9 9 4 . ▁Paul ▁Dav ies ▁argued ▁that ▁a ▁ 4 0 0 - qu bit ▁computer ▁would ▁even ▁come ▁into ▁conflict ▁with ▁the ▁cosm ological ▁information ▁bound ▁imp lied ▁by ▁the ▁h olog raph ic ▁principle . ▁ ▁Ob st acles ▁ ▁There ▁are ▁a ▁number ▁of ▁technical ▁challeng es ▁in ▁building ▁a ▁large - scale ▁quantum ▁computer . ▁Phys ic ist ▁David ▁Di V inc en zo ▁has ▁listed ▁the ▁following ▁requirements ▁for ▁a ▁practical ▁quantum ▁computer : ▁ ▁S cal able ▁phys ically ▁to ▁increase ▁the ▁number ▁of ▁qu bits ▁ ▁Q ub its ▁that ▁can ▁be ▁initialized ▁to ▁arbitrary ▁values ▁ ▁Quant um ▁gates ▁that ▁are ▁faster ▁than ▁de co her ence ▁time ▁ ▁Universal ▁gate ▁set ▁ ▁Q ub its ▁that ▁can ▁be ▁read ▁easily ▁ ▁S our cing ▁parts ▁for ▁quantum ▁computers ▁is ▁also ▁very ▁difficult . ▁Many ▁quantum ▁computers , ▁like ▁those ▁constructed ▁by ▁Google ▁and ▁IBM , ▁need ▁Hel ium - 3 , ▁a ▁nuclear ▁research ▁by product , ▁and ▁special ▁super con duct ing ▁c ables ▁that ▁are ▁only ▁made ▁by ▁a ▁single ▁company ▁in ▁Japan . ▁ ▁Quant um ▁de co her ence ▁▁ ▁One ▁of ▁the ▁greatest ▁challeng es ▁involved ▁with ▁construct ing ▁quantum ▁computers ▁is ▁cont rolling ▁or ▁removing ▁quantum ▁de co her ence . ▁This ▁usually ▁means ▁isol ating ▁the ▁system ▁from ▁its ▁environment ▁as ▁interactions ▁with ▁the ▁external ▁world ▁cause ▁the ▁system ▁to ▁de co here . ▁However , ▁other ▁sources ▁of ▁de co her ence ▁also ▁exist