Quantum (plural: quanta, often abbreviated to Q, so Quantum 6 for example would be commonly referred to as Q6) is a term used in the GURPS Infinite Worlds setting for a group of alternate earths grouped together. See Worlds of Infinite Worlds for a list of known alternate earths per quantum.
Its relatively easy to travel from a world within a quantum to another world on the same quantum, it gets progressively harder to travel to worlds on other quantums. Homeline and Centrum, which are the most advanced worlds with the technology needed to travel to other worlds (parachronics), both have technology that allow them to travel no further then two quanta away from their homes (for example, Homeline which is on Q5 can travel only to Q3 to Q7). For a graphical map of quantum levels, see TT87.
Together, Homeline and Centrum know of 7 Quantums (3 to 10). A single world on Quantum 1 has been described in official publications (Amalle in this issue of Pyramid). No worlds on Q2, below Q1 (if it exists...) or above Q10 has been described.