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.
At the moment, the idea of “quantum money” is very much on the drawing board. That hasn’t stopped researchers from pondering what encryption schemes they might apply for it, or from wondering how the technologies used to create quantum states could be shrunk down “to the point of fitting it in your wallet,” says Scott Aaronson, an MIT computer scientist who works on quantum money. “This is science fiction, but it’s science fiction that doesn’t violate any of the known laws of physics.”
The laws that govern subatomic particles differ dramatically from those governing everyday experience. The relevant quantum law here is the no-cloning theorem, which says it is impossible to copy a quantum particle’s state exactly. That’s because reproducing a particle’s state involves making measurements—and the measurements change the particle’s overall properties. In certain cases, where you already know something about the state in question, quantum mechanics does allow you to measure one attribute of a particle. But in doing so you’ve made it impossible to measure the particle’s other attributes.
This rule implies that if you use money that is somehow linked to a quantum particle, you could, in principle, make it impossible to copy: It would be counterfeit-proof.
The visionary physicist Stephen Wiesner came up with the idea of quantum money in 1969. He suggested that banks somehow insert a hundred or so photons, the quantum particles of light, into each banknote. He didn’t have any clear idea of how to do that, nor do physicists today, but never mind. It’s still an intriguing notion, because the issuing bank could then create a kind of minuscule secret watermark by polarizing the photons in a special way.
To validate the note later, the bank would check just one attribute of each photon (for example, its vertical or horizontal polarization), leaving all other attributes unmeasured. The bank could then verify the note’s authenticity by checking its records for how the photons were set originally for this particular bill, which the bank could look up using the bill’s printed serial number.