This article is from the magazine Les Indispensables de Sciences et Avenir n°210 of July/September 2022.
Welcome to a world where you can be here and anywhere else, a zoo where an animal can be alive and dead at the same time, a universe where “teleportation” is commonly practiced, this reality is that of quantum physics, that describes light and matter on a scale inaccessible to our eyes.
His concepts are so strange that the American physicist Richard Feynman said about it in 1964, during a very serious conference, in front of the hilarious room: “I think it’s safe to say that no one understands her.” It was a year before he received a Nobel for his contribution… to quantum physics. “We understand it very well scientificallyclarifies Julien Bobroff, professor of physics at the University of Paris-Saclay. On the other hand, it completely escapes our intuition.
How do you know what’s going on inside the box?
Imagine, for example, a cat, locked in a well-insulated box containing a vial of lethal gas and an atom of radon, whose nucleus is radioactive. If the radon decays, the particle it emits breaks the bulb, killing the animal. How do you know what’s going on inside the box? Our classical view of the world leads us to think that the cat is alive or dead, depending on whether or not the radon has decayed. But if we consider this experience through the prism of quantum physics, our brain is in a ball: any quantum object can be in one state or the other, but also in both at the same time.
In other words, a “quantum cat” can be alive, dead, or alive and dead at the same time! Only at the moment of observation will we know its destiny. But beware: quantum physics also tells us that observing a state disturbs or destroys it! Similarly, it prohibits creating a perfect copy: impossible to clone a report. On the other hand, nothing prevents him from being instantly teleported to the far reaches of our galaxy, if there is someone to watch him… Impressive! But you will understand why.
“Admit it’s hard to conceive!”
Take two photons emitted by an atom in two opposite directions. Resulting from the same physical process -the return to calm of an excited atom-, they are not strangers to each other: they are said to be intricate, entangled. “Everything then happens as if these two photons formed a single quantum object,” sums up Julien Bobroff. Quantum physics shows that any measurement on one will tell us about the other. A bit like two dice that would give each roll a random result but whose sum would always be 6.
When these objects are close together, one might imagine that something connects them, an unknown local interaction or hidden parameter. “What is surprising is that the entanglement is preserved when the objects are receded to a great distance, even more than a thousand kilometers! “, continues the physicist. If we observe our two tangled dice, one in Paris and one in Oslo, any measurement of a 2 in the first will be accompanied by the observation of a 4 in the second, and vice versa. Everything happens as if the state of the dice was transmitted instantaneously to the other at the moment of measurement, therefore at a speed greater than that of light…
“This action is immediate and doesn’t decrease with distance like other interactions do, eg gravitation. Admit that it’s hard to conceive!” So difficult that Albert Einstein himself never managed it… He even considered the entanglement as “remote action that sends shivers down the spine.” Since then, we have been able to manipulate photons separated by 1,400 kilometers and even entangle atoms. But would we be able to do this with macroscopic or even living objects?
In December 2021, a team based in Singapore claimed to have achieved this feat with a tardigrade, an infallible little animal: in extreme situations, especially cold or empty air, its metabolism is interrupted; it can stay that way for decades before coming back to life as soon as conditions are right…quantum. They placed the animal between the two: the electrical properties of the system were slightly modified.
Investigators deduced that the tardigrade had become entangled. The news made noise… without convincing. “For specialists, any insulating material would produce the same result, regardless of whether it is an animal, Justifies Julien Bobroff. Also, at the time of the experiment, the tardigrade is no longer alive. The only amazing thing is that it could then come back to life… But that’s biology, not quantum physics! “
We know how to use teleportation to transmit secret codes
Now that entanglement no longer holds any secrets for us, let’s return to quantum teleportation in more detail. “Nothing to do with Star Trek, warns Julien Bobroff. The term is poorly chosen, because it suggests that something tangible has moved. However, it is only the state of a particle that is instantly transmitted, and not this one. Rather we should talk about photocopies or facsimiles of the quantum state.” It’s already impressive!
Let’s summon two people, Alice and Bob, each of whom has one of the two previously entangled photons, called A and B. Alice entangles her photon A with another photon X. When Alice measures A, she will therefore know the state from X. Whatever its distance, Bob’s measurement of photon B will give the same state as X: by entangling this third photon, Alice has “faxed” its properties instantly!
A mechanism that we know how to control today to transmit information at a distance, for example secret codes. This is called quantum key distribution, which also offers the guarantee that this communication will remain secret: since the state of a photon cannot be cloned, nor observed unchanged, any eavesdropping would be detected immediately. China already has a network of several thousand kilometers of optical fibers, so it is secure. Their engineers also know how to distribute keys outdoors, via satellite.
While most quantum effects are only observed on a microscopic scale, they sometimes have visible consequences. Test by water… In principle, the colder a material is, the closer the elements -atoms or molecules- that compose it are: a solid substance is, therefore, denser than the same in liquid state. So when you throw a steel ball into a crucible of molten steel, it sinks. And yet, ice floats on water… and that’s a good thing: if it were denser than liquid, polar lakes and oceans would get caught up in the mass every winter, killing all animal life. This is again an oddity, a consequence of the specific quantum properties of the oxygen and hydrogen atoms that make up water: below 4°C, as the molecules are less agitated, they can exchange electrons with each other. Their geometry is then organized in a way that distances them from each other… and reduces the density of water and, a fortiori, of ice.
All these somewhat crazy properties have given scientists ideas. While certain quantum effects risk stopping the race for the miniaturization of electronic components, in particular the “tunneling effect”, which allows -another surprising property- electrons to pass through walls, why not trust this fun physics to reinvent the computer and achieve unimaginable achievements? computing capabilities? The idea is gaining ground, but the game is far from over.
In the meantime, let’s savor the existence of our smartphones, our solar collectors, and continue to use our MRIs for better treatment. Unknowingly, all of these devices are powered by quantum phenomena that govern the behavior of electrons on chips, create current from light, or allow us to visualize matter in its smallest details… No quantum, nothing! would exist at all!
The computer, generator… of errors
In the electronic puces, the bit of information can turn on the value 0 or 1. They are equivalent quantique, the qubit, for représenter ces deux names, but also tous ceux que sont compris between 0 and 1, even if the 0 and the 1 at the same time ! By perfectly controlling a hundred qubits, isolated from the outside world, we would theoretically obtain a power infinitely greater than that of our supercomputers. “In practice, quantum chips make a lot of mistakes, about one in every thousand operations, summarizes Julien Bobroff. Therefore, we must implement methods to correct them. So you will probably have to generate millions of qubits to produce a computation that is useful for anything. But the current record is 127 qubits!”
This number increases slowly, because the error rate skyrockets with the number of qubits. It’s a bit like a snake biting its own tail… We’ll probably see analog quantum computers first, designed to solve a particular problem, what we call a simulator. They could, for example, study the evolution of a complex quantum system, something we don’t know how to do today.
“It is easier to build than a general-purpose computer, since it would be enough to create a quantum system that reproduces the initial state of what we want to study, and then let it evolve spontaneously. A bit as if, to study the fall of a large rock from a mountain, we build a mountain on the Playmobil scale before dropping a marble. These simulators are starting to pay off. The same cannot be said for quantum computers. But given the efforts of the computer giants, starting with IBM, Google or Intel, there is reason to be hopeful.