Long lived entanglement
We have created maximally entangled states of two calcium-40 ions and measured the lifetime of these states. For these experiments we manipulate the atoms on their S1/2 to D5/2 transition by a series of laser pulses that entangle the first ion with the motional state of the ion crystal and subsequently transfer the entanglement from the ion motion to the second ion. In this way, we are able to create the Bell states y± = |S>|D>± |D>|S> and f± = |S>|S>±|D>|D>. Here, |S> represents the m=-1/2 sublevel of the S1/2, |D> stands for the m=-1/2 level of the metastable D5/2 level. The f± states decohere quickly since fluctuations of the magnetic field lead to random shifts of the relative phase of the superposition. In contrast, the y± states are immune against decoherence that acts collectively on both atoms. Therefore, these states can have extremely long lifetimes, exceeding the time needed for creating the entanglement by more than a factor of 1000. In fact, the entanglement eventually due to the finite lifetime of the metastable D5/2 level (t=1.16 s).
For details of the experiment, see the slide-presentation in English, pdf. Download
"Bell States of Atoms with Ultralong Lifetimes and Their Tomographic State Analysis", C. F. Roos, G. P. T. Lancaster, M. Riebe, H. Häffner, W. Hänsel, S. Gulde, C. Becher, J. Eschner, F. Schmidt-Kaler, R. Blatt, Phys. Rev. Lett. 92, 220402 (2004), pdf
We are financially supported by Universität Innsbruck, the Austrian research fund (FWF) within the program "Control and Measurement of Coherent Quantum Systems", the European networks QGATES and CONQUEST, IQI, ARO and the Austrian academy of sciences.