I see at least three interpretations of an "Experience Machine":
(1) Simulated worlds in which people from our world can temporarily participate.
(2) Simulated worlds to which we can "upload" people from our world, i.e., they can enter them permanently.
(3) Simulated worlds that have their own agents with their own experiences.
Of course, we are already developing (1) in video games and virtual reality, so that is presumably not what you mean. (2) concerns mind uploading; it is controversial whether that can be done at all and what it would mean (if we uploaded your mind would it really be you or just a copy of you?), but in any case we are nowhere close to understanding how one would achieve this, both from computational and neuroscientific perspectives. (3) is related to the idea of the simulation hypothesis which states that we ourselves are in a simulation. There too there has been philosophical discussion of whether this would in fact be possible -- and if not then presumably there wouldn't be much point to doing these simulations ourselves, at least not from the perspective of generating experiences anything like ours -- and there too, in any case we are nowhere close to achieving such simulations technically, in part because we do not yet understand how the brain generates phenomenal experiences (or qualia). Also (3) would not benefit us directly, in the sense that the experiences would not be ours, though perhaps we might like to do it out of altruism towards simulated creatures.