I am reading a paper on Quantum Annealing... and the paper writes the following:
"one of the fundamental challenges in building a fully programmable quantum annealer is the competing requirements of full controllable all-to-all connectivity and the quasi locality of the interactions between physical qubits."
I am not sure how to understand this sentence. In particular,
- What is meant by the 'quasi-locality' of the interactions, and
"Quasi" means "almost" or "seemingly."
"Quasi-local interactions" means that the interactions are "pretty much" local. For example, nearest neighbor interactions.
As another example, there could be next-nearest-neighbor interactions that have much smaller couplings than nearest-neighbor interactions and so these next-nearest-neighbor interactions could "effectively" be ignored.
- Why are these requirements competing?
As an example, suppose you have a planar square-lattice arrangement of atoms/ions (your qubits). It will likely be easier to get the qubits to interact with their nearest neighbors, e.g., the four atoms right next to them, than with the eight next-nearest-neighbor atoms (and so on). But in order to have some dynamics that allows all the atoms (all the qubits) to couple to one another, we would seemingly have to have them all interacting in some way.
Generally, the desire for good isolation is competing with the desire for lots of interaction, and particularly with long-range interactions. Since, if the qubits have long range interactions with each other, why wouldn't they have long range interactions with their environment? (And thereby be more susceptible to undesirable consequences like decoherence).