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Serge Haroche: “Quantum theory..."

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Serge Haroche: “Quantum theory, the universal substrate of knowledge about nature”

Serge Haroche, Photo : C.Lebedinsky-CNRS

Collège de France Professor Serge Haroche directs the Electrodynamics of Simple Systems team within the Kastler Brossel laboratoryNouvelle fenêtre (CNRS/ENS/UPMC). Founded in 1952, the “LKB” has since become one of the major players in quantum physics worldwide. Here the scientist, who has just received the 2009 CNRS gold medal, answers questions on the fields of optics and quantum information with luminous simplicity.


What is quantum optics and why study it?

“It is the study of the interaction between atoms and light. Most information comes to us through light, from information on stars and the universe to information on the objects that surround us. Of course, as well as looking at visible rays, we study infrared and ultraviolet light and the radio- and micro-waves shooting through space in all directions that give us a huge quantity of information! Quantum optics helps us to understand natural phenomena and to develop new technologies.


You have been passionate about this field since the very beginning of your career. Why?

Because it has seen a succession of extremely important revolutions. Light has long been used as a “probe”. When you receive the light from a star and break it down into its different wavelengths, you obtain a spectrum which forms a “signature” of the elements which diffused or absorbed light on the surface of the star. Spectroscopy is also an important part of the study of the structure of matter on Earth. But as early as the 1950s, Kastler and Brossel developed methods of optical pumping which make it possible to use light to place atoms in situations of “imbalance”, different from those in which they are observed in nature. These experiments have developed considerably and others have appeared, such as using lasers to cool atoms. In fact, for the last forty years the Laboratory has been using light not only as a “probe”, but as an instrument for manipulating matter.


How do you dissect atom-light interaction?

By using cavity quantum electrodynamics: fabricating a meeting between a single atom and one or more photons in an environment protected from any exterior perturbations. This makes it possible to observe the most elementary manifestations of their interaction.


What happens on such a small and protected scale?

The laws of quantum physics, which are so different from those of the macroscopic scale, really stand out. They obey the principle of the superposition of states; if a system can exist in states which correspond to several different classic situations, it can exist in all these states at once. For example, an atom or an electron can become suspended, so to speak, between several different positions or follow different trajectories at the same time to go from one point to another. But when you increase the size of the objects towards that of the macroscopic world, quantum superpositions disappear. That is the phenomenon of decoherence. Understanding this phenomenon of decoherence and the nature of the frontier between the classic and quantum worlds, through simple experiments on a few atoms and photons confined in a box, is an essential aspect of our experimental work. For example, we prepared a field of a few photons in a state of superposition, “suspended” between two classical realities, and observed how this superposition gradually fades, giving way to a system described by classical laws.


May I ask you for a simpler explanation?

It is practically impossible to explain using terms from everyday language, precisely because they are adapted to the observation of the macroscopic world. Metaphors can be pushed to the point at which they give a false impression. In mathematical language, the equations obeyed by the quantum system are by no means mysterious; they are simple and their interpretation is clear.


Which point is the laboratory currently focusing on?

Having passively observed decoherence, we are now trying to stretch it out by maintaining our field of a few photons in a state of quantum superposition for as long as possible. The experiment consists of using certain atoms to probe the system and observe decoherence, then other atoms which, depending on the information provided by the first atoms, modify the field to re-establish the quantum superpositions.
This way we are getting closer to situations where we could directly exploit quantum logic for computation or communication. This aspect of our research belongs to a rapidly developing field: quantum information.


What can quantum information contribute to computer science?

The computer would not exist had we not understood the quantum mechanisms which underpin the transistor and integrated circuits, and the physical properties of solids. Conversely, modern physics would be impossible without computers, as it requires such a large storage and calculation capacity. Quantum physics and the computer are therefore mutually indispensable from a practical viewpoint – it is a marriage of convenience. Quantum information does not try simply to use quantum physics to accelerate computing within the binary logic of the computer, but to make the computer adopt quantum logic in order to exploit the superposition of states. The marriage of convenience would then become a “spiritual” marriage. A machine which worked in this way could utilize quantum parallelism to perform certain types of computing much more efficiently than a classic computer. However, the obstacles which need to be overcome before this theory can be put into practice remain daunting. The most significant is the way decoherence contradicts quantum logic. Despite important advances correcting errors produced by this phenomenon, we are still way off target. Even if the dream quantum computer does not become reality, research in this field is likely to have as yet unforeseen practical consequences.

Can you give me an example of an important project you have worked on?

Classic methods of single photon detection destroyed the grain of light as soon it was detected, which obliged scientists to do a “post mortem” analysis. We worked for around fifteen years to create a photon box able to keep a photon alive for a fraction of a second. During this time, the atoms are sent into the cavity and take with them its imprint, which is what we detect. We could develop this more subtle technique for quantum information. In fact, if it is possible to conserve the information transported by the photon and share it with a large number of atoms, we could create extremely useful systems of quantum memory. To improve the quality of these experiments we need cold, more easily controlled atoms. Fortunately, we can call on the expertise of the other teams in our laboratory who work in this field.


So you don’t work on your own …

Certainly not. I benefit, of course, from exchanges with the Laboratory’s other groups. But I would like especially to emphasize the close collaboration within my own team, in terms of both understanding and performing experiments. I co-direct our research team with Jean-Michel Raimond, Director of the ENS physics departmentNouvelle fenêtre, professor at UPMC and the Institut Universitaire de FranceNouvelle fenêtre, and Michel Brune, research director at the CNRS. We are very close-knit and always work together, with the same passion for advancing knowledge of the behavior of atoms and photons. Nothing would have been possible without the collaboration we have maintained over many years.


Interview by Emmanuelle Manck