Einstein’s Quantum Riddle tells the remarkable story of perhaps the strangest phenomenon in science – quantum entanglement. It’s a story of mind-bending concepts and brilliant experiments, which lead us to a profound new understanding of reality. At the start of the 20th century Albert Einstein helped usher in quantum mechanics - a revolutionary description of the behaviour of tiny particles. But he soon became uncomfortable with the counterintuitive ideas at the heart of the theory. He hunted for flaws in the equations and eventually discovered that they predicted a seemingly impossible situation. Quantum theory suggested you could have two particles, which had interacted in the past, and even if you separated them by millions of miles they would somehow act in unison. If you measured one, forcing it to take on one of many properties, the other would instantly take on a corresponding property. Like rolling two dice, millions of miles apart, and as you look at one to see what number it landed on, the other instantly shows the same number. This bizarre prediction of magically connected particles became known as quantum entanglement. Einstein felt it couldn’t possibly be real – it seemed to break the rules of space and time. In 1935, with two of his colleagues, he published a paper that argued that this bizarre phenomenon implied the equations of quantum theory must be incomplete. No-one could think of a way to test whether Einstein was right, until in 1964, John Bell, a physicist form Northern Ireland, published an astonishing paper. He’d found a key difference between Einstein’s ideas and those of quantum theory. It all boiled down to entanglement. As Professor David Kaiser puts it: ‘We now know this was one of the most significant articles in the history of physics. Not just the history of 20th-century physics; in the history of the field as a whole.’ In 1972 John Clauser and Stuart Freedman built an experiment based on John Bell’s work and found the first