been transported between distinct locations while maintaining internal-state coherence [2].In this letter we report on the coherence properties and on a quantum state transportation of neutral atoms in a standing wave dipole trap. We present a scheme of preparation and detection of the electronic hyperfine ground states, which has been applied to ensembles as well as to single atoms. Along with the demonstration of 1-qubit-rotations, our system is a promising candidate for storing quantum information, since the hyperfine ground states exhibit long (∼200 ms) coherence times. In particular, the coherence even persists while transporting the atoms over macroscopic distances. This opens a route towards quantum gates via cavity-mediated atom-atom coupling.We trap cesium atoms in a standing wave dipole trap (λ = 1064 nm) with a potential depth of U 0 = 1 mK, loaded from a high-gradient magneto-optical trap (MOT) [3,4]. The single-atom transfer efficiency between the two traps is better than 95% [5]. The MOT is also used to determine the exact number of trapped atoms by observing their fluorescence. By shifting the standing wave pattern along the direction of beam propagation, we can transport the atoms over millimeter-scale distances. This is realized by mutually detuning the frequencies of the dipole trap laser beams with acoustooptical modulators. Additionally, we use microwave radiation at ω hfs /2π = 9.2 GHz to coherently drive the | F = 4, m F = 0 → | F = 3, m F = 0 clock transition of the 6 2 S 1/2 ground state with Rabi frequencies of Ω/2π = 10 kHz.The initial state is prepared by optically pumping the atom into | F = 4, m F = 0 . For this purpo...

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