For decades, our computers have been advancing by packing ever more transistors onto a single chip. But this race has its physical limits: we cannot reduce the size of components indefinitely. To take things further, scientists are exploring a radically different path: harnessing the laws of quantum physics, which govern the behaviour of matter at the atomic scale. In this infinitely small world, the rules change. A particle can exist in several states simultaneously – it can be both here and there, on and off at the same time. It is on this baffling principle that the promise of quantum computers rests: machines capable of solving certain complex problems at a speed that no classical computer will ever be able to match.

But building a quantum computer first and foremost involves knowing how to manipulate its building blocks: qubits. Whereas a classical bit can be either 0 or 1, a qubit can be both at the same time. To achieve this, physicists make use of a fundamental property of particles known as spin, a sort of internal orientation, like a microscopic compass. These spin qubits must be operated at temperatures close to absolute zero, around –273°C, to remain in their fragile quantum state. The slightest rise in temperature, even by one thousandth of a degree, is enough to disrupt everything.

Rather than simply putting up with this heat, can’t we also make the most of it?

This is the two-part question that Boris Brun-Barrière has been tackling since his arrival at the University of Grenoble Alpes in December 2023. By combining experiments in solid-state physics, theoretical simulations and quantum device engineering, he is seeking to understand, and control, thermal effects in the most promising quantum systems.

Understanding how heat affects qubits means tackling one of the most tangible obstacles on the path to quantum computers.

When a thousandth of a degree makes all the difference

Spin qubits are among the most promising candidates for building the quantum processors of the future. Manufactured from silicon, the same material as our everyday microchips, they have the advantage of being extremely miniaturisable and compatible with existing manufacturing techniques. However, they are also extraordinarily sensitive to their thermal environment.

When these qubits are manipulated using microwave fields, the immediate environment heats up. This heating, however slight, disrupts the qubits in ways that are not yet fully understood. To observe and measure this, the team has developed an ultra-fast thermometry technique capable of detecting temperature variations of the order of a millikelvin, that is, one thousandth of a degree, on timescales of the order of a microsecond (Champain 2024). A precision tool unprecedented at these nanometric scales and at these extreme temperatures.

The next step was to identify the physical origin of these effects. By studying qubits based on ‘holes’, the oppositely charged counterparts of electrons in silicon, the team was able to show that it is an interaction specific to holes that creates their particular sensitivity to heat (Champain 2025). This is a decisive discovery: it paves the way for qubits capable of withstanding these thermal disturbances, by identifying specific magnetic field configurations in which sensitivity to heat is almost completely eliminated. At the same time, the team also demonstrated that it is possible to shield these same qubits from parasitic electrical disturbances in their environment, another major source of errors, opening up the prospect of very high-quality spin qubits (Bassi, Nature Physics 2026).

A third area of research is now emerging: the interaction of hole qubits with the surrounding silicon nuclei, which carry a nuclear spin. This interaction, which is still poorly understood, is one of the remaining factors limiting the coherence of qubits, and is the subject of ongoing work within the team.

From obstacle to tool: the birth of a quantum bolometer

But heat is not just a constraint to be overcome. It can also become a signal to be harnessed. Alongside his work on qubits, Boris Brun-Barrière is developing a new generation of bolometers, extremely sensitive radiation sensors, based on two-dimensional germanium. The principle involves dissipating the energy of photons in a tiny absorber, the temperature of which is measured in real time. Thanks to their very low heat capacity, these materials form thermal detectors of exceptional sensitivity, in principle opening up the possibility of detecting single microwave photons, a missing building block in the field of quantum technologies.

The results now exceed the state of the art: a decisive turning point for this new field of research, which has brought together an international consortium around an ambitious European project.

A challenge for the digital age

This research forms part of what physicists call the ‘second quantum revolution’: our new ability to isolate, manipulate and read out individual quantum objects. This new paradigm promises sensors of unrivalled sensitivity, extreme miniaturisation, and, a prospect that is still distant but real, computers capable of solving certain classes of problems much more quickly than any classical machine.

Along the way, there is still much to understand about how quantum systems interact with their environment. It is precisely this friction between the quantum world and the real world, imperfect, noisy and hot, that this physicist from Grenoble has chosen to place at the heart of his work.

Outlook

Quantum thermodynamics is currently one of the most active fields in fundamental and applied physics. How can we manage energy dissipation as quantum processors become increasingly complex? How can we harness thermal effects rather than simply being at their mercy? What new quantum sensors will our understanding of microwave radiation enable us to build?

Building on close collaborations with the CEA and European partners, and drawing on his role as director of the international Master’s programme ‘Quantum Information Quantum Engineering’ at the University of Grenoble Alpes (UGA), Boris Brun-Barrière is also training the next generation of specialists in emerging quantum technologies, technologies which, in the future, could profoundly transform our relationship with computation and measurement.