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Since the invention of paper money, counterfeiters have churned out fake bills. Some of their handiwork, created with high-tech inks, papers, and printing presses, is so good that it's very difficult to distinguish from the real thing. National banks combat the counterfeiters with difficult-to-copy watermarks, holograms, and other sophisticated measures. But to give money the ultimate protection, some quantum physicists are turning to the weird quirks that govern nature's fundamental particles.