Testing hidden-variable theories of quantum physics using trapped ions

A new important building block for a future quantum computer has been realised by physicists at the Institute for Experimental Physics in Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI): a gate acting on three quantum bits, the so-called Toffoli gate, as has been reported in Physical Review Letters.

Entanglement—once only a subject of disputes about the foundation of quantum mechanics—has today become an essential issue in the emerging field of quantum information processing, promising a number of applications, including secure communication, teleportation and powerful quantum computation. Therefore, a focus of current experimental work in the field of quantum information is the creation and manipulation of entangled quantum systems. Here, we present our results on entangling two qubits in an ion-trap quantum processor not through a direct interaction of the ion qubits but instead through the action of a protocol known as entanglement swapping.

Choosing the rules of quantum physics as the physical basis for constructing models of computation allows for solving certain computational problems more efficiently as in models based on classical physics. In the quantum circuit model, information is encoded in quantum bits and manipulated by applying appropriate quantum operations acting on the joint state space of the qubits. Similar to what is done in classical computation, these quantum operations can be decomposed into a sequence of gate operations, consisting of single-qubit operations and entangling operations acting on pairs of qubits.

We make use of a decoherence-free subspace with specifically designed entangled states to demonstrate precision spectroscopy of a pair of trapped Ca⁺ ions; we obtain the electric quadrupole moment, which is of use for frequency standard applications.