quasiparticle poisoning

Quasiparticle poisoning is one of the sneakier ways a superconducting qubit suddenly forgets what it was doing. In a superconductor, the electrons that carry the lossless current travel as bound pairs called Cooper pairs. Now and then a pair gets broken, leaving two lonely unpaired electrons. Each of these stray broken pieces is called a quasiparticle, and when one wanders across a Josephson junction it can dump or steal a chunk of the qubit's energy. The result is an abrupt, hard-to-predict error: a qubit that was holding a 1 flips to 0, in the middle of a calculation, for no reason the control software can see.

Where do the strays come from? Partly from heat, but mostly from stray energy leaking in. A single stray infrared photon, a flash of cosmic radiation, or radioactivity in the chip's own packaging carries far more than enough energy to snap a Cooper pair, and once a few pairs break they can cascade and break more. Because each event is random and rare, it shows up as occasional glitches rather than a steady hiss, which makes it both maddening to chase and dangerous for error-correcting codes that assume errors stay independent and small.

Engineers fight back on three fronts. They shield the chip from light and radiation with light-tight cans, filtered wiring, and sometimes radiopure materials. They use gap engineering, deliberately making one side of a junction a slightly stronger superconductor so quasiparticles roll downhill away from the sensitive spot. And they add quasiparticle traps, small patches of normal metal or lower-gap superconductor that act like drains, soaking up strays before they reach a junction. None of this is fully solved, and a single cosmic-ray hit can still knock out many qubits at once, which is an active worry for anyone scaling up.