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Biological measurement beyond the quantum limit


Dynamic biological measurements require low light levels to avoid damaging the specimen. With this constraint on optical power, quantum noise fundamentally limits the measurement sensitivity. This limit can only be surpassed by extracting more information per photon by using quantum correlations. Here, we experimentally demonstrate that the quantum shot noise limit can be overcome for measurements of living systems. Quantum-correlated light with amplitude noise squeezed 75% below the vacuum level is used to perform microrheology experiments within Saccharomyces cerevisiae yeast cells. Naturally occurring lipid granules are tracked in real time as they diffuse through the cytoplasm, and the quantum noise limit is surpassed by 42%. The laser-based microparticle tracking technique used is compatible with non-classical light and is immune to low-frequency noise, leading the way to achieving a broad range of quantum-enhanced measurements in biology.

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Figure 1: Experimental layout.
Figure 2: Schematic of the particle tracking method.
Figure 3: Particle tracking spectra.
Figure 4: MSD data.


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The authors thank Ping Koy Lam for facilitating the squeezing experiments, Bill Williams and Nicolas Treps for useful discussions about microrheology and spatial squeezing respectively, and Magnus Hsu for input on the experiments. This work was supported by the Australian Research Council Discovery Project (contract no. DP0985078). B.H. acknowledges financial support from the Alexander von Humboldt Foundation.

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M.A.T., W.P.B. and H-A.B. designed the experiment. M.A.T., J.K., J.J., B.H. and V.D. constructed the apparatus. M.A.T., J.J. and V.D. performed the experiments. M.A.T. analysed the data. M.A.T. and W.P.B. wrote the paper, with assistance from all co-authors.

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Correspondence to Warwick P. Bowen.

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The authors declare no competing financial interests.

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Taylor, M., Janousek, J., Daria, V. et al. Biological measurement beyond the quantum limit. Nature Photon 7, 229–233 (2013).

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