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arXiv:quant-ph/9806012 · Physical Review Letters

Deterministic Entanglement of Two Trapped Ions

Q. A. Turchette, C. S. Wood, B. E. King, Christopher J. Myatt, Dietrich G. Leibfried, Wayne M. Itano, Christopher R. Monroe, David J. Wineland

Published Oct 26, 1998• 638 citations
arXivHTMLPDF

Abstract

We have prepared the internal states of two trapped ions in both the Bell-like singlet and triplet entangled states. In contrast to all other experiments with entangled states of either massive particles or photons, we do this in a deterministic fashion, producing entangled states on demand without selection. The deterministic production of entangled states is a crucial prerequisite for large-scale quantum computation.

DOI
10.1103/physrevlett.81.3631

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We have prepared the internal states of two trapped ions in both the Bell-like singlet and triplet entangled states. In contrast to all other experiments with entangled states of either massive particles or photons, we do this in a deterministic fashion, producing entangled states on demand without selection. The deterministic production of entangled states is a crucial prerequisite for large-scale quantum computation.
Since the development of quantum mechanics almost a century ago, there has been considerable controversy over the interpretations and predictions of quantum theory. Owing to the counterintuitive predictions of quantum mechanics, Einstein himself even wondered if this theory should be considered complete. While these questions have troubled many physicists over the past century, the development of the new field of quantum information science and the applications that may result from large scale quantum systems have brought many of these fundamental questions of quantum mechanics to the mainstream of not only theoretical but also experimental physics. This thesis deals with a system at the heart of these questions - the first demonstration of quantum entanglement of two individual massive particles at a distance. I describe a theoretical and experimental framework for entanglement of two particles using trapped atomic ions. Trapped ions are among the most attractive systems for scalable quantum information protocols because they can be well isolated from the environment and manipulated easily with lasers. Using our trapped ion system, I show the first explicit demonstration of quantum entanglement between matter and light using a single ion and its single emitted photon. Further, by combining two such ion-photon entangled systems, I demonstrate the entanglement of two remotely located ions. These entanglement protocols, together with the recent developments of trapped ion quantum computing, can be used to create a platform for a scalable quantum information network, and perhaps confront some of the strangest features of quantum mechanics.
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Summary form only given. Entanglement is one of the most striking features of quantum mechanics and was a source of many debates among the early founders of this theory. More recently, entanglement was considered as one of the key points in the field of quantum information processing. Those potential applications require however the deterministic production of entanglement implying a large number of subsystems. Spontaneous phenomena, generally used to entangle 2 systems, are therefore not easily extendable towards those requirements. Recent works have succeeded in producing entangled state of three or more particles by using spontaneous parametric down-conversion or a unitary operation on a ensemble of trapped ions. We have created an entangled state between two atoms and a mode of the electromagnetic field stored in a cavity. The particles are separated by centimetric distances, which allows for individual addressing. The entangled state is prepared by unitary reversible operations acting on one or two of the particles at the same time. The analysis of the state is performed by detecting correlations between individual observables.
We review recent experiments on entanglement, Bell's inequality, and decoherence-free subspaces in a quantum register of trapped \be ions. We have demonstrated entanglement of up to four ions using the technique of Mølmer and Sørensen. This method produces the state |down down> + |up up> for two ions and the state |down down down down> + |up up up up> for four ions. We generate the entanglement deterministically in each shot of the experiment. Measurements on the two-ion entangled state violates Bell's inequality at the 8σ8σ8σ level. Because of the high detector efficiency of our apparatus, this experiment closes the detector loophole for Bell's inequality measurements for the first time. This measurement is also the first violation of Bell's inequality by massive particles that does not implicitly assume results from quantum mechanics. Finally, we have demonstrated reversible encoding of an arbitrary qubit, originally contained in one ion, into a decoherence-free subspace (DFS) of two ions. The DFS-encoded qubit resists applied collective dephasing noise and retains coherence under ambient conditions 3.6 times longer than does an unencoded qubit. The encoding method, which uses single-ion gates and the two-ion entangling gate, demonstrates all the elements required for two-qubit universal quantum logic.