Quantum money is a type of currency that employs the strange laws of quantum mechanics to make it both uncopyable and easily verifiable. These properties make it an ideal medium of exchange, similar to regular cash, but without the risk of counterfeiting.
The idea was first developed in 1970 by physicist Stephen Wiesner using the idea that trying to measure an unknown quantum state inevitably destroys it. By comparison, the process of measuring a known quantum state holds it.
Wiesner realized that if the details of the quantum state were kept secret, for example by a central bank, this property could be used to guarantee the authenticity of quantum money while ensuring that it could never be copied. .
Since then, the idea of quantum money has been very influential, forming the basis of numerous experiments and quantum cryptography techniques that are carried out on a daily basis.
However, Wiesner’s formulation of quantum money has one drawback. The verification process can only be performed by trusted bodies such as central banks that keep the details of the quantum state secret.
However, with the emergence of decentralized currencies such as Bitcoin and Ether, attention is focused on currency systems that do not require centralized control.
Now, Andrei Kesin and Peter Scholl of the Massachusetts Institute of Technology and Jonathan Lu of Harvard University (both in Cambridge) are working to create quantum money that can be verified by anyone without requiring a blockchain to securely record transactions. I found a way to create
The new approach gets its security from a form of post-quantum cryptography that is resistant to attack by quantum computers. The key to post-quantum cryptography is finding problems that are difficult even for quantum computers to solve.
One of the most promising has to do with the mathematical idea of a lattice, a kind of multidimensional grid formed by a set of vectors. The points in this grid are connected by vectors of various lengths that are easy to compute. However, the problem of finding the shortest vector in a lattice turns out to be difficult, especially if the lattice is random.
One approach is to compute the distance between all points in a random grid. This will eventually find the shortest. But as grids get larger or contain more dimensions, this problem becomes dauntingly difficult, even for quantum computers.
An approach that Keshin et al. came up with is to encode a random lattice, perhaps as an atomic array, into the quantum properties of the units of quantum money. Anyone who wants to copy this money must recreate this random lattice. But this can only be done if the shortest vector is known, and even quantum computers can’t beat it.
It guarantees money security. Also, the quantum states of the lattice have certain properties that make them easy to verify because anyone can test them.
The result is a physical system that cannot be copied, but can be easily checked. “Because our money state is physical, it can function as tangible but unforgeable paper money, but it can also be transferred as digital money through quantum channels,” he said. Khesin et al.
And all this is done by buyers and sellers without the need for a record of the transaction in the same way that ordinary cash is used today. “Validation of ownership can be done locally and offline, without the need for global synchronization through mechanisms such as blockchain,” the team says.
It’s an interesting job with important implications. One of the drawbacks of decentralized cryptocurrencies is the enormous energy costs required to encrypt and maintain the blockchain. For Bitcoin, this is currently believed to exceed the electricity consumed by all of Argentina, which is clearly unsustainable in the long term.
Quantum money could work without this overhead. Also, like cash, there is naturally anonymity, making it a popular property. “Our quantum money also offers advantages that cannot be achieved with traditional cryptocurrencies or physical bills,” say the researchers. However, it will only become available if the infrastructure exists to transmit quantum information easily and cheaply. In other words, quantum money first requires a full quantum internet. This is a technology that is steadily but slowly emerging.
There may be another application that is more likely to come to fruition first. Khesin et al. raise the possibility that the same technology could provide copy protection in the quantum world.
And they have plans in this direction. “The next step is to adapt the quantum money algorithm to an anti-piracy protocol that protects quantum computations (circuits) from cloning.”
Notice this space. Quantum copy protection could soon become a reality, if not quantum money.
Reference: Publicly Verifiable Quantum Money from Random Lattice: arxiv.org/abs/2207.13135