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Accueil du site > Séminaires > Autres > Séminaire exceptionnel : Optical precursor : speed limit of photons
by Shengwang Du, professor at Hong Kong University of Science and Technology (HKUST)

Séminaire exceptionnel : Optical precursor : speed limit of photons
by Shengwang Du, professor at Hong Kong University of Science and Technology (HKUST)

Mercredi 3 juillet à 14h, salle D5 / L278. Ecole Normale Supérieure, département de physique, 24 rue Lhomond 75005 Paris

Abstract :

The principle of invariant light speed c in vacuum lays down the foundation of Einstein’s special relativity and causality. However, in a dispersive medium, light wave propagation is described by phase and group velocities which, in both principle and reality, can exceed c, the speed of light in vacuum.

Therefore, whether the information carried by a light pulse or a single photon can travel faster than c becomes an important and fundamental question to understand the special relativity and causality. Answering this question led to the study of optical precursor initiated by Sommerfeld and Brillouin in 1914. They theoretically predicted that the optical precursor, which refers to the propagation of the front of a step optical pulse, always travels at c in any dispersive medium. Since then, seeking for the existence of optical precursor has become a great challenge to experimentalists. In 2009, for the first time, we experimentally observed freely-running optical precursors and successfully separated them from delayed main fields using square laser pulses passing through a dense laser-cooled atomic ensemble with electromagnetically induced transparency.

Thus we fully verify the theoretical prediction about 100 years ago. Our result confirms that the optical precursor always travels at c, even in a so called superluminal medium where the group velocity is faster than c or negative. With stacked optical precursors, we generated short optical pulses having peak powers about 10 times the input power through a passive and absorptive system without gain. Most recently (in 2011), we generated and observed the precursor of a single photon and verify that the optical precursor, which travels at c in any medium, is always the fastest part of a single-photon wave packet. There is no probability for a single photon moving faster than c, the speed limit, even in a superluminal medium.

Our result closed the long-standing debate about the speed of a single photon and suggests that there is no way to accelerate a photon to exceed its vacuum speed limit and violate the causality. Our observation of the optical precursor of a single photon provides a physical picture in understanding correctly the transmission speed limit of optical quantum information.