2009 CNRS Gold Medal
Serge Haroche : Photon Tamer
Serge Haroche is the recipient of the 2009 CNRS Gold Medal, the highest French scientific award. This outstanding physicist was able to carry out some of the most elaborate thought experiments ever imagined by the inventors of quantum physics, over a hundred years ago, in order to observe the infinitesimally small.
© C. Lebendinsky/CNRS Photothèque
Niels Bohr once argued that truth and clarity could not simultaneously be achieved, but Serge Haroche's work shows us that they can be,” says MIT Professor Daniel Kleppner, talking about the 2009 CNRS Gold Medal laureate.
As the leading pioneer of quantum theory, Niels Bohr, together with Werner Heisenberg, Erwin Schrödinger, and a few other scientific heavyweights, revealed the strange world of the infinitesimally small. They imagined a number of thought experiments to illustrate the state superposition of particles, and Serge Haroche, now 65, has worked hard over the last 30 years to turn these experiments into reality.
At such a scale, one can neither measure nor accurately predict both the velocity and position of a particle at the same time: it can be both here and there, or spin clockwise and counterclockwise. The quantum systems are so to speak “suspended” between different classical realities, their state being described by so-called “quantum superpositions.” “In our work, we observe the state superpositions of systems made of a few particles,” Haroche explains. “Our goal is to catch the moment when a system ceases to be quantum and becomes classical. This transition from one world to the other is called decoherence. It happens because the coherence of the system is somewhat disturbed by its macroscopic environment which forces it to take an unambiguous stand in the classical realm.”
Haroche has become a master in the art of catching quantum coherence, i.e., the state superpositions of particles. To observe this phenomenon, he confines a few photons to a trap (see box
), forcing them to bounce back and forth, more than a billion times, between two highly reflective mirrors. In this device, called an electromagnetic cavity, the captive photons survive more than a tenth of a second before decaying into the mirrors. This gives Haroche enough time to observe their quantum state and witness the decoherence phenomenon.
© C. Lebendinsky/CNRS Photothèque
Serge Haroche and his colleague Igor Dotsenko are setting up a cavity experiment.
In the 1970s, he was the first to suggest using highly excited atoms as probes to detect radiation with very high sensitivity. In the cavity experiments, these probes, called Rydberg atoms, cross the cavity and carry away with them a print of the photonic state, just like X-rays can take prints of a body's internal structures. In this way, the photons' state is revealed as an abstract picture containing all the information about its quantum features. Such studies of single quantum systems would have amazed Bohr, Heisenberg, or Schrödinger.
Haroche's interest in science began when he arrived in Paris in 1956, after a childhood spent in Casablanca (Morocco). In high school, he discovered a passion for math and realized with amazement that nature obeyed mathematical laws. Admitted to the prestigious “Ecole Normale Supérieure” (ENS) in 1963, he particularly appreciated the fact that he could immediately start working in a research lab.
Three years later, he began working on his PhD in the Laboratoire de Spectroscopie Hertzienne at ENS (now renamed Laboratoire Kastler Brossel1
), which was—and still is—at the forefront of quantum physics research. At the time, major leaders in the field were working there, like Jean Brossel, who was later awarded the CNRS Gold Medal (1984), or Alfred Kastler, the winner of the 1966 Nobel Prize in Physics. His thesis advisor, Claude Cohen Tannoudji (1997 Nobel Prize in Physics) guided Haroche's first steps into the world of atoms and photons. After an early appointment at CNRS, Haroche moved to Pierre and Marie Curie University where he became physics professor in 1975, while keeping his research activity at ENS. Between 1981 and 1993, he also held both visiting and part time positions at Stanford, MIT, Harvard, and Yale.
Today, Haroche is head of the “Electrodynamics of simple systems” group at the Kastler Brossel laboratory and professor of quantum physics at the Collège de France. He has authored more than 170 publications and has received a number of scientific awards. As for the CNRS Gold Medal, “I consider it an acknowledgment of a team's work and a mark of CNRS' commitment to basic research,” he says.
Haroche is already thinking about the future, with a full program ahead: “Our objective is to improve the cavity experiments in order to control decoherence: we want to find ways to delay it as much as possible and better understand why it occurs,” he adds. He also wants to further some of the possible applications of his work in quantum information physics. “This field has been thriving over the last 15 years and has led to the development of innovative technologies such as quantum cryptography.”
Haroche is a team player and never misses an opportunity to pay tribute to his students and co-workers, primarily Jean-Michel Raimond and Michel Brune, who have been his students and are now his colleagues and co-workers at the Kastler Brossel laboratory. “I could have done nothing without them,” he says. “The work rewarded today is their achievement as much as it is mine.”
Cats in a Cavity
Cavity experiments carried out by the team at the Laboratoire Kastler Brossel (LKB) study the strange nature of the principle of superposition by performing what physicists call a “Schrödinger's cat” experiment, in reference to a famous thought experiment formulated by Erwin Schrödinger in 1935.
To illustrate the superposition of particle states, Schrödinger imagined the following set-up: in a sealed box are placed a cat and a flask of poison whose condition depends on the state of a radioactive atom. If the atom decays, the flask shatters, killing the cat. If it doesn't decay, the flask remains intact, and the cat alive. According to quantum theory, there is a time during which the state of the atom is uncertain: it is just as likely to have decayed as to have remained intact. In other words, the atom is in a superposition of two states, and so is the cat: there is no way of knowing whether it is dead or alive, it is both things at once.
In the experiments at LKB, the cat is replaced by a handful of photons (from the microwave region of the electromagnetic spectrum). To capture the superposition of states, the team seals up the photons in a cavity, or more specifically between two circular walls 5 cm in diameter that face each other. The walls are covered with ultra-high-reflectivity mirrors cooled to a near absolute zero temperature. In this set-up, the photons bounce back and forth between the walls over a billion times, which means that they travel 40,000 km, the same distance as the circumference of Earth. This considerably delays the moment at which they are absorbed by the mirrors. Their lifespan is extended to 130 milliseconds, which is enough time to observe the quantum system before decoherence takes place.
But how can the quantum system be prepared in a Schrödinger cat state and then be observed? The key is to prepare and detect it with special probe atoms—atoms placed in a Rydberg state through laser excitation1—which are extremely sensitive to microwave photons. Crossing the cavity one by one, the Rydberg atoms manage to carry an imprint of the field state without absorbing light energy or significantly disturbing the system. From these imprints, the team creates a “quantum map,” a kind of image of the state of the field trapped in the cavity. Two peaks can be distinguished, the signatures of classical states (the equivalent of the “dead” and “alive” states of Schrödinger's cat) between which are interference fringes, signs of quantum superposition. When a quantum map of the field as a function of time is created, the fringes progressively fade away, revealing the process of quantum decoherence.
1. Named after the physicist Johannes Rydberg, one of the founders of atomic spectroscopy.
1. CNRS / ENS Paris / Université Paris-VI.