The difficulty captured by the EPR thought experiment was that by perturbing the angular momentum state of either of the two particles in a spatially distributed singlet state, the quantum state of the remaining particle appears to be "instantaneously" altered, even if the two particles have over time become separated by light years of distance. A critical insight made decades later by John Stewart Bell, who was a strong advocate of Einstein's locality-first perspective, showed that his Bell's theorem could be used to assess the existence or non-existence of singlet entanglement experimentally. The irony was that instead of disproving entanglement, which was Bell's hope, subsequent experiments instead established the reality of entanglement. In fact, there now exist commercial quantum encryption devices whose operation depends fundamentally on the existence and behavior of spatially extended singlets.