A d ive rse class o f p h ysica l syste m sÑ in clu d in g su p e rco n d u cto rs, su p e rflu id helium, lasers a n d q u a sio n e d im e n sio n a l ... in teg ral of th e velocity aro u n d th e ring) is q u an tized : C = ¥d nh_ m. H ere n is an in teg er an d m ., th e m ass of a h eliu m ato m .