One of the most famous more recent quotes about the nature of money is by the fictional Baltimore drug dealer D’Angelo Barksdale in the US series The Wire: “Money be green…” he says, “money feel like money!”, reacting to a scam where a buyer has passed poorly counterfeited bills. Money feels like money… except when it doesn’t. Money of the future will no longer be bills and will not be the digital money of today, either. Scientists at Palacký University in Olomouc have demonstrated that in all likelihood it will be quantum, ultra-secure and impossible to clone.
Associate Professor Karel Lemr at the university, involved in the research which led to a prototype quantum “bill”, told me more about the quantum world and how far quantum money had come since it was first thought up.
“When the quantum money protocol was proposed for the first time the situation was very different from what we know today. One example: when it comes to experiments in the lab, we weren’t able to implement even the basics of quantum gates at that time. We have very much moved forward since then. Also, today we are capable of distributing quantum information at much longer distances, of dozens of kilometres, for instance, so we are slowly getting there. That is the reason why this concept was re-visited. In short, it is getting more important now.”
Is it the case that with quantum computing, the necessity for quantum money will grow? Quantum computers will make it easier to hack digital money now…
“That’s exactly right. The advances in quantum information processing wouldn’t in way jeopardize classical paper money of course but will jeopardise the digital currency Bitcoin. Bitcoin is based on classical cipher algorithms and with quantum computers you will be able to break those algorithms much faster. Quantum computing can be a threat not only to classical digital currency but also to classical secure communications, such as the RSA cipher, for instance. With the advances in quantum computing we will need a more secure concept and that is what quantum physics can provide when it comes to be money as well as cryptography.”
“In the quantum world there are different rules than in the classical world and what we experience on a daily basis.
What are the advantages that quantum physics offer in making ultra-secure money?
“Well of course in the quantum world there are different rules than in the classical world and what we experience on a daily basis. In the classical world, any state can be perfectly duplicated, at least in principle. In principle, you can take a sheet of paper and make a perfect duplicate in your copy machine. But in the quantum world, things are completely different. There is a fundamental law that prohibits making copies of an unknown quantum state: if I give you a photon or whatever system in an unknown, to you, quantum state, you won’t be able to perform a perfect replica. That is the cornerstone of security behind quantum money and quantum cryptography.”
The basic particle which is used is the photon, used in the project at the university to create a quantum “bill”, is that correct?
“That’s right. Of course, quantum money has nothing in common with anything like a physical banknote with some atoms implemented on top of it – that was maybe the idea back in the 1970s. Now we reviewed the concept and came to the conclusion that if quantum money becomes a future ‘thing’ it will most likely be a string of photons sent via wireless or optical fibre lines. Light is a perfect carrier for quantum information: photons are very robust against ‘noise’ so they can be sent over large distances over optical fibres. The national bank, the money issuer, would issue a random sequence of photons, it wouldn’t be a bill at all, but a random sequence of photons or states in which they were prepared, this sequence would be secure and known only by the issuing bank.”
I understand that there is also a built-in problem or potential risk which is essentially that when the quantum information is sent it never arrives in the exact same composition as when it left. Is that a risk?
“With quantum money, the bank will have to accept a slightly damaged banknote. Some photons can go missing.”
“It is. In the real world, you have to take into account imperfections. With paper money, you can have a corner torn off or a bill slightly torn but the central bank still recognizes the money as authentic and will accept it. In the theoretical concept, you can predict that the distribution of photons encoding the quantum bill would be perfect but in the real world you have imperfections. With quantum money, the bank will have to accept a slightly damaged banknote. Some photons can be missing or part of the sequence can be different. That’s just how it is and the bank would have to accept the bill. So what is crucial is that there has to be an acceptable threshold to close the opening gained for a potential counterfeiter.
“If the threshold is too low, that enables potential counterfeiters to create a passable note, not a perfect copy – that is forbidden – but an approximate duplicate within quantum mechanical laws and sell it as a genuine. To set the threshold so that counterfeiting is impossible but slightly damages notes still be accepted - that’s the real trick. Luckily, it is possible and that is something we established in our paper, laying out possible encoding schemes.”
Anyone interested in producing a counterfeit quantum bill would presumably have to have hi-tech equipment themselves…
“Hi-tech, maybe, but certainly it is achievable even with the current level of technology. WE managed to do it, so it is certainly doable but other labs around the world.”
“We are still waiting for quantum memory, where quantum money can be stored.”
If quantum money is not quite there yet, how long a timeframe are you giving it? A decade? Two decades?
“More likely two decades. You see, we are still waiting for quantum memory, where quantum money can be stored. We are now capable of distributing quantum money, we are perfectly able to do that, it is very similar to quantum cryptography which has been available commercially for a decade. But for quantum money we would need something like a quantum ‘purse’ – storage. And that’s tricky. While we are slowly getting there, it is very slowly. So maybe two decades or half a century? It’s hard to tell.”