Subtracting a photon of light sometimes adds a photon: What?!

William Atkins
Wednesday, 17 October 2007 20:06

Science - Energy

According to research by Italian physicist Marco Bellini, when he used a laser to remove a photon of light from a light pulse, the result sometimes produced more photons.                 

{mosloadpostion william}

In the September 28, 2004 issue of Science magazine, Bellini published his article “Quantum-to-Classical Transition with Single-Photon-Added Coherent States of Light.” He and his collaborators (Alessandro Zavatta and Silvia Viciani) found that if they added a photon of light to an existing light pulse with a laser, the pulse ended up with more photons. What they would normally expect in the real world.

However, when they took away a photon, the pulse sometimes contained MORE photons rather then less. A result that doesn’t seem to add up.

The abstract of their paper states: “Single-photon–added coherent states are the result of the most elementary amplification process of classical light fields by a single quantum of excitation. Being intermediate between a single-photon Fock state (fully quantum-mechanical) and a coherent (classical) one, these states offer the opportunity to closely follow the smooth transition between the particle-like and the wavelike behavior of light. We report the experimental generation of single-photon–added coherent states and their complete characterization by quantum tomography. Besides visualizing the evolution of the quantum-to-classical transition, these states allow one to witness the gradual change from the spontaneous to the stimulated regimes of light emission.” [Science magazine: http://www.sciencemag.org/cgi/content/abstract/306/5696/660?ck=nck]

They also performed the experiment as a pair: (1) first adding and then subtracting single photons to a light pulse and (2) first subtracting and then adding single photons to a light pulse.

Bellini found that the order of the operations—whether adding first and then subtracting, or subtracting first and then adding—makes a difference to the outcome.

They state that such a conclusion—the order of the operation—is a basic tenet of quantum theory, which is used within the field of quantum mechanics.

Quantum theory is not observed in the real world; that is, in large things we normally see everyday. It is only applicable in very tiny things such as within objects the size of atoms and smaller. When looking at these subatomic objects, quantum mechanics studies the relationship between matter and packets of energy (called quanta), or radiation.

According to Science News magazine (October 13, 2007), Bellini states, “Although [this law] underlies the entire quantum behavior of light, it had never been checked directly.”

Bellini and the collaborators are from the Department of Physics, University of Florence (Italy) and the European Laboratory for Nonlinear Spectroscopy.