▁that ▁further ▁adv ances ▁in ▁physics ▁could ▁lead ▁to ▁even ▁faster ▁computers . ▁For ▁instance , ▁it ▁has ▁been ▁shown ▁that ▁a ▁non - local ▁hidden ▁variable ▁quantum ▁computer ▁based ▁on ▁Boh m ian ▁Mechan ics ▁could ▁implement ▁a ▁search ▁of ▁an ▁N - item ▁database ▁in ▁at ▁most ▁ ▁steps , ▁a ▁slight ▁speed up ▁over ▁Gro ver ' s ▁algorithm , ▁which ▁runs ▁in ▁ ▁steps . ▁Note , ▁however , ▁that ▁neither ▁search ▁method ▁would ▁allow ▁quantum ▁computers ▁to ▁solve ▁N P - complete ▁problems ▁in ▁polynomial ▁time . ▁The ories ▁of ▁quantum ▁gravity , ▁such ▁as ▁M - theory ▁and ▁loop ▁quantum ▁gravity , ▁may ▁allow ▁even ▁faster ▁computers ▁to ▁be ▁built . ▁However , ▁defining ▁computation ▁in ▁these ▁theories ▁is ▁an ▁open ▁problem ▁due ▁to ▁the ▁problem ▁of ▁time ; ▁that ▁is , ▁within ▁these ▁physical ▁theories ▁there ▁is ▁currently ▁no ▁obvious ▁way ▁to ▁describe ▁what ▁it ▁means ▁for ▁an ▁observer ▁to ▁submit ▁input ▁to ▁a ▁computer ▁at ▁one ▁point ▁in ▁time ▁and ▁then ▁receive ▁output ▁at ▁a ▁later ▁point ▁in ▁time . ▁ ▁See ▁also ▁ ▁References ▁ ▁Further ▁reading ▁ ▁Text books ▁ ▁Academ ic ▁papers ▁▁ ▁Table ▁ 1 ▁lists ▁switching ▁and ▁de ph asing ▁times ▁for ▁various ▁systems . ▁ ▁External ▁links ▁▁▁ ▁Stan ford ▁Encyclopedia ▁of ▁Philosoph y : ▁" Quant um ▁Comput ing " ▁by ▁Am it ▁H agar ▁and ▁Michael ▁E . ▁C uff aro . ▁▁▁ ▁Quant um ▁computing ▁for ▁the ▁very ▁curious ▁by ▁Andy ▁M atus ch ak ▁and