Albert Einstein's Quantum Riddle is related to the phenomenon known as quantum entanglement. This phenomenon refers to the way in which particles that have interacted with each other become 'entangled', such that the state of one particle is instantly connected to the state of the other, no matter how far apart they are. This appears to violate Einstein's theory of relativity, which states that nothing can travel faster than the speed of light.
Einstein, along with physicists Boris Podolsky and Nathan Rosen, highlighted this paradox in a 1935 paper, often referred to as the EPR paradox. They argued that quantum mechanics must be incomplete because it allows for 'spooky action at a distance'. However, numerous experiments have since confirmed that quantum entanglement is a real phenomenon, although its interpretation continues to be a topic of debate among physicists.