▁phase ▁sh if ters ) ▁Diam ond - based ▁quantum ▁computer ▁( qu bit ▁realized ▁by ▁the ▁electronic ▁or ▁nuclear ▁spin ▁of ▁nit ro gen - v ac ancy ▁cent ers ▁in ▁diam ond ) ▁B ose - E in stein ▁cond ens ate - based ▁quantum ▁computer ▁Trans istor - based ▁quantum ▁computer ▁– ▁string ▁quantum ▁computers ▁with ▁ent rain ment ▁of ▁positive ▁holes ▁using ▁an ▁electro static ▁trap ▁R are - ear th - met al - ion - d oped ▁in organ ic ▁cry stal ▁based ▁quantum ▁computers ▁( qu bit ▁realized ▁by ▁the ▁internal ▁electronic ▁state ▁of ▁dop ants ▁in ▁optical ▁fib ers ) ▁Met all ic - like ▁carbon ▁n anos pher es ▁based ▁quantum ▁computers ▁ ▁A ▁large ▁number ▁of ▁candidates ▁demonstr ates ▁that ▁quantum ▁computing , ▁despite ▁rapid ▁progress , ▁is ▁still ▁in ▁its ▁inf ancy . ▁ ▁Rel ation ▁to ▁comput ability ▁and ▁complexity ▁theory ▁ ▁Comput ability ▁theory ▁ ▁Any ▁computational ▁problem ▁sol v able ▁by ▁a ▁classical ▁computer ▁is ▁also ▁sol v able ▁by ▁a ▁quantum ▁computer . ▁Int uit ively , ▁this ▁is ▁because ▁it ▁is ▁believed ▁that ▁all ▁physical ▁phen omena , ▁including ▁the ▁operation ▁of ▁classical ▁computers , ▁can ▁be ▁described ▁using ▁quantum ▁mechan ics , ▁which ▁under lies ▁the ▁operation ▁of ▁quantum ▁computers . ▁▁ ▁Con vers ely , ▁any ▁problem ▁sol v able ▁by ▁a ▁quantum ▁computer ▁is ▁also ▁sol v able ▁by ▁a ▁classical ▁computer , ▁or ▁more ▁formally ▁any ▁quantum ▁computer ▁can ▁be ▁sim ulated ▁by