Physicist Serge Haroche has dedicated his life to studying quantum mechanics, the world of subatomic particles where intuitive-defying phenomena occur.
Their main interest is photons, the particles that carry light and allow us to detect the entire observable universe.
His research in this field earned him the Nobel Prize in Physics in 2012, which he received together with his colleague and friend David J. Wineland.
One of the big problems with studying particles smaller than an atom, such as photons, is that they lose their quantum properties as soon as they interact with the outside world.
Haroche and Wineland, however, figured out how to observe and measure individual quantum particles without destroying them.
Achievements like this have meant a breakthrough in one of the great promises of technology: quantum computing.
In this interview, Haroche talks about the importance of light in solving the riddles of the universe, the mysteries that photons still hold, and how close we are to a true revolution in quantum computing.
Haroche was born in Casablanca, Morocco, in 1944 and has French nationality.
He is professor emeritus at the Collège de France and is the author of the book “The Light Revealed: From Galileo’s Telescope to Quantum Strangeness”.
You argue that to answer the deepest questions of the universe you have to understand the nature of light. What do you mean?
First of all, most of the information we have from the outside world comes from light.
Whether it’s visible light from stars and galaxies, or invisible light like radio waves, ultraviolet rays, x-rays, or everything else on the electromagnetic spectrum.
Second, all the progress we have made in modern science, I would say since the sixteenth and seventeenth centuries, comes from the understanding of natural phenomena involving light.
And third, the two great revolutions of the twentieth century, the quantum revolution and the theory of relativity, come from big questions we’ve asked ourselves about the interaction of light with matter.
And this has led us to a technological revolution, because all the instruments that have changed our lives such as computers, lasers, internet, GPS, all come from the knowledge of the microscopic world that we get from quantum physics and the theory of relativity.
What are the big questions about light that still have no answer?
Quantum theory gives us a good explanation of the forces of nature: electromagnetic, strong and weak nuclear.
But there is one force that is outside that model: gravity (which is described in Einstein’s theory of general relativity).
And so, if you want to understand in depth what light is, we need a theory that unites quantum physics with Einstein’s theory of general relativity.
That is what we need.
We don’t know exactly what happens, for example, when light is trapped in a black hole. We don’t know what happens to a photon when it is subjected to such an amount of extreme gravity.
We need a theory that reconciles gravity with quantum physics.
And it’s interesting, because both theories were pioneered by Einstein in the early twentieth century, and are not yet compatible.
There are people who believe that something must be changed in both, or at least in one of the two, to make them compatible and achieve a theory of everything.
This is a very important topic in cosmology, because they are questions related to the early stages of the universe. That would help us better understand the origin of the universe and its evolution.
You study the enigmas of light, paradoxically two of the great mysteries of physics are at the other extreme: dark matter and dark energy. Can light help us unveil what these components of the universe are all about?
Those terms are interesting.
If we talk about dark matter and dark energy it means that they are something that has no light.
In fact, dark matter is a form of matter that doesn’t interact with light, so we haven’t detected it yet.
Then, light plays an important role in understanding what is happening. For now it’s an unanswered question.
That’s why it’s so important to find the connection between general relativity and quantum physics, we think understanding that would be helpful in achieving an answer.
It is a situation very similar to the one we had in the nineteenth century, when people talked about ether, a supposed mysterious medium in which waves propagated.
InstallIn solved the mystery, said that the ether did not exist, that we do not need it to understand the universe.
Perhaps something similar will happen with dark matter.
If we modify that theory, the dark matter issue will fade away or be replaced by something else.
But we don’t know yet, and that’s a big deal.
We need evidence and finding it is difficult, because it may require the use of much more powerful particle accelerators than those currently being built.
But we are making progress, and we are hopeful that in the coming decades we will be able to answer these questions.
Quantum computing holds promise for the future of technology. Every now and then we see that a computer company claims to have achieved “quantum supremacy”. Beyond commercial competition, what is the true state of quantum computing?
In quantum computing there is a lot of hype.
Over the last 20 or 30 years we have learned to manipulate individual quantum systems, isolate an atom, or isolate a photon and make them interact under very precise conditions.
The next step is to do that with a large number of atoms or particles.
Let’s see, each atom can have a bit of information.
An atom can be in two different states, which we call 1 or 0, but when you make them interact you get a quantum machine that forms cubits of information.
The difference between a quantum machine and a classical one is that the quantum machine can be in state 1 and 0 at the same time, which is called a superposition of states.
That phenomenon, in principle, can be used to build machines that would be much more powerful than classical computers, which can only be in state 1 or 0, but never in overlap.
The problem with quantum machines is that it is very difficult to maintain that state of superposition.
When you lose that ability to be in two states at once, we call it decoherence, because you lose quantum coherence (and you go to the classical world).
That decoherence is the fundamental phenomenon that explains why the macroscopic world is classical. It’s what explains why it’s impossible for you to see me in two different places at the same time.
So a quantum computer must be able to do something very paradoxical: it must preserve quantum coherence, which means protecting the system from the classical world.
But, at the same time, the machine must be able to connect with the classical world, because we are a macro world.
What happens, then, is that as soon as we try to interact with this quantum machine we cause a lot of decoherence.
So far, no one knows how to solve that problem.
Machines are being made that I would call “toy machines.” Google, for example, created a 53-cubit machine, and up to a few hundred cubits can be achieved, but to make it useful, millions of cubits are needed.
The greater the number of cubits, the more the decoherence is complicated. At present, no one knows how to overcome decoherence.
My feeling is that we are still in the realm of basic science, not applied science, and we don’t know what will be possible or how long it will take.
This research is fascinating and we learn a lot along the way, but we should not do so much hype because it can have the opposite effect if the promises are not kept.
Companies use grandiloquent terms like “quantum supremacy,” which don’t have much meaning beyond the definitions they use to show that their machines are better than others.
I saw at a conference that you were referring to multiverses, a term inspired by quantum physics widely used in science fiction.
You’re very astute if you found a talk where I talk about it, because it’s not an idea I like.
What happens is that in quantum physics every time you have an interaction between particles, the result of that interaction is random.
You can’t accurately predict what will happen. The only thing you can do is predict the chances of some things happening.
So if you excite an atom, it will emit a photon. But the position in which that photon is at the time of detection will be random. The only thing you can do is find probabilities within a time frame.
Some say that what is detected is only one of the possible outcomes, and that perhaps the other results occur in another universe.
And that’s what this idea of multiverses that develop simultaneously and according to which we simply inhabit one of those universes, consists of.
There is another idea about multiverses, which refers to us living in a bubble, but there are other bubbles in which other universes develop.
But again, these are just speculations.
For me, as an experimental physicist, the main criterion for encoTo enter the truth, if there is a truth, is to be able to make experiments and observations.
And speaking of science fiction, do you think humans will ever manage to travel at the speed of light?
No.
The theory of relativity is quite sound, it has been tested with great precision.
What it tells us is that the speed of light is the maximum possible speed, and it also tells us that to approach that speed you need a lot of energy.
If you want a large object to reach 99% of the speed of light, you would need all the energy generated by all physical phenomena on Earth.
So, there are physical limitations, but even if we achieve the speed of light, keep in mind that the nearest star to us is four light-years away, and stars where, for example, there might be another civilization, may be millions of light-years away.
So my feeling is that traveling outside the solar system, let alone outside our galaxy, is a science fiction dream.
Now, that’s not to say we can’t have results and observations.
Exploring exoplanets, for example, is a fantastic way to know if we’re the only ones in the universe.
And that’s a good example of what I call “useless science,” because it may be useless try to travel at the speed of light, but by trying, you can create instruments that can be useful.
In the same way, all the applications that have changed our lives, such as lasers, magnetic resonance imaging, GPS, cell phones, come from understanding how matter works at microscopic scales.
All this would have been impossible without the basic sciences, which are often considered useless.
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