What are the main qubit technologies and their pros and cons?

A "qubit" is a concept, and there are several competing technologies being used to build them. Each has its own strengths and weaknesses, and a "winner" has not yet been decided.

The main technologies are:

Superconducting Qubits (Google, IBM):

Pros: They are extremely fast at processing. They are also built using silicon-chip manufacturing techniques, which our industry is already very good at, making them easier to scale.

Cons: They are very "noisy" and unstable. Qubits lose their quantum state (decohere) very quickly. They must be kept in massive refrigerators at temperatures near absolute zero (milli-Kelvin), which is expensive and complex.

Trapped-Ion Qubits (Quantinuum, IonQ):

Pros: These are the most stable qubits. They have very high-fidelity (low error rates) and can hold their quantum state for seconds or even minutes.

Cons: They are much slower at processing (gates) than superconducting qubits. While very stable, scaling them up (entangling hundreds of ions) is a significant engineering challenge.

Photonic Qubits (Xanadu, PsiQuantum):

Pros: These qubits are photons (particles of light). They are extremely fast (travel at light speed) and, most importantly, can often operate at room temperature, removing the need for extreme cooling.

Cons: It is very difficult to make two photons interact and entangle, which is essential for computation. They are also difficult to "hold still" to store information.