▁cannot ▁be ▁communic ated ▁between ▁two ▁distinct ▁locations ▁L 0 ▁and ▁L 1 ▁without ▁the ▁transmission ▁of ▁any ▁physical ▁systems , ▁despite ▁any ▁quantum ▁ent ang lement ▁shared ▁between ▁L 0 ▁and ▁L 1 . ▁This ▁implies , ▁in ▁particular , ▁that ▁without ▁the ▁transmission ▁of ▁any ▁physical ▁systems ▁between ▁L 0 ▁and ▁L 1 , ▁quantum ▁correlation ▁between ▁L 0 ▁and ▁L 1 ▁cannot ▁be ▁used ▁to ▁transmit ▁information ▁between ▁L 0 ▁and ▁L 1 , ▁even ▁if ▁they ▁are ▁non - loc ally ▁caus al ▁and ▁viol ate ▁Bell ▁in equal ities . ▁According ▁to ▁relativ ity ▁theory , ▁physical ▁systems ▁cannot ▁travel ▁faster ▁than ▁the ▁speed ▁of ▁light . ▁Thus , ▁it ▁follows ▁from ▁the ▁no - sign alling ▁principle ▁that ▁information ▁cannot ▁travel ▁faster ▁than ▁the ▁speed ▁of ▁light . ▁This ▁is ▁called ▁the ▁no - super l um inal ▁sign alling ▁principle . ▁ ▁The ▁principle ▁of ▁no - super l um inal ▁sign alling ▁is ▁the ▁key ▁physical ▁principle ▁explo ited ▁in ▁relativ istic ▁crypt ography . ▁It ▁guarante es ▁that ▁the ▁outcome ▁x ▁of ▁a ▁random ▁variable ▁X ▁obtained ▁at ▁some ▁spac etime ▁point ▁P ▁cannot ▁influence ▁the ▁probability ▁that ▁a ▁random ▁variable ▁Y ▁takes ▁some ▁value ▁y ▁at ▁a ▁spac eli ke ▁separated ▁spac etime ▁point ▁Q . ▁Thus , ▁for ▁example , ▁if ▁two ▁parties ▁Alice ▁and ▁Bob ▁have ▁each ▁two ▁agents , ▁with ▁the ▁first ▁agent ▁of ▁Bob ▁sending ▁a ▁secret ▁message ▁x ▁to ▁a ▁first ▁agent ▁of ▁Alice ▁at ▁the ▁spac etime