A qubit is any addressable 2-level quantum system. Think spins, oscillation modes, energy levels, polarization, etc. All of these have been implemented and demonstrated to varying degrees, with superconductors and trapped ions (this article) being the farthest along.
Noise is indeed a problem, and right now these systems can’t execute more than a few hundred operations at most before decohering to the point of randomness. There is an error rate threshold (~1e-3 - 1e-4) at which error correction becomes possible, and the field has been hovering around that threshold for the past 3 years or so. I think we’ll know within the next 5 years whether or not it’s possible to engineer these error correction schemes in a scalable way.
Noise is indeed a problem, and right now these systems can’t execute more than a few hundred operations at most before decohering to the point of randomness. There is an error rate threshold (~1e-3 - 1e-4) at which error correction becomes possible, and the field has been hovering around that threshold for the past 3 years or so. I think we’ll know within the next 5 years whether or not it’s possible to engineer these error correction schemes in a scalable way.