A coherent quantum annealer with Rydberg atoms
There is a tremendous ongoing effort in realizing quantum computing with different physical platforms and different paradigms, and one of the most prominent is quantum annealing. It is a quantum computing paradigm with the aim to solve generic optimization problems, usually requiring to find the maximum or minimum of a complex cost function. Quantum annealing is currently being implemented by some of the largest players in the industry and it might show practical advantages, due to the global control on all qubits at the same time compared to the (in principle equivalent) standard gate-based model.
The main challenge in quantum annealing is to realize a fully programmable quantum device, which is capable of processing the stipulated coherent adiabatic quantum dynamics. The method requires individually programmable long–range interactions, which is in contradiction to the decaying nature of interactions in cold atoms and molecule setups.
In the paper “A coherent quantum annealer with Rydberg atoms”, dating back to 2017, the researchers Alexander W. Glaetzle, Rick van Bijnen, Peter Zoller and Wolfgang Lechner presented a novel setup aimed at overcoming this notorious problem. They demonstrated that combining the well-developed quantum simulation toolbox for Rydberg atoms with the ParityQC (LHZ) architecture makes it possible to build a prototype for a coherent adiabatic quantum computer with all-to-all Ising interactions.
The variables of an optimization problem are usually encoded as logical spins resulting in a spin glass Hamiltonian. The core idea of the LHZ architecture is now to introduce physical spins as relative alignment of the logical spins. If two logical spins are aligned in parallel, the corresponding physical spin is in state “+”, while if the logical spins are aligned anti-parallel, then the physical spin is in state “-”. The major advantage of this approach is that the interaction energy of a pair of logical spins can now be implemented with a local field acting on a single physical spin.
This paradigm shift allows the cost function of a general optimization problem to be encoded in the LHZ architecture in local fields. This spin model can be emulated in a natural way with Rubidium and Caesium atoms in a bipartite optical lattice involving laser-dressed Rydberg–Rydberg interactions. This platform makes the proposed quantum annealing protocols experimentally accessible by applying current state-of-the-art techniques.
The paper was published in April 2017 in Nature Communications.