Some quantum computers might need more power than supercomputers

Large quantum computers may be able to solve problems impossible for even the best traditional supercomputers – but in order to do so, some of them might need far more energy than those supercomputers.

Existing quantum computers are relatively small, with most having fewer than a thousand building blocks called qubits. They are also prone to making errors during operation because of how fragile those qubits are. This makes these computers incapable of solving the economically and industrially relevant problems they have been predicted to excel at, such as aiding drug discovery. Researchers largely agree that really useful quantum computers must have radically larger qubit counts and an ability to correct errors – making them fault-tolerant quantum computers (FTQCs). But getting there is still a formidable engineering challenge, partly because there are several competing designs.

Olivier Ezratty at the Quantum Energy Initiative (QEI), an international organisation, says that one overlooked concern of building utility-scale FTQCs is their potential energy consumption. At the Q2B Silicon Valley conference in Santa Clara, California, on 9 December, he presented preliminary estimates of it. Strikingly, several FTQC designs surpassed the energy footprint of the world’s largest supercomputers.

The world’s fastest supercomputer, El Capitan at the Lawrence Livermore National Laboratory in California, needs about 20 megawatts of electrical power, which is approximately triple the energy consumption of the nearby 88,000-resident city of Livermore. In Ezratty’s estimate, two designs for FTQCs, scaled up to 4000 logical, or error-corrected, qubits, would require even more. The most power-hungry among them might need as much as 200 megawatts of power.

Basing his estimates on publicly available data, proprietary information from quantum computing firms and theoretical models, Ezratty has identified a wide spectrum of possible energy footprints for future FTQCs, which ranges from 100 kilowatts to 200 megawatts. Notably, in Ezratty’s estimation, three FTQC designs that are currently being developed would ultimately require less than 1 megawatt of electricity, which is comparable to typical supercomputers used by research facilities. In his view, this spectrum could influence the evolution of the industry, for instance making the quantum computing market larger if the less power-hungry designs come to dominate.

The broad difference in projected energy consumption primarily reflects the diversity of competing ways in which quantum computer firms build qubits and put them to use. In some cases, energy consumption is driven by the need to keep different parts of the device cold, for instance for some light-based qubits where sources and detectors of light work less well when warm. Ezratty says that this can be especially power-consuming. In other cases, such as for qubits made from superconducting circuits, whole chips must be put in giant fridges, while quantum computers based on trapped ions or ultracold atoms require energy for the lasers and microwaves that control the qubits.

Oliver Dial at IBM, which makes superconducting quantum computers, says that he expects the firm’s large-scale FTQC to require just under 2 or 3 megawatts to operate. Dial says this is only a fraction of what is projected to be needed for hyperscale AI data centres, and could be even lower if the FTQC were integrated with an existing supercomputer. The team at ultracold atoms quantum computing company QuEra estimates that its FTQC would require around 100 kilowatts, falling on the lower end of Ezratty’s spectrum.

Xanadu, which builds light-based quantum computers, and Google Quantum AI, whose quantum computers are based on superconducting qubits, declined to comment. PsiQuantum, which also makes qubits from light, didn’t respond to New Scientist’s request for comment.

Ezratty says there are also many costs associated with traditional electronics that are used to direct and monitor qubits, especially when it comes to FTQCs where qubits receive extra directions to catch and correct their own errors. This complicates the situation further because it means that details of error-correction algorithms also contribute to the devices’ energy footprint. And then there is the issue of how long a quantum computer must run to complete an operation, because energy savings that come from using fewer qubits could be counteracted if they must run for longer.

To untangle all these factors – the basic energy cost of making qubits, the cost of cooling and controlling them and the cost and time of running quantum software – the industry ought to develop standards and benchmarks for determining and reporting the energy footprint of their machines, says Ezratty. This is part of the mission of QEI. He says there are related projects under way both in the US and the European Union.

In the same way that the whole quantum computing industry is still developing, Ezratty says his work is in early stages and should lead to efforts to better understand FTQC’s energy consumption and draw on that understanding to lower it. “There are many, many technical options that could work in favour of reducing the energetic footprint.”