A carbon nanotube-based superconducting quantum bit

For nearly 25 years, miniaturized electrical circuits cooled to extremely low temperatures have become some of the most important players in quantum technology. Invented in research laboratories, these circuits are now used as basic components (quantum bits) in prototype quantum computers developed by companies such as IBM and Google. Alongside these developments seeking to apply quantum physics, fundamental research continues using these superconducting circuits as tools for exploring the quantum world. 

The work of the QCMX team, and in particular that conducted by Hannes Riechert during his PhD, has just demonstrated a new architecture for a superconducting quantum circuit associated with a carbon nanotube, which functions as a quantum bit. This is a first. The article is published in Nature Communications.

Like an artificial atom

What is a quantum bit? A classical bit is a physical system whose characteristic can take two distinct values, which can be used to encode the “0” and “1” of computer language (for example, two different levels of voltage or electric current in a transistor within our computer processors). Similarly, a quantum bit (or qubit) is a physical system in which two levels are isolated. But quantum physics adds the possibility of putting this system into states of superposition “at the same time” in one level and in the other. Quantum bits can be created in several ways: with atoms, ions, photons... or even superconducting circuits.

These devices are actual electrical circuits measuring just a few centimeters, engraved in niobium using electronic lithography techniques. They generate electromagnetic fields due to electrical currents and voltages, just like conventional electrical circuits. The difference lies in quantum effects: the electromagnetic energy contained in the circuit can only change in “packets”; it cannot take on any value. There are therefore separate energy levels. “Unlike an atom, which has fixed properties, here we can precisely design the characteristics of the circuits. This is one of their great advantages,” emphasizes Jean-Damien Pillet, a researcher at QCMX.

One of the key ingredients for the emergence of these quantum phenomena is superconductivity: at very low temperatures, the electrons in certain materials behave collectively rather than individually. This superconductivity is necessary both because it prevents energy dissipation that would interfere with the preservation of quantum states and also for the central element of the quantum bit, called a Josephson junction. This allows researchers to manipulate exactly two energy levels that act as “0” and “1.”

Caption: On the left, two superconducting circuits. The electrical tracks are blue and gray. The quantum bit itself is outlined in orange. On the right, a schematic representation of the quantum bit itself. The cylinder representing the carbon nanotube appears in black in the middle.

A new hybrid architecture: superconductivity and carbon nanotubes 

The innovation developed at École Polytechnique is the use of a carbon nanotube as the building block for this Josephson junction. A carbon nanotube consists of a hollow cylinder with a diameter of one nanometer formed from a single layer of carbon atoms. “As this nanotube is a semiconductor, it allows the properties of the quantum bit to be controlled using a simple electrical voltage,” explains Landry Bretheau, a researcher at QCMX. One of the challenges was to insert the highly sensitive nanotube into the circuit while preserving the functioning of the whole.

In the scientific publication, the researchers demonstrate that they can effectively control this quantum bit and place it in different state superpositions. They also measure the lifetime of these quantum states, which are fragile by nature, and conduct a systematic study to understand how to improve it in the future.

“It's possible that this original work, which is driven by curiosity, could eventually be used for practical applications,” says Jean-Damien Pillet. "But our goal is more fundamental: carbon nanotubes are very curious objects in themselves. With a diameter of only one nanometer and a length of one micrometer, electrons are forced to pass through one after the other. It's like a one-dimensional world, with exotic physical effects that we hope to see in future experiments."

Article reference:

Riechert, H., Annabi, S., Peugeot, A. et al. The carbon nanotube gatemon qubit. Nat Commun 16, 7197 (2025). https://doi.org/10.1038/s41467-025-62283-y

*PMC: a joint research unit CNRS, École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France