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2012 Nobel Prize in Physics: Serge Haroche

Jean-Michel Raimond et Michel Brune
p. 15

Notes de la rédaction

Jean-Michel Raimond and Michel Brune have worked alongside Serge Haroche at the Kastler Brossel laboratory, at the École normale supérieure, for 35 years and 27 years, respectively.

Source: La lettre, no. 35, December 2012

Texte intégral

When Thought Experiments Become Real

1The world is made of atoms which emit, absorb and scatter light, the crucial vehicle of the information we receive from our environment. At the beginning of the last century, quantum theory revealed the strange laws which govern matter and radiation on a microscopic scale, in a counter-intuitive world where notions of wave and particle are intimately intertwined.

2Light is both a continuous wave and a set of discrete photons! This “strange” physics is founded on a principle of superposition. A microscopic system can exist in several possible states at the same time, so to say suspended between different classical realities. In their famous discussions, the founders of quantum theory resorted to thought experiments, in which they manipulated atoms and photons virtually. These experiments, long dreamt, are finally being materialized. Juggling with atoms and photons, making them interact in a controlled way is now a flourishing field of experimental research. Serge Haroche is one of its pioneers. He forced an atom to interact with a few photons in a “photon box” once conceived of by Bohr and Einstein, with almost ideally reflective walls. He thus observed atom-light interaction at its most fundamental. Serge Haroche and the ENS team (which he currently heads with the co-authors of this article) were among the initiators of cavity quantum electrodynamics, which has been developed fast over the last thirty years.

3Simple in principle, the ENS’s experiments are technically complex. Cavities, which resonate in the domain of microwaves, are made of superconductive mirrors facing each other. These are the best mirrors produced to date, of which light bounces several billion times before being absorbed or scattered.

4The atoms which interact with these photons are also very parti­cular. These are atoms in which an electron has been positioned on a highly excited circular orbit, with a radius (0.1µm) 2 500 times greater than that of the atom in its fundamental state. These “Rydberg atoms” have motivated extensive work over the last 30 years. Serge Haroche was one of the pioneers of these studies in the 1970s, demonstrating these atoms’ extreme sensitivity to microwaves and developing methods­ to prepare, manipulate, and detect them.

5With these unprecedented tools, Serge Haroche and the ENS team recently revolutionized, for instance, the way that photons are counted. Whereas conventional detectors (inclu­ding our eyes) destroy the photons they count, they elaborated a “transparent” detection process in which photons interact with the counting tool without being absorbed. The experiment consists in making the field, trapped in the cavity, interact with “probe” atoms. They move across the cavity one by one and, without absorbing the luminous energy, take away with them an imprint of the state of the field. The information on the ­number of photons is acquired gradually, as the successive atoms are detected, each one providing a partial contribution to the determination of the state of the field. When a photon subsequently disappears, absorbed by the mirrors’ imperfections, the field’s energy undergoes a sudden and discontinuous varia­tion, detected by the atoms. These quantum leaps, fundamental quantum processes, had never been observed in light before this experiment.

Photons thus travel forty thousand
kilometres in the narrow space (3 cm) between
the mirrors, leaving experimenters with one tenth of a second to manipulate
and observe them.

6The Zeno effect is another spectacular quantum manifestation illustrated by these experiments. In a paradoxical argument, this Greek philosopher denied the existence of an arrow’s movement since, he said, at each instant it was in a place and therefore immobile. A succession of immobilities cannot result in movement. While this sophism is of course false in a macroscopic world, it can become true in quantum physics where observation influences the object measured. The ENS team showed that the evolution of a field to be injected into a cavity is frozen if its number of photons is repeatedly and non-destructively counted.

7Atoms and cavities can also serve to explore the boundary between the classical and quantum worlds. In a crucial experiment, Serge Haroche and his colleagues monitored with an atom the state of a field containing a few photons. The field is in a quantum superposition of two radically different states. While in practice the atom controls the oscillation phase, it amounts to the same and is simpler to consider that it controls the amplitude. After interacting with the atom, the field is in a superposition of a state where it oscillates intensely (high amplitude) and a state where it does not oscillate at all (zero amplitude). This is an impossible situation in the classi­cal world, but a legitimate one according to quantum law. Such states are called “Schrödinger’s cats”, with reference to a thought experiment where it is imagined that a cat, trapped in a box with a radioactive atom, is placed in the uncomfortable situation of being suspended in a quantum manner between life and death. In real life, cats are either dead or alive! This is where deco­herence comes in. Under the coupling effect with the environment, macroscopic objects see their superpositions of states disappear very rapidly. Quantum ambiguity subsides to give way to the classical world of daily experience. The ENS team was able to monitor this phenomenon live, by observing the evolution of a Schrödinger’s cat made of a few photons. It showed that the decoherence time decreases as the number of photons increases. This explains why macroscopic systems, formed of a gigantic number of particles, always appear as classi­cal. Beyond the performance of thought experiments, cavity electrodynamics plays an important role in the development of quantum information, as science seeks to exploit the strange logic of the quantum world to process information. In conventional computers, information is coded in the form of classical “bits” that take on two mutually exclusive values, 0 and 1. Quantum information uses “quantum bits” or “qubits” that can exist in a superposition of states 0 and 1. The principle of superposition considerably expands the possibilities. Machines juggling with such qubits could perform certain computations much faster than current computers, or make the secrecy of information communication unbreakable. Very early on, the ENS team was able to produce basic building blocks of these machines. Although Rydberg’s atoms will probably not be the qubits of tomorrow’s tools, they have demonstrated the feasibility of operations now used with systems that are easier to integrate, like “circuit electrodynamics”, which uses microwave resonators made of parallel threads on a “chip” and super­conductive junctions instead of Rydberg’s atoms.

8Serge Haroche’s research has just been rewarded with the 2012 Nobel Prize, shared with David J. Wineland from the NIST (USA). There is a fine duality between these two adventures. The ENS team traps a few basic specs of light, photons, and manipulates them with basic specs of matter, atoms. The NIST team traps a few specs of matter (ions) and manipulates their quantum state with lasers, photon beams. These two teams have made very similar advances, sometimes simultaneously. Their works is driven by pure curiosity. Making thought experiments real requires complex methods and continuous effort which in both cases was only possible because these teams benefited from stable financial support, and from the contribution of generations of exceptional students. As it belongs to the field of pure research, their studies cannot however be envi­saged without a constant feedback loop between fundamental and applied research. It relies on technological advances and, in turn, will inspire the development of new devices.

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Jean-Michel Raimond et Michel Brune, « 2012 Nobel Prize in Physics: Serge Haroche », La lettre du Collège de France [En ligne], 7 | 2015, mis en ligne le 29 octobre 2015, consulté le 21 février 2017. URL :

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Michel Brune

Director of Research at the CNRS

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Jean-Michel Raimond

Professor at the UPMC Honorary member of the Institut universitaire de France (IUF)

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