En mécanique quantique, selon le principe de superposition, un même état quantique peut posséder plusieurs valeurs pour une certaine quantité observable (spin, position, quantité de mouvement, etc.)
Ce principe résulte du fait que l’état – quel qu’il soit – d’un système quantique (une particule, une paire de particules, un atome, etc.) est représenté par un vecteur dans un espace vectoriel nommé espace de Hilbert (premier postulat de la mécanique quantique).
Comme tout vecteur de tout espace vectoriel, ce vecteur admet une décomposition en une combinaison linéaire de vecteurs selon une base donnée. Or, il se trouve qu’en mécanique quantique, une observable donnée (comme la position, la quantité de mouvement, le spin, etc.) correspond à une base donnée de l’espace de Hilbert.
En conséquence, si l’on s’intéresse, par exemple, à la position d’une particule, l’état de position doit être représenté comme une somme d’un nombre (infini!) de vecteurs, chaque vecteur représentant une position précise dans l’espace. Le carré de la norme de chacun de ces vecteurs représente la probabilité de présence de la particule à une position donnée.
Principe de superposition quantique — Wikipédia
Le phénomène dit de l’étrangeté quantique (quantum veirdness) désigne des événements inconnus dans la physique newtonienne dite aussi macroscopique. On cite l’intrication quantique, la non-localité quantique, la superposition quantique, le principe d’incertitude, la dualité onde-particule, le caractère probabiliste de l’effondrement de la fonction d’onde
Or il a été depuis longtemps suggéré que la conscience trouvait son origine dans des phénomènes quantiques se produisant dans les neurones du cerveau , plus précisément lorsque la superposition quantique s’y effondrait. Mais comment prouver cette affirmation? N’était ce pas remplacer un mystère par un autre ?
Nous publions ici sur ce sujet un article en anglais que vient de faire paraitre dans le NewScientist du 4 janvier 2025, p.40 Hartmut Neven chef du laboratoire Google Quantum AI https://quantumai.google/ responsable chez Google de la construction des plus puissants calculateurs quantiques à ce jour.
Il a été depuis longtemps été suggéré que la conscience trouvait son origine dans des phénomènes quantiques se produisant dans les neurones du cerveau , plus précisément lorsque la superpositon quantique s’y effondrait. Mais comment prouver cette affirmation? N’est ce pas remplacer un mystère par un autre ?
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The suggestion that consciousness has its origins in quantum weirdness has long been viewed as a bit, well, weird. Critics argue that ideas of quantum consciousness, the most famous of which posits that moments of experience arise as quantum superpositions in the brain collapse, do little more than merge one mystery with another. Besides, where is the evidence? And yet there is a vocal minority who insist we should take the idea seriously.
Hartmut Neven, who leads Google’s Quantum Artificial Intelligence Lab, is among them. He originally trained as a physicist and computational neuroscientist before pioneering computer vision – a type of AI that replicates the human ability to understand visual data. Later, Neven founded Google Quantum AI, which in 2019 became the first lab to claim its quantum computers solved calculations that are impossible on a classical computer, a milestone known as quantum supremacy. In December 2024, his team announced another step forward with its new quantum processor, Willow, which it claims is more powerful and reliable than previous chips.
But Neven is also interested in the relationship between mind and matter. And now, in a use case for quantum computers that no one saw coming, he reckons they could be deployed to put the idea of quantum consciousness to the test. Neven spoke to New Scientist about his belief that we live in a multiverse; why Roger Penrose’s theory of quantum consciousness is worth pursuing, albeit possibly with a new twist; and how we can test such ideas by entangling quantum computers with human brains.
Thomas Lewton: How has working at the forefront of quantum computing altered your view of what reality is?
Hartmut Neven: We recently ran a computation on our new quantum processor, named Willow, that would take the best classical supercomputer an astounding amount of time to complete: 1025 years. This mind-boggling number exceeds known timescales in physics and vastly exceeds the age of the universe. To me, this result suggests that quantum processors are tapping into something larger than just our universe, lending credence to the notion that their computation occurs in many parallel universes.
Over the years, I’ve come to appreciate that the most straightforward reading of the equations of quantum mechanics is that, indeed, we live in a multiverse: that every object, including myself or the cosmos at large, exists in many configurations simultaneously. This view of reality has profoundly shaped my everyday outlook on life.
In what way?
My general stance when describing the world is physicalism, which states that every phenomenon we witness can be explained as a manifestation of matter. But the only phenomenon that we are certain exists is conscious experience. Everything starts from experience; without mind, nothing matters.
So then the task you have as a physicalist is to identify the locus of consciousness. Here, I think, quantum mechanics has a unique advantage over classical mechanics – and it is directly related to the multiverse picture.
If the multiverse picture is correct, then there are a vast number of parallel worlds. But right now, you and I coexist in a definite, classical branch of the multiverse. So why do we witness this configuration and not the other ones? This is an opportunity to place consciousness in your physicalist theory. An attractive conjecture is that consciousness is how we experience the emergence of a unique classical reality out of the many that quantum physics tells us there are.
Consciousness seems like a very different kettle of fish to quantum physics. How can one be accommodated into the other?
I’m a disciple of Roger Penrose, who, in his 1989 book The Emperor’s New Mind, put forth the idea that consciousness involves a state of matter in quantum superposition, where a quantum object exists in multiple configurations at the same time. When the superposition collapses during a “measurement” process, one classical branch gets selected out of many possible branches and this implements a conscious moment. I always thought this was beautiful because then qualia – specific subjective experiences such as the redness of a rose or the feelings that music evokes – can naturally be encoded into the state that [the superposition] collapses into.Is there any way to test the idea that consciousness is quantum in origin?
There are already some insights coming from experiments with anaesthesia. Anaesthetics reversibly knock out your consciousness. You are still breathing, your heart is still pumping, but you can’t report conscious experiences anymore. However, even though anaesthetics are a medical godsend and in use for almost 180 years, we still have no clue how they work. Nobody understands it.
Interestingly, the simplest anaesthetics are inert gases like xenon. Even more peculiarly, there are reports that different isotopes of xenon, each of which has slight differences in mass and a quantum property called spin, have different anaesthetic potency. If that can be confirmed, then you can’t possibly explain this without considering quantum mechanics. I feel this is a smoking gun experiment.
And you have proposed another kind of experiment in a recent paper. Can you tell us a bit more about that?
Let’s first picture our brain as containing qubits, which are the basic units of information in quantum computing. I think that’s rather uncontroversial. Some researchers – like our colleague Stuart Hameroff, [the director of the Center for Consciousness Studies at the University of Arizona] – suggest that large protein structures in neurons called microtubules act as qubits. But any biophysicist or biochemist would say that, at the very least, on the level of molecules with electron clouds, there are quantum states in our brain – so we can be said to have qubits in our brains.
Then let’s say we have “N” qubits in our brain and “M” qubits in an external quantum computer, with the letters referring to a certain number of qubits. If a person could entangle their brain with this quantum computer, they could create an expanded quantum superposition involving “N+M” qubits. If we now tickle this expanded superposition to make it collapse, then this should be reported by the person participating in this experiment as a richer experience. That’s because in their normal conscious experience, they typically need “N” bits to describe the experience, but now they need “N+M” bits to describe it.
I call this the “expansion protocol”, as it would allow us to expand consciousness in space, time and complexity. In fact, if we can find a way to set up this experiment, and someone reports these richer experiences, then this would support our explanation that quantum processes generate consciousness.
What do you imagine it would be like to experience this expanded consciousness?
The number of bits per second that we are consciously aware of is not very large. Many things that you could potentially be consciously aware of you’re not. Let’s say the James Webb Space Telescope shoots a beautiful picture, we make a screensaver out of it and we admire it. We are not able to consciously behold all the information that’s in the myriads of photons streaming into the James Webb telescope. That’s an experience we are not able to have.
So, in principle, we could generate way richer experiences than we normally have using our default biological brain. Some extraordinary states of consciousness, such as those experienced under psychedelics, for example, may be sort of a preview of what you could expect here. Entangling one’s brain with a quantum computer could potentially unlock higher levels of consciousness, creativity and understanding.
Would this help you to understand the relationship between mind and matter?
We could use this experimental set-up to identify which quantum states of matter correlate to different qualia. We can do this by asking a person whose brain is entangled with a quantum computer about the specific characteristics of their feelings and measuring the qubits associated with those feelings.
How has considering this experimental concept changed how you think about the origin of conscious moments?
When I started to think about this experimental programme, I realised, oh, wait a minute, there’s actually an issue with Roger’s ideas. If I measure a qubit in the quantum computer, the superposition collapses into a state that instantly goes hand in hand with an experience in the person who is entangled with the computer. If this were to happen, then I could use this entanglement – a unique phenomenon in quantum mechanics where two or more particles become intrinsically linked – as a channel to transmit information faster than light.
So, when Roger associated conscious moments with the collapse of superpositions, this opened the possibility of faster-than-light communication, which goes against fundamental rules of physics. I don’t like this – I’m the more orthodox physicist on this point. But if, instead, we say that a conscious moment is experienced when a superposition forms, not when a superposition collapses, then this challenge with faster-than-light communication goes away.
In our experimental set-up, we could test which of these ideas is correct. In Penrose’s version, the richer experience would be felt when the superposition collapses. However, if conscious moments occur when superpositions form, then the richer experience would be felt as soon as the qubits in someone’s brain become entangled with the qubits of the quantum computer.
What other problems does flipping Penrose’s idea on its head help to solve?
The role of entanglement in the formation of conscious moments naturally explains our unified experience of reality. This is a well-known issue in neuroscience called the binding problem.
When we see an object, such as a face, neurons in the brain’s primary visual cortex fire in response to certain features being present, such as edges in certain orientations, creating the rough outline of a face. Then, this brain activity propagates to the higher visual cortices where richer facial features are represented. Our experience is distributed through the brain, rather than single neurons existing that fire to, say, represent your grandmother. We perceive holistically. So there is a disjoint between what we experience and the structure of our material brains. This is called the binding problem.
We can solve this by proposing that entanglement between qubits creates a unified conscious experience. Entanglement is the only true binding agent we have in physics, as it allows for the creation of holistic states where individual components are fundamentally interconnected. Thus, entanglement offers an elegant solution to the binding problem.
Will it ever be practically possible to entangle a human mind with a quantum computer?
At this point, the expansion protocol is technically still very challenging. But we can do a simpler warm-up experiment.
In recent years, researchers have become adept at growing little balls of human brain cells called brain organoids. We could use two qubits coupled via a brain organoid and carry out a Bell test – a quantum experiment that checks whether or not two systems are entangled. If we were to find that entanglement is needed to explain the results of this Bell test, then we can conclude that the brain organoid, at least in part, deserves a quantum mechanical description.
Maybe all the ideas I’ve been talking about turn out to be incorrect. But if it works, then you could ask, how is the quantum coupling best realised? Do you want to use photons? Do you want to use spin – a quantum property that atomic nuclei or electrons have? Or perhaps you want to use collective modes in microtubules.
How are these ideas about the quantum nature of consciousness received in your circles?
It’s an acquired taste. I’m often surprised how, among scientists, the nature of consciousness is considered a question one shouldn’t ask or be involved with. Whereas I think, look, when I have a toothache, this experience is very real, much more real than, say, the big bang or other constructs of science.
The philosopher of science Thomas Kuhn, in his book The Structure of Scientific Revolutions, said that before “normal science” can begin, there’s a “pre-paradigmatic” phase where we are still searching for the right framework to understand a phenomenon. I believe consciousness research has reached this inflection point. Our conjecture on what creates consciousness and our proposal on how to test it show that the nature of consciousness might be addressed with the methods of experimental science.
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