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Why Quantum Money Could Replace Blockchain-Based Cryptocurrencies

Quantum currency is a form of currency that uses the strange laws of quantum mechanics to ensure that it cannot be copied but at the same time can be easily verified. These properties make it an ideal means of exchange, just like ordinary cash, but without the risk of counterfeiting.

The idea was first developed by physicist Stephen Wiesner in 1970 using the notion that any attempt to measure an unknown quantum state inevitably destroys it. By comparison, the process of measuring a known quantum state preserves it.

Wiesner realized that if the details of the quantum state were kept secret, by a central bank for example, this property could be used to guarantee the veracity of quantum currency while ensuring that it could never be copied .

Since then, the idea of ​​quantum currency has become hugely influential, forming the basis of many quantum cryptography experiments and techniques that have become routine.

Quantum downside

However, Wiesner’s quantum money formulation has a downside. The verification process can only be performed by a trusted authority, such as a central bank, which otherwise keeps the details of the quantum states secret.

But the emergence of decentralized currencies like Bitcoin and Ether has drawn attention to monetary systems that do not require centralized control.

Today, Andrey Khesin and Peter Shor of Massachusetts Institute of Technology and Jonathan Lu of Harvard University, both in Cambridge, have found a way to create quantum money that anyone can verify, which will makes it fully decentralized without the need for a blockchain to securely record transactions.

The new approach derives its security from a form of post-quantum encryption that resists attacks from quantum computers. The key to post-quantum encryption is finding problems that even a quantum computer struggles to solve.

One of the most promising involves the mathematical idea of ​​a lattice, a sort of multidimensional grid formed by a set of vectors. The points of this grid are connected by vectors of different easy-to-calculate lengths. However, the problem of finding the shortest vectors in the network turns out to be difficult, especially when the network is random.

One approach is to calculate the distance between all points in a random network, which will eventually find the shortest. But as the grid gets larger or includes more dimensions, this problem becomes incredibly difficult, even for a quantum computer.

The approach that Khesin and co have proposed is to encode the random lattice into the quantum properties of a quantum currency unit, perhaps in the form of an atomic lattice. Anyone who wants to copy this money must reproduce this random lattice. But that can only be done if the shortest vectors are known, a task that will defeat even a quantum computer.

This guarantees the security of the money. It is also easily verifiable since the quantum state of the network has specific properties that any user can test.

The result is a physical system that cannot be copied but is easily verified. “Because our monetary states are physical, they can serve as tangible but unforgeable invoices, but they could also be transferred through quantum channels as digital currency,” say Khesin and co.

And all of this is done by the buyer and seller without the need for a record of transactions, just like regular money is used today. “Ownership verification can be done locally and offline, without the need for global synchronization through mechanisms such as blockchains,” the team explains.

blockchain bust

This is interesting work with important implications. One of the disadvantages of decentralized cryptocurrencies is the huge energy cost required to encrypt and maintain the blockchain. For bitcoin, it is currently believed to be more energy than the entire country of Argentina consumes and this is clearly unsustainable in the long term.

Quantum money has the potential to operate without this overhead. It is also naturally anonymous, just like cash, which will be a valued property. “Our quantum currency also offers benefits that are not available to conventional cryptocurrencies or physical bills,” the researchers say. But it will only become possible to use it when the infrastructure exists to send quantum information easily and cheaply. In other words, quantum money first requires a full quantum internet, a technology that is surely but slowly emerging.

There might be another application that is likely to come to fruition first. Khesin and co raise the possibility that the same technique may also provide copy protection in the quantum world.

And they have plans in that direction. “A next step is to adapt the quantum money algorithm to an anti-hacking protocol that protects quantum computations (i.e. a circuit) from duplication.”

Watch this space – quantum copy protection, if not quantum money, could soon be a reality.


Ref: Publicly verifiable quantum currency from random networks: arxiv.org/abs/2207.13135

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