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Quantum Electronics and Laser Science Conference

Observation of long decoherence times in a quantum logic gate using trapped Ba137 ions

Ralph G. DeVoe, Christian Kurtsiefer

Published Nov 13, 2002• 1 citation
Publisher

Abstract

Summary form only given. Trapped ion quantum logic gates following the model of Cirac and Zoller have been constructed by the NIST and Innsbrueck groups using /sup 9/Be/sup +/ and /sup 40/Ca/sup +/ ions respectively. We are operating a new quantum logic gate using the /sup 137/Ba/sup +/ ion confined in a microscopic trap of 80 microns radius. Raman cooling to the ground state, with N < 1, is shown by the asymmetric sidebands shown. We have measured the decoherence time of a such a cooled phonon mode to be greater than 1 millisecond, which indicates that the anomalously fast decays seen by the NIST group is not dominant here.

DOI
10.1109/qels.2001.962184

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Summary form only given. Trapped ion quantum logic gates following the model of Cirac and Zoller have been constructed by the NIST and Innsbrueck groups using /sup 9/Be/sup +/ and /sup 40/Ca/sup +/ ions respectively. We are operating a new quantum logic gate using the /sup 137/Ba/sup +/ ion confined in a microscopic trap of 80 microns radius. Raman cooling to the ground state, with N < 1, is shown by the asymmetric sidebands shown. We have measured the decoherence time of a such a cooled phonon mode to be greater than 1 millisecond, which indicates that the anomalously fast decays seen by the NIST group is not dominant here.
We report on experiments with strings of laser-cooled Ca+ ions for quantum information processing. An experiment using the isotope 43Ca+ was set up from scratch. We devised techniques for loading and laser-cooling 43Ca+ ion strings, initializing the ions in the hyperfine clock states and reading out the quantum state with high efficiency. Coherence times of the qubits are about 1 s. The single-qubit gates were implemented using microwave and Raman excitation, and coupling to the motional state was demonstrated. Very recently, a pair of 43Ca+ ions were entangled for the first time. In a second line of experiments, we demonstrated simple quantum algorithms with few 40Ca+ ions as well as the implementation of a high-fidelity gate operation entangling a pair of ions with a fidelity of 99.3(1)%. Among the milestone experiments are the demonstration and quantum process tomography of a Cirac-Zoller gate with mean fidelity of about 93%, the demonstration of entanglement in a decoherence-free sub space lasting for 20s, the demonstration of deterministic quantum teleportation, a Toffoli gate, deterministic entanglement swapping a partial measurement of a GHZ state preserving entanglement in the remaining qubits, creation and analysis of an 8-ion W-state, the application of entangled ions for precision spectroscopy.
The intrinsic decoherence from vibrational coupling of the ions in the Cirac-Zoller quantum computer [Phys. Rev. Lett. 74, 4091 (1995)] is considered. Starting from a state in which the vibrational modes are at a temperature TTT, and each ion is in a superposition of an excited and a ground state, an adiabatic approximation is used to find the inclusive probability P(t)P(t)P(t) for the ions to evolve as they would without the vibrations, and for the vibrational modes to evolve into any final state. An analytic form is found for P(t)P(t)P(t) at T=0T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0T=0, and the decoherence time is found for all TTT. The decoherence is found to be quite small, even for 1000 ions.