Dr Silvia Bergamini

Entanglement has always been regarded as a necessary resource for quantum information processing. The computational power given by entanglement is however unavoidably hindered by decoherence, which makes this asset extremely fragile. Quantum system in dissipative environments can only sustain entangled states for a short time: it has been recently demonstrated that even interactions with the environment that affect only mildly single qubits, have devastating effects on entanglement provoking its "sudden death". Sudden death is considered to be beyond repair, as no error-correction protocol could restore entanglement that abruptly estinguishes. In all the proposal and experimental demonstration of entanglement-based quantum logic operations, there is the inherent open challenge that decoherence needs to be limited and controlled. Whilst some entangled states (like W states) are more robust than others towards decoherence, for the true implementation of macroscopic quantum machines entanglement is perhaps not the ideal quantum property to exploit. Entangled states are a particular set of highly correlated quantum systems that belong to a more geneal class for which the quantum mutual information always supersedes purely classical correlations. This difference, called discord, is quantum in nature and always positive. Although discord includes entanglement, there is a subset of quantum states that have no entanglement but positive discord. These states are of particular interest as they have been proved to be a valuable resource for speed up of specific computational tasks and for being robust towards decoherence. Despite the renewed interest in discord and few theoretical propositions on how discord is an optimal resource for certain tasks, the experimental verification is certainly lagging behind. However photonic qubits have been employed to demonstrate that discord can efficiently solve computational tasks that are classically intractable even with zero entanglement. Furthermore for specific tasks separable states with discord have been proved to be more efficient than entangled ones. However the more general demonstration that discord can provide enhancement for any computation has not been provided yet. This makes discord a very controversial asset for quantum information processing, although it is widely recognised that if one day it could be made of use for quantum computation, the impact would be truly groundbreaking.

Ogden Science Officer

The aim of the project is the investigation of the interactions between cold Rydberg atoms in small atomic ensembles and the exploitation of such interactions to implement fast quantum logic gates. We will study dipole-dipole and van-der-Waals interactions between cold Rydberg atoms in disordered samples of atoms stored in optical dipole traps. Dipole blockade will be observed in the spectra of collective excitations of ensembles of Rydberg atoms arranged in spatially separated dipole traps. A theoretical model describing the spectroscopic properties of long-range interactions of cold Rydberg atoms will be developed. Subsequently, fast quantum phase gate with cold Rydberg atoms will be implemented, which is the next important step towards scalable quantum computing.

Phase III Quantum Computing Hub outline bid. Oxford University leading.

Develop modules for continued professional development for specialist teacher training (Physics). This work is in partnership with the IOP and DfE.