Coherent quantum transport in waveguide lattices

The coherent transport of quantum states between distant qubits is one of the key milestones towards the realisation of large-scale quantum computers. For static qubits, this state transfer is often envisioned to be carried out only by the internal dynamics of the system, which has the great advantage that detrimental influences of the environment are minimised. A chain of spin-1/2-qubits with ferromagnetic coupling has been suggested as an implementation of a coherent transport which is perfect in theory. In this scheme, a precise engineering of the coupling strengths between adjacent spins is crucial. To date, such a quantum state transfer has only been achieved for the modest system size of three qubits, employing nuclear magnetic resonance. However, this concept is by no means restricted to spin chains; it is therefore possible to resort to another physical platform, such as optics, to investigate the capabilities of the transferring Hamiltonian. Results show that 84% fidelity of transfer across the 19 waveguides used in the study was achieved. The errors in the presented scheme are mostly induced by imperfections in the fabrication and excitation, whereas full coherence is always maintained.These results demonstrate the feasibility of state transfer schemes on static qubits and highlight how these ideas can be implemented in the domain of optics.