A variational eigenvalue solver on a photonic quantum processor
A paper co-authored by several researchers including Dr Xiao-Qi Zhou and Professor Jeremy O'Brien from the Centre for Quantum Photonics has been published in Nature Communications this month.
The paper presents an alternative approach to finding eigenvalues that greatly reduces the requirements for coherent evolution and combines the quantum phase estimation algorithm method with a new approach to state preparation based on ansätze and classical optimization. They have implemented the algorithm by combining a highly reconfigurable photonic quantum processor with a conventional computer. Then they experimentally demonstrated the feasibility of this approach with an example from quantum chemistry—calculating the ground-state molecular energy for He–H. The proposed approach drastically reduced the coherence time requirements, enhancing the potential of quantum resources available today and in the near future.
The paper which was published on the 23rd July 2014 can be found ont he Nature Communications website.
(a) Experimentally computed energy <
› (coloured dots) as a function of the optimization step j. The colour represents the tangle (degree of entanglement) of the physical state, estimated directly from the state parameters 
. The red lines indicate the energy levels of (R). The optimization algorithm clearly converges to the ground state of the molecule, which has small but non-zero tangle. The crosses show the energy calculated at each experimental step, assuming an ideal quantum device.
(b) Overlap |‹ψj |ψG› between the experimentally computed state |ψj› at each optimization step j and the theoretical ground state of , |ψG›. Error bars are smaller than the date points