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Frontiers in Optics

Quantum position measurement of a shadow: beating the classical limit

Ermes Toninelli, M. Edgar, Paul‐Antoine Moreau, Graham M. Gibson, Giles D. Hammond, Miles J. Padgett

Published Jan 1, 2017• 0 citations
Publisher

Abstract

The precision with which the position of a shadow can be measured is classically limited by shot-noise. We achieve sub-shot-noise position sensitivity by jointly detecting correlated photons with a simple split-detector scheme.
DOI
10.1364/fio.2017.ftu5e.3

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The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector's quantum efficiency.
The limit of precision in optical measurements is due to quantum fluctuations, which are governed by Poissonian statistics. With a clever setup, however, the authors achieve a precision in transmission measurements that beats this limit, without the selective analysis of recorded data (post-selection). This shows that the single-photon-detector approach, used by many groups worldwide, is viable for real-world optical quantum metrology, not just proof-of-principle tests. In turn, this demonstration unlocks an array of techniques for quantum-state engineering that crucially depend on single-photon detection.
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.
Advancements in physics are often motivated/accompanied by advancements in our precision measurements abilities. The current generation of atomic and optical interferometers is limited by shot noise, a fundamental limit when estimating a phase shift with classical light or uncorrelated atoms. In the last years, it has been clarified that the creation of special quantum correlations among particles, which will be called here useful entanglement, can strongly enhance the interferometric sensitivity. Pioneer experiments have already demonstrated the basic principles. We are probably at the verge of a second quantum revolution where quantum mechanics of many-body systems is exploited to overcome the limitations of classical technologies. This review illustrates the deep connection between entanglement and sub shot noise sensitivity.
Advancements in physics are often motivated/accompanied by advancements in our precision measurements abilities. The current generation of atomic and optical interferometers is limited by shot noise, a fundamental limit when estimating a phase shift with classical light or uncorrelated atoms. In the last years, it has been clarified that the creation of special quantum correlations among particles, which will be called here useful entanglement, can strongly enhance the interferometric sensitivity. Pioneer experiments have already demonstrated the basic principles. We are probably at the verge of a second quantum revolution where quantum mechanics of many-body systems is exploited to overcome the limitations of classical technologies. This review illustrates the deep connection between entanglement and sub shot noise sensitivity.