Quantum annealing and gate-based quantum computing are both methods of performing quantum computation, but they are based on different principles and have different strengths and weaknesses.
Quantum annealing is a method of solving optimization problems by encoding the problem into the energy levels of a physical system and then letting the system evolve towards the global minimum of the energy landscape. The most common physical implementation of quantum annealing is using superconducting circuits. The system is initialized in an initial state, typically the ground state of a trivial Hamiltonian. Then it evolves towards the ground state of the final Hamiltonian, which encodes the solution to the problem. Quantum annealing algorithms are based on the adiabatic theorem, which states that if the evolution is slow enough, the system will remain in the ground state throughout the process.
On the other hand, gate-based quantum computing is a method of performing quantum computation by applying a sequence of quantum gates to a set of qubits. The qubits are initialized in a known state, and the gates are used to manipulate the state of the qubits, encoding the problem and performing the computation. Gate-based quantum computing algorithms are based on the principles of quantum circuits and quantum algorithms.
Key differences between gate-based and annealing quantum computing
One key difference between the two methods is that quantum annealing is mainly used for optimization problems, while gate-based quantum computing can be used for a broader range of problems. Quantum annealing is traditionally used to solve optimization problems (although it can solve other problems), while gate-based quantum computing is more versatile.
Gate-based: A quantum circuit is constructed using a series of quantum gates, each of which acts on a small number of qubits. The gates are used to manipulate the quantum state of the qubits and perform quantum algorithms. Many different types of gates can be used to construct quantum circuits, and those who are familiar with traditional electronics will understand the similarities. For an introduction to quantum gates and how they work, see our intro guide into the quantum gates that everyone should know.
Another difference is that quantum annealing is considered to be relatively robust against certain types of errors, such as noise and decoherence. In contrast, gate-based quantum computing is more sensitive. Currently, we are in NISQ regime of quantum computing, which stands for: NISQ stands for Noisy Intermediate-Scale Quantum computing. Current quantum computers have to find ways in which they can recover from the vast number of errors that propagate as computations progress. Quantum computers use a variety of error correction techniques.
Commercialising Quantum Annealing Computing
Companies like D-Wave Systems, Cambridge Quantum Computing and 1QBit work on quantum annealing, while IBM, Google, Rigetti and IonQ work on gate-based quantum computing. Although D-wave, which was traditionally an annealing-only company, has relatively recently announced that it will move into gate-based quantum computing.