Use of Squeezed Light to perform Distance Measurement below the Standard Quantum Limit
(from Left to Right) Nicolas Treps, Brahim Lamine and Claude Fabre
A team of researchers (B. Lamine, N. Treps and C. Fabre) from the Laboratoire Kastler Brossel (LKB) at the University Pierre and Marie Curie (Paris, France) have shown how to use squeezed light to perform distance measurement below the standard quantum limit imposed by the quantum nature of light .
Any distance measurement involves the propagation of light between two observers and the measurement of its phase (interferometric measurement, which gives distance within a wavelength) or its amplitude (time of flight measurement, giving absolute measurement). The quantum nature of light introduces fluctuations in the phase and the amplitude of the light used for ranging, therefore leading to a noisy measurement. The scientists have shown how to combine both a time of flight and a phase measurement, using frequency combs and homodyne detection, to minimize the effects of this quantum noise.
When classical light is used, then the sensitivity cannot go below what is called a standard quantum limit, which is smaller than previously existing standard quantum limits based either on interferometric or phase measurement. More interestingly, when squeezed frequency combs are used to perform the measurement, the sensitivity can significantly dive below the previous standard quantum limit. Squeezing light consists in tailoring its quantum fluctuations.
Ranging using frequency combs have already been proposed in the past  while it has long ago been realized that quantum resources is a way of improving ranging  (in particular entanglement and squeezing). Nevertheless the combination of both technology in an adapted optimal scheme is a major first.
Potential applications could be for future space-based experiments such as DARWIN (to detect Earth-like exoplanets) or LISA (to detect gravitational waves), and even for precise dispersion measurement. Indeed, when dispersion occurs, it does not affect in the same way the phase and the envelope --an effect which can be seen in the detection scheme proposed by the scientists.
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