Who are experts in technology who use their knowledge to break into computers and computer networks, either for profit or just motivated by the challenge?

Those experts are called White Hat Hackers

Explanation:

White hat hackers are those whose hacking activities are for a noble cause. They are also known as ethical hackers. Their primary motivation is usually not for personal or financial gain.

Also, they cannot be involved in cyber espionage, system hacking without the permission of the system owner.

They could be paid employees or contractors working for companies as security specialists that attempt to find security holes via hacking.