- bin ▁encoding ▁is ▁done ▁by ▁having ▁a ▁single - ph ot on ▁go ▁through ▁a ▁Mach – Z eh nder ▁inter fer ometer ▁( M Z ), ▁shown ▁in ▁black ▁here . ▁The ▁phot on ▁coming ▁from ▁the ▁left ▁is ▁gu ided ▁through ▁one ▁of ▁two ▁paths ▁( sh own ▁in ▁blue ▁and ▁red ); ▁the ▁gu iding ▁can ▁be ▁made ▁by ▁optical ▁fi ber ▁or ▁simply ▁in ▁free ▁space ▁using ▁mirror s ▁and ▁polar ising ▁cub es . ▁One ▁of ▁the ▁two ▁paths ▁is ▁longer ▁than ▁the ▁other . ▁The ▁difference ▁in ▁path ▁length ▁must ▁be ▁longer ▁than ▁the ▁coh er ence ▁length ▁of ▁the ▁phot on ▁to ▁make ▁sure ▁the ▁path ▁taken ▁can ▁be ▁un ambigu ously ▁distinguished . ▁The ▁inter fer ometer ▁has ▁to ▁keep ▁a ▁stable ▁phase , ▁which ▁means ▁that ▁the ▁path ▁length ▁difference ▁must ▁vary ▁by ▁much ▁less ▁than ▁the ▁w avel ength ▁of ▁light ▁during ▁the ▁experiment . ▁This ▁usually ▁requires ▁active ▁temperature ▁stabil ization . ▁ ▁If ▁the ▁phot on ▁takes ▁the ▁short ▁path , ▁it ▁is ▁said ▁to ▁be ▁in ▁the ▁state ▁; ▁if ▁it ▁takes ▁the ▁long ▁path , ▁it ▁is ▁said ▁to ▁be ▁in ▁the ▁state ▁. ▁If ▁the ▁phot on ▁has ▁a ▁non - zero ▁probability ▁to ▁take ▁either ▁path , ▁then ▁it ▁is ▁in ▁a ▁coh er ent ▁super position ▁of ▁the ▁two ▁states : ▁▁▁▁ ▁These ▁coh er ent ▁super pos itions ▁of ▁the ▁two ▁possible ▁states ▁are ▁called ▁qu bits ▁and ▁are ▁the ▁basic ▁ing red ient