Quantum computers are expected to be disruptive and potentially impact many industry sectors. So researchers in the United Kingdom and the Netherlands decided to explore two very different quantum problems: breaking the encryption of Bitcoin (a digital currency) and simulating the molecule responsible for biological nitrogen fixation.
In AVS Quantum Science, from AIP Publishing, the researchers describe a tool they created to determine how big a quantum computer needs to be to solve problems like these and how long it will take.
“The majority of existing work within this realm focuses on a particular hardware platform, superconducting devices, like those IBM and Google are working toward,” said Mark Webber, of the University of Sussex. “Different hardware platforms will vary greatly on key hardware specifications, such as the rate of operations and the quality of control on the qubits (quantum bits).”
An error-corrected quantum computer will be necessary for many of the most exciting quantum advantage use cases. By correcting for internal mistakes in the quantum computer, error correction makes it possible to perform longer computations, but it requires more physical qubits.
The energy-intensive process of extracting nitrogen from the atmosphere to produce ammonia for fertilizers could be improved, which would have an effect on both the global food problem and the climate crisis. Even the world’s fastest supercomputers are now unable to simulate important molecules, but next-generation quantum computers should be able to do so.
“Our tool automates the calculation of the error-correction overhead as a function of key hardware specifications,” Webber said. “To make the quantum algorithm run faster, we can perform more operations in parallel by adding more physical qubits. We introduce extra qubits as needed to reach the desired runtime, which is critically dependent on the rate of operations at the physical hardware level.”
Because only qubits that are adjacent to one another may interact directly, most quantum computing hardware platforms are constrained. Other platforms, like certain trapped ion designs, let the qubits to be physically moved about rather than being locked in place, allowing each qubit to interact with a large number of other qubits directly.
“We explored how to best take advantage of this ability to connect distant qubits, with the aim of solving problems in less time with fewer qubits,” said Webber. “We must continue to tailor the error-correction strategies to exploit the strengths of the underlying hardware, which may allow us to solve highly impactful problems with a smaller-size quantum computer than had previously been assumed.”
Quantum computers are significantly more capable of breaking various encryption algorithms than conventional computers. For the majority of its secure communications, the world employs RSA encryption. Bitcoin’s elliptic curve digital signature technique and RSA encryption are both susceptible to quantum computing attacks in the future, but currently, not even the fastest supercomputer can seriously compromise security.
The researchers estimated the size a quantum computer needs to be to break the encryption of the Bitcoin network within the small window of time it would actually pose a threat to do so — in between its announcement and integration into the blockchain. The greater the fee paid on the transaction, the shorter this window will be, but it likely ranges from minutes to hours.
“State-of-the-art quantum computers today only have 50-100 qubits,” said Webber. “Our estimated requirement of 30 [million] to 300 million physical qubits suggests Bitcoin should be considered safe from a quantum attack for now, but devices of this size are generally considered achievable, and future advancements may bring the requirements down further.
“The Bitcoin network could perform a ‘hard-fork’ onto a quantum-secure encryption technique, but this may result in network scaling issues due to an increased memory requirement.”
The researchers emphasize the rate of improvement of both quantum algorithms and error-correction protocols.
“Four years ago, we estimated a trapped ion device would need a billion physical qubits to break RSA encryption, requiring a device with an area of 100-by-100 square meters,” said Webber. “Now, with improvements across the board, this could see a dramatic reduction to an area of just 2.5-by-2.5 square meters.”
A large-scale error-corrected quantum computer should be able to solve important problems classical computers cannot.
“Simulating molecules has applications for energy efficiency, batteries, improved catalysts, new materials, and the development of new medicines,” said Webber. “Further applications exist across the board — including for finance, big data analysis, fluid flow for airplane designs, and logistical optimizations.”
Reference: “The impact of hardware specifications on reaching quantum advantage in the fault tolerant regime” by Mark Webber, Vincent Elfving, Sebastian Weidt and Winfried K. Hensinger, 25 January 2022, AVS Quantum Science.