Traditionally, “quantum superiority” is sought from the point of view of raw computing power: we want to calculate (much) faster.
However, the question of its energy consumption could also now warrant research, with current supercomputers sometimes consuming as much electricity as a small town (which could actually limit the increase in their computing power). Information technologies, at its end, accounted for 11% of global electricity consumption in 2020.
Why focus on the energy consumption of quantum computers?
Since a quantum computer can solve problems in a few hours, while a supercomputer could take several tens of billions of years, it is natural to expect that it will consume much less energy. However, manufacturing such powerful quantum computers will require us to solve many scientific and technological challenges, possibly over one to several decades of research.
A more modest goal would be to create less powerful quantum computers capable of solving computations in a time relatively comparable to supercomputers but using much less energy.
This potential energy advantage of quantum computing has already been discussed. Google’s Sycamore quantum processor consumes 26 kilowatts of electrical power, much less than a supercomputer, and runs a test quantum algorithm in seconds. Following the experiment, scientists presented classical algorithms to simulate the quantum algorithm. The first proposals for classical algorithms required much more energy – which seemed to show the energy advantage of quantum computing. However, they were soon followed by other proposals that were much more energy efficient.
Therefore, the energy advantage is still open to question and is an open research topic, especially since the quantum algorithm made by Sycamore has no identified “useful” application so far.
Superposition: the delicate phenomenon at the heart of quantum computing
To know whether quantum computers can be expected to have an energy advantage, it is necessary to understand the fundamental laws according to which they work.
Quantum computers manipulate physical systems called qubits (for quantum bits) to do a calculation. A Qubit can take two values: 0 (the “basic state”, of minimum energy) and 1 (the “excited state”, of maximum energy). It can also occupy a “superposition” of 0 and 1. How we interpret superpositions is still the subject of heated philosophical debates, but, simply put, this means that the qubit can be “both” in state 0 and state 1 with certain associated “probability amplitudes”.