Acta Physica Polonica A
Remarks on Entanglement, its Sources and Applications
Marek Żukowski
Abstract
R em ar k s on En tan gle m en t , its Sou r ces an d A p p lic ation s M .`uk ow sk i I n st yt ut F izy ki T eorety cznej i Astro Ùzyki , Uni wersytet G da¥ski W i ta Stw osza 57, 80-952 G da ¥sk, Po l and and Insti tut f ur Ex peri m enta l physi k, Uni versi t at Inns bruck, 6020 Innsbruck, Austri a W e shall discuss the cent ral r ole pla yed in the quantu m information theory by entan glement .A generali zed version of Bell ' s theorem w ill be presented , as well as metho ds in multiphoton interf erometry to obtain correlations violatin g the Bell inequali tie s.Finally , w e shall present some quantum inf ormationa l applicati on s of entanglement.PAC S numb ers: 03.65.U d, 03.67.{a, 42.50.A r 1.I n t r o d u ct io n Current adv ances i n the op erati ona l contro l of qua ntum pheno m ena enabl e us to use the para do xi cal aspects of the qua ntum wo rl d as sources of pra cti cal appl i cati ons.The mai n areas i n whi ch qua ntum pheno m ena can b e harnessed to p erform ta sks im p ossibl e for classical theo ry of i nf orm ati on are qua ntum crypto gra phy (a l ready a rel i abl e techno l ogy), qua ntum com muni cati on (a t the sta ge of bi rth, e.g. the tel eporta ti on exp eri ments) and qua ntum com puta ti on (stunni ng theo reti cal resul ts, brea ki ng the l i m its of the T uri ng -t yp e machines, but sti l l far away fro m op erati onal real i zati ons).The basic no ti on i n the qua ntum i nform a ti on theo ry i s tha t of the qua ntum bi t or the qubi t.Cl a ssical bi ts can ta ke one of the two p ossibl e values 0 or 1 ( \ on " or \ o˜" , etc.).The qubi ts (the simpl est quantum system s wi th pure sta tes descri bed by a two -dim ensional Hi l b ert space, e.g.spins 1/ 2), since they are qua ntum enti ti es, they can b e in an arbi tra ry sup erp ositi on of two ba sis states j 0 i and j 1 i : j q u bi t i = ˜j 0 i + Ù j 1 i :(1 ) (2 1)