Quantum Annealing vs Gate-Based Computing
A gate-model QPU runs an ordered circuit of gates and then measures. A quantum annealer is different. It physically settles toward low-energy answers of a scoring function and returns samples. The two kinds of machine give qubits completely different jobs. Annealers also spend many physical qubits on each problem variable. So raw qubit counts across the two kinds cannot be ranked.
What are the two ways to program a quantum machine?
A gate-based quantum processor (also called a gate-model QPU) runs a circuit. A circuit is an ordered list of steps, called gates, done to qubits. At the end, a measurement reads each qubit as a 0 or a 1. Everything in the QPU137 Lab uses this model. It is like following a recipe, one step after another.
A quantum annealer is programmed in a totally different way. You never write a circuit. Instead you describe your problem as an energy model. This is a scoring rule that gives a number, called the energy, to every possible choice of the variables. It is set up so that better answers score lower. The machine then physically settles toward low-energy choices. It returns samples: candidate answers, drawn many times over.
These are two different kinds of machine. The word qubit does a different job in each. An annealer with 5,000 qubits and a gate-model QPU with 100 are not two points on one scale. The counts describe different things, so they cannot be ranked against each other. This lesson shows exactly why.
- What does an annealer try to make small? A worked energy model
- What does a gate-model run look like?
- Run it: an ordered two-gate circuitINTERACTIVE
- Change one thingINTERACTIVE
- Why are 5,000 annealer qubits not the same as 5,000 gate qubits?
- On real hardware
You’ve read the opening of chapter 7. Pro unlocks the other 6 sections — plus every chapter of every course, with circuits you can run right on the page. That’s $11.99 a month, about the price of a coffee, or $99.99 a year (save 30%). The first chapter of every course, and the whole math course, stay free.