The PS3 version of Quantum Conundrum will be made available to download at no extra cost to US PlayStation Plus subscribers this week. The game will be added to the list of 'Instant Game Collection' titles as part of tomorrow's US PS Store update.Quantum Conundrum is a first-person physics based puzzle game, designed by one of the original Portal developers, Kim Swift. The game scored a solid 8/10 in CVG's review, which called it "an intelligent, original, and charming puzzler that's held back by awkward platforming and erratic physics." Also for US PS Plus members this week will be a 30 per cent discount on point-and-click adventure Machinarium, which will bring its price down from $9.99 to $6.99.