(PhysicsWorld) Physicists from the University of Sussex, UK recently set out to answer this question for two pragmatic computational tasks: breaking the encryption used in Bitcoin transactions and simulating the behaviour of an agriculturally important nitrogen-fixing molecule. By estimating the number of quantum bits, or qubits, that different types of quantum computers would need for each task, members of the team say their theoretical study should help other researchers decide which designs to pursue.
Two of the most popular ways of engineering qubits involve superconductors and trapped ions. In either case, the number of qubits available to perform quantum operations is significant, explains Mark Webber, a PhD student at Sussex who is also involved in a Sussex spin-out called Universal Quantum.
The Sussex team began by considering common algorithmic methods that could be implemented on a quantum computer.
The first problem the team examined concerns a molecule called the FeMo cofactor (FeMo-co) that bacteria use to extract nitrogen from air and create ammonia. This same process is performed on an industrial scale in the fertilizer industry, but in a way that is much less efficient and accounts for almost 2% of the world’s energy use.
In the second part of the study, the team calculated the number of physical qubits needed to break the encryption used for Bitcoin transactions. This question – whether cryptocurrencies are safe against quantum computer attacks – comes with additional constraints not present in the FeMo-co simulation. While a 10-day computation time may be acceptable for FeMo-co simulations, the Bitcoin network is set up so that a hacker armed with an error-correcting quantum computer would have a very limited time to decrypt information and steal funds.
According to Webber and collaborators, breaking Bitcoin encryption within one hour – a time window within which transactions may be vulnerable – would take about three hundred million qubits. Based on this result, Narozniak concludes that “Bitcoin is pretty safe”, although he warns that not all cryptocurrencies operate the same way. “There are other cryptocurrencies that work differently, and they have different algorithms that could be more vulnerable,” he says.
Tim Byrnes, who leads Narozniak’s quantum research group at NYU, says that scaling these machines to the millions of qubits discussed in the Sussex team’s work is not an impossible goal. “That sounds big, but there are commercial companies where these qubit numbers are the target. Certainly not today, but given some years, this is not too out of reach,” he says.
For now, Bitcoin encryption is safe from quantum computers
