A fully funded 3.5 year PhD position is available in the Ion Trap Cavity-QED and Molecular Physics (ITCM) Group in the Department of Physics & Astronomy at the University of Sussex.

Entanglement is at the heart of quantum information processing. Up to 14 qubits have been entangled in ion strings [1] using collective phonon excitations. An alternative to the vibrational coupling of ions is provided by photons. By analogy with phonon-based gates, the internal states of ions can be entangled with the states of photons, which in turn can establish a long-range interaction between ions in the string. To enhance the coupling between ions and single photons, a high-finesse optical cavity is required [2]. In this setting, gates between arbitrary ions coupled to the cavity mode can be realized [3]. The goal of the project is the first implementation of a photon-based gate for ion qubits. In the Ion-Trap Cavity-QED group, we have set up a miniature trap-cavity system optimized for strong ion-photon coupling. A linear ion trap is combined with an optical cavity whose axis is orthogonal to that of the trap (Fig. 1). In this way, any ions in a string interacting with the cavity field can be coupled (see Fig. 2). Which ions participate in a quantum gate is controlled by addressing them with a laser injected from the top. The simplest gate to realize entangles two ions in the cavity through the partial exchange of a single photon. Initially, one ion is prepared in the ground state, the other in a metastable state. As soon as coupling to the cavity is established, the ions perform a cavity-assisted Rabi-oscillation via the Raman transition connecting the two states. Stopping the Rabi-oscillation after a p/2-pulse, each ion is in a superposition of the two basis states such that the state of the system is entangled. The entanglement must be verified and quantified. Entangled states respond to qubit-rotations in a characteristic way, so that the amount of entanglement can be estimated. More complex gates are possible by making use of the Zeeman substructure of calcium ions.

[1] T.Monz et al., Ph.Rev.Lett. 106, 130506 (11).

[3] M. Feng, Phys. Rev. A 66, 054303 (2002).

[2] M. Keller et al., Nature 431, 1075 (2004).