Since 17 June 2026, deep inside a refrigerated cabinet at Bruyères-le-Châtel, a silicon wafer a few centimetres across has been waiting its turn, sealed in a housing the CEA's engineers will not open: warming the whole assembly would take them three days, cooling it again as long, and the least speck of heat would ruin weeks of tuning.

The golden stages of a suspended dilution refrigerator, threaded with coaxial cables, in a laboratory with a tool board visible at the back.
The cryostat in Alice & Bob's laboratory. The golden plates are the successive cooling stages; the chip sits right at the bottom, at ten millikelvin. (Photographie Nilhope, 2024, Wikimedia Commons, licence CC BY-SA 4.0, source page)

France signed that day for eighteen qubits, to cross two walls no quantity of classical processors will ever cross.

Breaking a cipher. Simulating a molecule.

Nobody, though, ever says what a qubit is, materially. What is this component made of, and why must it live so close to absolute zero?

What exactly is a state?

It all starts with a word everyone uses without defining it: the state. A state is a stable physical situation, distinct from the others, that we know how to read. Nothing more. All of computing has rested on that for seventy years.

In an ordinary computer a transistor carries the information. The component works like a tiny switch, driven by a voltage applied to one of its terminals, and depending on whether that voltage sits above or below a threshold, it lets the current through or blocks it dead.

Two voltage levels are then enough, the lowest in the circuit and the highest. The first is called zero, the second one. A bit comes down to that: a voltage in a wire, at a given place, at a given moment.

A superconducting qubit leaves that ground entirely, and the comparison stops there: the information no longer travels in a resting voltage but in a moving current, which circulates endlessly in a small loop and crosses, on every lap, a barrier so thin it measures a few atoms, the Josephson junction.

A word before going further, because what follows describes one family and not the whole field. Superconducting is one way of making a qubit; others exist, sharing neither its physics nor its constraints. France funds five of them side by side, and the last episode comes back to that. The machine it bought belongs to this one.

On the left a switch open then closed, driven by a voltage; on the right a loop with its junction, and two currents oscillating at different rhythms.
A bit is a voltage level. A superconducting qubit is a way of oscillating. In both cases the state is a stable physical situation we know how to read. (Schéma Shy Robotics)

A current that turns for ever

The word superconducting keeps coming back and it is far from obvious: an ordinary metal always resists the passage of current a little, and that resistance dissipates the motion as heat, whereas a superconducting metal, cooled below a certain temperature, stops resisting altogether.

Altogether, in the literal sense. Send a current round a superconducting ring, unplug the supply: it keeps turning.

Without that property no qubit would exist. A bit makes do with a voltage sitting still, which asks only to stay where it is. A qubit rests on motion, and motion in an ordinary metal would die away in a fraction of a second. That current oscillates at very high frequency, and it can do so in two distinct ways, two vibration regimes physicists know how to prepare and read: those two ways of oscillating play the parts of zero and one.

The whole of what follows lies in that difference. A transistor holds a voltage, and a voltage does not move on its own. A qubit holds a motion, and motion drifts.

Superposition is not ignorance

Alice & Bob makes cat qubits, named after Schrödinger's thought experiment, and the name deserves its explanation here.

Schrödinger never claimed a cat could be dead and alive at once: he was showing the opposite, by absurdity.

Slide a coin under an opaque cup, flip the whole thing onto the table with one gesture, then lift it barely: the coin has fallen one way, you do not know which, but an answer does exist under the cardboard. Schrödinger called that ignorance, so as not to confuse it with superposition.

A system in superposition hides no value. Its two possibilities exist together, like two waves, and can cancel one another out. A hidden answer never cancels anything.

Three columns: a cup upturned on the table with the coin hidden beneath, an atom whose two possibilities are two waves, and a cat struck from every direction.
Ignorance on the left, superposition in the middle, and what destroys it on the right. Schrödinger built the experiment to show the absurdity, not to assert it. (Schéma Shy Robotics)

A cat does not live cut off from the world: the air strikes it, light hits it, its own warmth betrays it.

Each of those collisions carries off a little of the answer. A photon bounces off the cat and leaves carrying, in its trajectory, the information about the state it has just brushed past. Nobody reads it, nobody catches it: it is enough that it has gone. The environment monitors the system without any observer taking part.

An isolated atom whose two possibilities still meet, a photon bouncing off and carrying the answer away, then a cat riddled with impacts from every direction.
We are not the ones looking. Each collision carries the answer away, and the two possibilities stop meeting. At a cat's scale the collisions run into the billions every second. (Schéma Shy Robotics)

And as soon as the answer circulates outside, the system stops hesitating and behaves like any everyday object: one state, a single one, the one the environment has already carried off.

It all comes down to a matter of number. A well-isolated atom, in vacuum and cold, meets almost nobody. A cat counts billions of billions of particles, each of them constantly struck from outside. The CEA gives the order of magnitude for quantum objects bound together: with N constituents, superposition holds roughly N times less long than with one.

Does growing protect, or expose?

You will object, and rightly: if growing makes things fragile, how can a company claim to build a sturdier qubit by making it bigger?

Because the word large does not mean the same thing on each side. A cat exposes billions of billions of particles to the outside. A cat qubit encloses more photons in an isolated resonator, in vacuum, at ten millikelvin.

On the left an object riddled with arrows from all sides; on the right two states moving apart, with loss arrows escaping upwards.
Two ways of being large. The cat takes everything at once. The cat qubit trades one error type for the other, deliberately. (Schéma Shy Robotics)

They get no gift in return, whatever a press release might suggest. Adding photons pushes the two states apart, which makes a first kind of error exponentially rare. But those photons get lost, and the more of them there are the more often it happens: the second kind of error gets worse.

The cat of the thought experiment loses on every front at once. The cat qubit chooses which front it loses on.

Alice & Bob keeps one thing from the thought experiment, and it is not the animal. It is the distance between the two states.

Go back to the two states playing the parts of zero and one. In an ordinary qubit they are the two lowest energy levels of the circuit, separated by a gap so small that anything can tip one into the other. At Alice & Bob they take the form of two oscillations of opposite phase, held as far apart as possible.

That separation can be tuned, and there engineering comes in: the more photons the resonator holds, the further the two oscillations move apart, so that for a zero to become a one by accident the system would have to cross the whole distance in one go, with no intermediate step.

Théau Peronnin and Raphaël Lescanne founded Alice & Bob in 2020 on that bet, and it went against the grain. The sector was stacking ordinary qubits and hoping error correction would follow. They wanted a component whose physics removes half the problem outright, even if the whole architecture then goes to the other half.

Neither copper nor gold: tantalum on silicon

For ordinary electricity everyone knows the material: copper, sometimes gold. In the laboratory where a quantum chip is etched, engineers want something else: a metal that, once cooled, stops resisting the current entirely. Copper still resists at ten millikelvin.

Every metal has its own critical temperature, below which it stops resisting, and for aluminium as for tantalum it sits a few degrees above absolute zero. Hence the fridge. Hence the ten millikelvin.

They make the chip almost like an ordinary one: the support is a silicon wafer, on top of which they etch tracks in a metal film a few hundred nanometres thick, tantalum carrying the information because it loses less signal than aluminium, the latter kept for the junctions. The Josephson junction stacks three tiny layers: two layers of superconducting metal separated by an insulating oxide layer, so thin that electrons cross it anyway, by tunnelling.

The process explains the Al/AlOx/Al formula found in the papers: a first aluminium layer is laid down, its surface is exposed to oxygen just long enough to oxidise to a minute depth, and the second layer goes on top, so the insulator never comes from anywhere else. Aluminium makes it itself by oxidising at the surface, the way a pan tarnishes in air.

On the left the chain from the generator to the chip; on the right a cross-section of the chip showing silicon, the metal film and the junction.
The full chain, and the cross-section of the chip. The resonator is not a box: it is an etched track, folded back on itself, where the wave travels to and fro. (Schéma Shy Robotics)

The cavity is not a box

One part of that chip will hold the information: the resonator. On Alice & Bob's chip that part stores, and the microwave photons living in it carry the zero and the one. Everything else, junctions included, prepares it, protects it and reads it.

It accepts one note only. Blow into an organ pipe: however you blow, out comes a precise pitch, set by the length of the pipe. Here the length of the etched track sets the frequency, and the wave travels back and forth inside instead of escaping. They call it a cavity all the same, and the word misleads: the first resonators of this kind were genuine hollow metal boxes, and the name stuck. On Alice & Bob's chip there is no box, only a tantalum track, etched and folded, lying flat on the silicon.

That leaves the generator. At room temperature, beside the machine, they install ordinary electronics that produce microwave pulses; these travel down coaxial cables through the fridge's stages to the chip, and the same cables bring back the reflected signal, the one used to read the state.

The object can finally be described. An etched silicon wafer, in a housing, at the bottom of a refrigerator the size of a wardrobe, connected to the world by cables. What circulates in it is not direct current but microwaves, at a few gigahertz.

And that is where the question gets interesting.

What can you possibly do with microwaves that transistors cannot?

The next episode answers, and it starts by taking apart the sentence you have heard a hundred times: no, the machine does not try every answer at once.

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