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Beyond cryptocurrencies: how long will it take to have quantum money?

El Salvador has become the first country to accept the cryptocurrency Bitcoin as legal tender . It happened in June 2021, and, two months later, Cuba followed in his footsteps. In September, by contrast, China declared that transactions with any cryptocurrency are illegal.

It is clear that cryptocurrencies are all the rage, and more and more people are investing their savings in these assets in the hope of making much more money than in the stock market. Although there are voices that call them a pyramid scam and predict a resounding failure in a not very long term, for now the industry boasts of breaking record after record in the prices of these coins.

The biggest attraction of cryptocurrencies (aside from the huge growth they experience) is that they are not dependent on any central bank. On the contrary, euro banknotes are only legal if this is confirmed by the European Central Bank, which is in charge of protecting the banknotes with security measures that make it difficult to copy them (the same happens for any other currency in the corresponding bank). Bank transfers and card purchases must also be verified by the banks that issue and receive them.

By dispensing with a central bank to verify the authenticity of money exchanges, it is essential to have reliable security systems to replace them. In fact, even the security of our banknotes and bank transactions is not inviolable. Photocopying a ticket or committing bank fraud is very, very difficult (and trying is not recommended). But, in principle, given enough time and resources, nothing prevents reproducing a ticket atom by atom or hacking a credit card to make fraudulent purchases.

Is there a way out? Can we design a coin that is perfectly safe? The answer is yes, and it was invented even before cryptocurrencies. The trick is to take advantage of quantum effects .

Quantum money was first proposed around 1969, although the article was not published until 1983. This design creates quantum banknotes that are impossible to counterfeit . This is an absolute impossibility, not just practical: a fundamental theorem of quantum theory dictates that quantum objects cannot be cloned.

Here is the main difference between quantum banknotes and current ones: by examining in depth a euro banknote we can get to know all its properties and find a description that captures all the information contained in it. But quantum objects are more elaborate. As much as we look at them, if we do not know part of their description in advance it is not possible to complete it without altering them.

The safest banknotes

That is why there has to be a central bank that knows the descriptions of the quantum bills and can verify the authenticity of each copy. Using a part of the description (identified with the serial number of the corresponding banknote) the banknote can be observed to obtain the missing part , and thus check that it corresponds to the correct description.

Still, because it requires a central bank, quantum money seems less attractive than cryptocurrencies in this regard. Unlike normal banknotes, verifying their authenticity is not immediate. A normal banknote can be looked at under ultraviolet light to check that the paper does not shine and the stars of the European flag glow orange, or it can be seen that the image of the hologram changes as we move the banknote. But to make sure that a quantum bill is authentic, you need knowledge (the partial description of the bill) that only the central bank has.

To overcome these drawbacks, there are other proposals that do not require this knowledge. In “public key quantum money”, the central bank creates the banknotes together with a numerical code which is also printed on them, and which allows the authenticity of each banknote to be verified. Thus, there is no need to contact the central bank to find out if a note is authentic .

However, what you gain in independence is lost in security : no public-key quantum money proposition is unconditionally secure. By being able to find the complete description of each bill without consulting a bank, anyone could create new bills that have the same description. The theorem that prevents cloning quantum objects only applies if we do not know the description of the object.

That is why other security measures are added to these bills that make it very difficult to copy them. They are measures similar to those that our credit cards have: guessing the complete number, the three digits behind it (the CVV), the expiration date and the pin code (the data necessary to make a purchase) is not strictly impossible. But it would take so long that it’s not worth trying to figure out combinations of numbers until you find the right one. It would be very difficult to guess before the card was blocked. That is, security is based on practicalities.

Of course, current technology is still far from being able to generate quantum money . To achieve this, the banknotes need to be stored in a quantum memory for a long time. This hurdle was already mentioned in the first quantum money proposal. The best current memories do not last more than a few minutes, although they are actively investigated to make them more durable.

Beyond technology, for quantum money to become a reality, banks and businesses would have to agree to accept this new currency. Considering the journey of cryptocurrencies, this is perhaps the most difficult obstacle to overcome .

DO NOT HAVE IT:

  • At present, the security of our banking transactions is not unconditional either, it is only practical. It is based on certain mathematical problems that are so difficult to solve that it would take too long to do it (the same happens if we try to guess the numbers on a credit card). Some of these problems become easy if we have a quantum computer at hand, that is why we are already working to develop alternative systems (called “post-quantum”) that are safe against quantum attacks.
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Two Wheeler

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