Would you welcome spooky banknotes? (Image: Mark Wilson/Getty)

In God we trust; all others must pay cash

Sign on a market stall

THERE is something special about cold, hard cash. Perhaps it is that its value is guaranteed by the government of the day, or that you can stash it under the bed when a banking collapse threatens. Maybe it is the freedom that cash allows: the ability to live without banks or credit cards or taxes.

Quantum physicists think a lot about cash. Not just any old money, you understand. They think about quantum cash. Quantum banknotes aren’t like credit cards or dollar bills. They are simply information: a mixture of bits – the 0s and 1s that we use to send electronic transactions – and quantum bits, or qubits, that are governed by the laws of quantum mechanics and can be both a 0 and 1 at the same time.

Since quantum money is just information, it can be stored and transmitted just like a digital picture or a text file. But because it has quantum properties too, it cannot be copied. It is this combination that makes quantum cash so attractive: whoever is in possession of it has exclusive and unequivocal ownership of it, just as with hard, physical cash and unlike a credit card. That is not the only use for quantum cash, though. To physicists, quantum cash is a toy problem, a sort of test case with which to study the strange properties of quantum mechanics.

Now the theoretical foundations are almost in place that could one day allow quantum cash to become a reality. These techniques …