Towards fault-tolerant quantum computing with trapped ions

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.

Precision spectroscopy with entangled states

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.

Scalable multiparticle entanglement of trapped ions

We report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography, using individual control and detection of the ions.

Deterministic quantum teleportation with atoms

For the first time, our team at the Institute for Experimentalphysik at Innsbruck University in collaboration with Daniel James from Los Alamos Laboratory in the USA succeeded at teleporting the quantum state of a trapped calcium ion to another calcium ion. This is the first time teleportation has been achieved with atomic particles, as opposed to beams of light, in an entirely deliberate, controllable manner.

Long lived entanglement

Arbitrary atomic Bell states with two trapped ions are generated in a deterministic and preprogrammed way. The resulting entanglement is quantitatively analyzed using various measures of entanglement. For this, we reconstruct the density matrix using single qubit rotations and subsequent measurements with near-unity detection efficiency. This procedure represents the basic building block for future process tomography of quantum computations.