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American Journal of Physics

Quantum noise detection: A portable and educational system

Jean‐François Morizur, M. Colla, Hans‐A. Bachor

Published Oct 14, 2008• 3 citations
Publisher

Abstract

Quantum noise is a key feature of laser beams. It is both a limiting effect in contemporary optical measurements and a manifestation of the quantum nature of light. Its properties distinguish it from classical noise. We demonstrate a simple, reliable, and portable apparatus using low cost commercial lasers and electronics that provides evidence of these properties.

DOI
10.1119/1.2969722

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Quantum noise is a key feature of laser beams. It is both a limiting effect in contemporary optical measurements and a manifestation of the quantum nature of light. Its properties distinguish it from classical noise. We demonstrate a simple, reliable, and portable apparatus using low cost commercial lasers and electronics that provides evidence of these properties.
Both quantum and classical noise are fundamental features of laser beams. This work introduces a quantum optics model to distinguish the two types of noise in a Michelson interferometer experiment for undergraduate education. By measuring photon flux noise in an interferometer, this lab enables undergraduates to observe quantum fluctuations in real data, distinguish quantum noise from classical noise, and develop hands-on skills in optical alignment and quantum measurement. By turning abstract quantum concepts into tangible experiences, it provides foundational training for emerging quantum technologies.
Commercial optical communication is presently based on incoherent detection, while the application of more complex coherent detection schemes is still being developed. In both cases laboratory systems are approaching a limit that seems to tie the hands of the communication engineer – a purely quantum effect known as shot noise. It has been realised, however, that this is not as fundamental a limit as it may appear. New possibilities become available using light with a special property called 'squeezed noise'.
For many years the "shot-noise limit" has been considered as the fundamental limitation on measurements made with light beams. Shot noise is due to the quantum nature of light: the number of photons detected in a given time from a "stable" light beam (for instance, a laser beam) is an intrinsically random number which has fluctuations of order N1/2 if the average result is N. So if the number of photons, N, is limited by time or power considerations, shot noise places a fundamental limitation on the precision to which intensity, and changes in intensity, can be measured. This is particularly critical for "one-shot" ultra-sensitive experiments, such as the detection of a gravitational burst using large-scale interferometers.
Photons and laser beams provide an ideal situation to discuss some of the mysteries of quantum physics. There are many additional fundamental features which are not accessible with a classical system such as an electric current, which include: quantum noise, the uncertainty relation and correlations between different laser beams. Not only is the difference between the quantum behavior of photons and classical waves a curious effect, it is also important in many technical applications and forms the basis of future technologies, such as improved optical sensors or quantum cryptography. Using the example of a simple beam splitter and its effect on a laser beam one can explore these mysterious quantum effect with undergraduate students. We will discuss a systematic series of cases, using the beam splitter, which identify the difference between the quantum and the classical world. We will present the technical details of an experiment suitable for third year students that demonstrate genuine quantum noise effects.
We present detailed instructions for the construction of a pyramidal-style laser cooling and trapping apparatus. This scheme requires only a single beam, rather than the three pairs of orthogonal beams of the standard magneto-optical trap, which greatly simplifies the geometry and substantially reduces the cost. The trap is based largely on low-cost commercially available items and is simple to construct. It is remarkably insensitive to alignment and reliable to operate. Using a single laser beam with an intensity of 1.3 mW/cm2 we cool and trap more than 4 million rubidium atoms.