It’s hard to forge what you can’t observe Equinox Graphics/Getty Images

A quantum upgrade could make old-fashioned cheques the most secure way to send money. Researchers have proven that quantum computers could in theory create and cash cheques that are nearly impossible to forge.

Quantum computers store information using qubits which, unlike the ones and zeros of classical computing, can exist in two states simultaneously. This is known as quantum superposition.

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But it’s impossible to observe a qubit while it’s in a superposition – it collapses into either a one or zero as soon as you measure it. This is what makes quantum cheques so secure. If anyone intercepted a cheque and had a peek inside, they would only be able to see the qubit in its collapsed state.


Quantum currency

The idea of quantum currency has been kicking around for decades, but until last year, very few computer scientists had access to quantum computers to test their theories. Now, researchers using IBM’s cloud-based quantum computer have run the numbers.

Although this study is only a proof of concept, quantum technology hasn’t got far to go until these systems are workable says Prasanta Panigrahi, who led the study at the Indian Institute of Science, Education and Research in Kolkata. Even existing five-qubit quantum computers – like the one used in this experiment – could eventually be used to issue and verify quantum cheques, he says. But for now, these kinds of transactions can’t be scaled to a wide population and they aren’t exactly the most convenient way of moving cash.

Say Alice wants to pay Bob using a quantum cheque. She would have to go to the bank, verify her identity and then the bank would issue her with two qubits taken from its central quantum computer. These qubits are inextricably linked to the remaining qubits within the bank’s central computer – a quality known as quantum entanglement. Measuring the state of any one qubit in an entangled system will reveal the state of all qubits within that system. The bank can use this entanglement to verify that its coffers were the origin of a quantum cheque.

Alice can then take her qubits and encode one of them with the amount of money she wants to give to Bob. She then gives this qubit to Bob and he takes it to the bank to cash. The bank verifies that it’s definitely a qubit from its own system, and that it has been encoded by Alice, and cashes the cheque.

Not quite tamper proof

Although you can’t forge a quantum cheque, there is one weak point in this system, says Subhayan Roy Moulick, a researcher at the University of Oxford who originally proposed the experimental proof. To encode her qubit, Alice has to access it using a passcode, so if someone stole that passcode and her qubit, they could theoretically tamper with the qubit. But as long as the passcode is memorised or securely locked away the risk of tampering is extremely low, says Moulick.

Then there’s the problem of transporting qubits. Some of our current quantum computers need huge cooling systems. It is possible to store qubits at room temperature by using diamonds, but Moulick says that a quantum cheque is more likely to be a laptop-sized black box than something you can slip into your pocket.

Scott Aaronson at the University of Texas suggests applying the concept in a slightly different way. If the bank managed the entire transaction, he says, it would still be ultra secure, but there would be no need for anyone to carry around quantum cheques.

However, the real application is a way off, Aaronson says. At the moment, the qubits in the IBM system only last for microseconds at a time. “Ideally,” Aaronson says, “one would like cheques that can last longer than that before being cashed or deposited.”

Besides, in this experimental scenario, the bank gives away two qubits every time it issues a cheque. Even if the bank only issued one cheque a day, it would need hundreds of qubits, and quantum computers of that size are still decades away from becoming reality. The current most powerful quantum computers only have 20 qubits, although Google is on track to build a 49-qubit machine by the end of this year.

Erika Andersson at Heriot-Watt University in Edinburgh, UK, thinks that the whole idea is unnecessarily complicated. Instead, quantum computers could be put to better use creating secure keys instead of physical cheques, she says. The bank could authenticate transactions using a technique called quantum key generation, where quantum computers are used to generate shared security keys to verify the identity of the parties involved in the transaction. This would cut out the need for anyone to carry qubits around.

And, unlike qubit cheques, quantum keys are already catching on in the real world. The technology was used to help keep the results of the 2007 Swiss election secure while in 2012 the Chinese government used it to help keep the discussions at the 2012 National Congress away from prying eyes.

Read more: Google on track for quantum computer breakthrough by end of 2017