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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Entangling two and more ions with collective laser-ion interactions

Christian F. Roos, Gerhard Kirchmair, F. Zähringer, Jan Benhelm, R. Gerritsma, R. Blatt

Published Jun 1, 2009• 0 citations
Publisher

Abstract

Laser-ion interactions that collectively couple the pseudo-spins of a string of trapped ions to one of the string's vibrational modes [1] have been used by us to entangle a pair of40Ca+ions with very high fidelity [2–4]. In addition, we experimentally confirmed that this gate mechanism allows for entangling ions without having recourse to ground state cooling [3]. This opens the way to performing an entangling gate after having applied an operation which does not preserve the ions' motional state like for example state detection.

DOI
10.1109/cleoe-eqec.2009.5192789

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Laser-ion interactions that collectively couple the pseudo-spins of a string of trapped ions to one of the string's vibrational modes [1] have been used by us to entangle a pair of40Ca+ions with very high fidelity [2–4]. In addition, we experimentally confirmed that this gate mechanism allows for entangling ions without having recourse to ground state cooling [3]. This opens the way to performing an entangling gate after having applied an operation which does not preserve the ions' motional state like for example state detection.
Coupling internal and vibrational states of a string of trapped ions has proven to be an effective way of entangling the ions' internal states. This mechanism can be used for high-fidelity quantum gates, QND measurements of spin correlations and creation of large entangled states. However, spin-motion interactions are also of interest for the purpose of quantum simulations where the motional state no longer acts as an auxiliary quantum system only. Here, we describe an experiment where a laser-cooled trapped ion is set to behave as a free relativistic quantum particle.
Ion trap quantum computers are based on modulating the Coulomb interaction between atomic ion qubits using external forces. However, the spectral crowding of collective motional modes could pose a challenge to the control of such interactions for large numbers of qubits. Here, we show that high-fidelity quantum gate operations are still possible with very large trapped ion crystals by using a small and fixed number of motional modes, simplifying the scaling of ion trap quantum computers. We present analytical work that shows that gate operations need not couple to the motion of distant ions, allowing parallel entangling gates with a crosstalk error that falls off as the inverse cube of the distance between the pairs. We also experimentally demonstrate high-fidelity entangling gates on a fully connected set of seventeen 171Yb+^{171}\mathrm{Yb}^{+}171Yb+ qubits using simple laser pulse shapes that primarily couple to just a few modes.