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Serge Haroche "Einstein’s thought experiments brought to life"

Translated Tuesday 20 November 2012, by Kristen Boydstun and reviewed by Kristen Boydstun

Nobel Prize in Physics (2012), French National Center for Scientific Research Gold Medal (2009), professor and director at the Collège de France, Serge Haroche has devoted his life to exploring the strange quantum world. Using groundbreaking experimental methods, he has achieved Einstein’s dream: to isolate and study an atom or photon.

You received the Nobel Prize in Physics on October 9th (with American David Wineland). What changes does this bring for you?

Serge Haroche. Since the announcement, I am often interviewed by journalists who, like you, want to ask me questions about quantum physics! Usually the general public is not at all interested in our research. This prize serves as a wonderful sounding board for our work. It’s challenging to explain the main ideas of the fundamental research that we conduct. It could be an interesting anthropological study to understand why this prize gives, all of the sudden, such an audience to the scientists who receive it.

Is anything about this perspective specific to France?

Serge Haroche. My American colleague, who shares the Nobel Prize with me, is also under strong media pressure, but less so since there are more Nobel Prizes awarded in the United States. This media pressure will lessen, but the pressure from the scientific community will not, and my problem these days is knowing how to say no and how to choose my priorities. I was appointed director of the Collège de France a month before receiving the Nobel Prize, so I will have to balance the duties of a Nobel Laureate with those of a director!

In 2009, you received the Gold Medal from the French National Center for Scientific Research (CNRS). At the time you confided a passion for mathematics and physics dating back to high school. You stated, "I was fascinated by the fact that nature can be understood by mathematical laws, and I was quickly drawn to physics, which adds a major constraint to mathematics: reality.”

Serge Haroche. While I was in high school at the Lycée Louis-le-Grand in Paris, my friends were all math enthusiasts. I was fascinated by the fact that nature obeys mathematical laws: I wondered how it was possible, given that the laws were so simple. You can actually write down on a sheet of paper the equations that govern the physical world around us! I loved the idea that we can have a grasp on the real world from mathematics. It was the time of the first satellites, of launching rockets into space... And with the simple knowledge of Newton’s laws, it was possible to calculate the speed a rocket must achieve in order to orbit the Earth!

How did you choose your research area?

Serge Haroche. I was interested in research to understand the world better and potentially develop practical applications. I chose a particular research area by chance. When I entered the École normale supérieure, I was captivated by certain professors’ courses, which led me towards atomic physics and in particular quantum optics. It was the era of the first lasers, which were extraordinary tools for manipulating atoms and testing nature. Therefore I had the opportunity to be involved in this great optical revolution.

You received the Nobel Prize in Physics for your "ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems," according to the Nobel Committee. More specifically, you invented a "trap" that let you capture and observe individual photons. In what way is this particle so important?

Serge Haroche. The photon is an elementary particle of light whose existence was recognized by Einstein at the start of the 20th century. He was the first to understand that light is both a wave and a collection of particles, which we call photons. The emission and absorption of light by matter are fundamental phenomena since 99% of the information that we receive from the outside world is transported by light: we see ourselves when there is light; we observe the universe by receiving light from the stars... this is visible light. There is also invisible light: cell phones work by receiving microwave radiation, which is light but at a wavelength that is not visible. We are trying to understand the fundamental interaction between matter and light. For this, we need to understand how atoms emit and absorb photons. These are the concerns of quantum optics.

Photons are part of the infinitely small world and thus obey the laws of quantum physics; is that why it is so difficult to observe them?

Serge Haroche. Quantum laws govern the particle world. They are counter-intuitive laws: they escape the logic of the classical world. For example, if we allow an atom to pass through two holes, it will pass through the two holes at the same time, while a classical particle will choose only one of the two holes. In the quantum world, systems can exist in a superposition of states. An atom can thus be in two places at once, have two speeds at the same time. A photon can have two polarizations at a time. These concepts are hard to understand classically, but they become apparent once we are able to isolate very small quantum systems.

Forty years ago or so, when you started to perform your research, lasers already allowed the study of atoms. Why wasn’t this enough to observe the behavior of light particles?

Serge Haroche. Indeed, I was working in the Kastler-Brossel Laboratory, named after its two founders who invented optical pumping. This involves studying the behavior of atoms by using light to put them in unusual energy levels, that is, in situations not seen in nature. But in these vast systems holding large numbers of atoms and photons, quantum effects are masked beneath piles of disturbances. To highlight the particles’ quantum behavior, we therefore had to be able to manipulate smaller systems. For example, a single photon. For this we needed to come up with more sophisticated methods. With the development of computers and lasers, we devised techniques to manipulate individual atoms so we could observe, in an almost surgical way, the quantum behavior of the systems.

How did you manage to trap and observe a single photon?

Serge Haroche. We invented a photon box which allowed us to isolate the system in a setting where quantum effects would show up in the cleanest way possible. We had to preserve these particles of light for as long as possible in order to see them and study their behavior. The most challenging part was detecting them without destroying them. When we detect light, we destroy it: this is the photoelectric effect discovered by Einstein. All methods developed before ours destroyed the photons upon detection. Over the past two decades, our dream has been to trap a photon in a cavity and send in atoms one by one, such that each atom makes a measurement of the photon without destroying it. This dream became a reality in 2006, when we indisputably saw a photon for the first time. The Nobel Prize recognized two experiments that we performed, one in 1996, where we managed to keep photons for under a thousandth of a second, and one in 2006, where we saw a single photon for a tenth of a second. Between these two experiments, we improved our photon trap technology considerably. This led us to our spectacular result, without which we wouldn’t have won the Nobel Prize.

What do these observations teach us?

Serge Haroche. Our experiments confirm the validity of quantum laws as the theory was established from 1920-1930. At the time, physicists dreamed up experiments that would give them insight into the concepts. Our achievement is having carried out these experiments that the great physicists deemed impossible. Next, learning to tame the atom and the photon at the level of individual particles could allow us to develop applications.

So in your opinion, what is the difference between fundamental research and applied research? Is this distinction pertinent?

Serge Haroche. Our primary goal is fundamental research. We are motivated by intellectual curiosity or by performing an experiment that has never been done before. We don’t promise an application, and we don’t work towards a utilitarian goal. Of course, there are a multitude of problems for which new applications need to be developed, but these can only be born from the basis of fundamental research. The majority of modern technologies, very often founded upon quantum physics, result by chance and from a combination of fundamental research which was not directed towards this goal. An example: when the laser was discovered, we had no idea what its use would be. Now, however, by sending laser light through ultra-transparent fibers, which were only made after the invention of the laser, we developed optical communication technology, allowing an enormous amount of information to be transmitted over the internet at the speed of light... This is an application that couldn’t have been predicted at the time the laser was discovered. To develop new applications, you must allow time for luck and chance, essential elements in fundamental research.

Fundamental research is no stranger to challenges. Did you face any obstacles?

Serge Haroche. Personally, I was lucky. I got started in research in a privileged environment, the Kastler-Brossel Laboratory at the École normale supérieure, where the directors recognized the quality of the researchers and provided them the means to work without being forced to write reports and fill in bureaucratic forms all day. When I took off on my own, I had already established my reputation, and therefore I didn’t have any trouble obtaining funding.

Going beyond your experience, what is your view on the conditions for fundamental research in France?

Serge Haroche. I see two major issues. First, the structure of the research system is too complex, we have too much bureaucratic red tape, and research programs often require scientists to pretend that their work will have applications because it’s the only way to get money. This situation puts researchers in a counter-productive bind. Next, the salary of young researchers in France is too low. It’s not acceptable, after ten long years of difficult study, to have a salary less than 2,000 euros per month. We should also increase their opportunities to start ambitious projects funded in the long-term. In a broader perspective, we need to resolve the imbalance between the long timescale of research, crucial for addressing the major challenges facing mankind, and the timescale of politics, which is much shorter. At the Collège de France, we are finishing a renovation program, a large part of which is designed to welcome physics and chemistry laboratories. We are going to create an incubator for young researchers in partnership with the CNRS. We will welcome and encourage young teams who will be given the time – up to eight years – to start ambitious research projects.

An excellent track record 

Physics Nobel Prize in 2012, with American David Wineland, 2009 Gold Medal recipient from the CNRS, the highest scientific distinction in France, Serge Haroche has always studied and sought to understand the transition from the infinitely small world, the quantum world, to the macroscopic world in which we live. To do this, he invented innovative experimental devices. Born in 1944 in Casablanca, Morocco, Serge Haroche studied at the École normale supérieure, where he completed his thesis under the supervision of Claude Cohen-Tannoudji, 1997 Nobel Laureate in Physics. He started his career at the CNRS. Appointed professor in 1975 at the University Pierre et Marie Curie, he also lectured at the École Polytechnique, normale supérieure, Yale, Stanford and Harvard. Professor and holder of the chair in Quantum Physics at the Collège de France since 2001, he directs the Electrodynamics of Simple Systems group at the Kastler-Brossel Laboratory, in the physics department of the École normale supérieure. Member of the French Academy of Sciences, he was named director of Collège de France on September 1st last year.

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