GENERAL SCIENTIFIC SUMMARY
Introduction and background. Being able to remove a single photon from a light field can grant access to fundamental physics experiments and is an important tool towards future quantum applications. We have shown that, depending on the photon statistics of the initial light state, the subtraction of a photon may result in weird behaviors, very far from classical intuition.

Main results. In our experiments, single-photon subtraction from a state with a well-defined number of photons resulted in the intuitive decrease of their number by one. Surprises appeared when subtracting a single photon from a thermal state, the most common form of light (both the sun and ordinary light bulbs emit chaotic thermal light). The mean number of photons in the field after subtraction was doubled compared to the initial one. Finally, when subtracting a photon from a coherent state (the most classical, wave-like, state of light), we found that it stayed the same. Since their introduction by Nobel laureate Roy Glauber in the 1960s, coherent states have been a cornerstone in the quantum description of light. Being invariant under single-photon subtraction is a fundamental part of their definition, but it had never been verified so directly in an experiment.

Wider implications. Apart from providing a beautiful demonstration of the inner workings of quantum mechanics, the techniques used in these experiments can be exploited to arbitrarily engineer light. This capability will open the way to 'tailor-made' quantum light for future technologies, like the secure exchange of information or the development of novel protocols for quantum-enhanced measurements and communications.

Figure. Schematic of the single-photon subtraction process. An input light state is passed through a beam splitter of very low reflectivity. Whenever the single-photon detector in the reflected path clicks, a photon-subtracted state is conditionally generated in the transmitted path. We have shown that, for particular input light states, the photon-subtracted version can be exactly identical to the input, or even contain more photons than at the beginning, on average.

42.50.Xa Optical tests of quantum theory

42.50.Dv Quantum state engineering and measurements

Optics, quantum optics and lasers

Issue 12 (December 2008)

Received 13 October 2008

Published 9 December 2008