Generation of a squeezed state of an oscillator by stroboscopic back-action-evading measurement

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Continuous observation of an oscillator results in quantum back-action, which limits the knowledge acquired by the measurement. A careful balance between the information obtained and the back-action disturbance leads to the standard quantum limit of precision. This limit can be surpassed by a measurement with strength modulated at twice the oscillator frequency, resulting in a squeezed state of the oscillator motion, as proposed decades ago1,2,3. Here, we report the generation of a squeezed state of an oscillator by a stroboscopic back-action-evading measurement. The oscillator is the spin of an atomic ensemble precessing in a magnetic field. The oscillator initially prepared nearly in the ground state is stroboscopically coupled to an optical mode of a cavity. A measurement of the output light results in a 2.2 ± 0.3 dB squeezed state of the oscillator. The demonstrated spin-squeezed state of 108 atoms with an angular spin variance of 8 × 10−10 rad2 is promising for magnetic field sensing.

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Figure 1: Magnetic oscillator.
Figure 2: Outline of the experimental set-up.
Figure 3: Noise characterization of the oscillator state prepared without any conditional evolution.
Figure 4: Conditional preparation of a squeezed oscillator.


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This work was supported by the ERC grant INTERFACE, DARPA project QUASAR and EU grants SIQS and MALICIA. K.J. acknowledges support from the Carlsberg Foundation. G.V. gratefully acknowledges help and support from P. Karadaki.

Author information

G.V. and H.S. contributed equally to this work. G.V., H.S., K.J., D.S. and B.C. performed the experiments and contributed to the analysis, M.B. and D.S. fabricated the microcell. G.V., H.S., K.J. and E.S.P. wrote the paper and all the authors provided feedback to the manuscript. E.S.P. supervised the research.

Correspondence to E. S. Polzik.

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Vasilakis, G., Shen, H., Jensen, K. et al. Generation of a squeezed state of an oscillator by stroboscopic back-action-evading measurement. Nature Phys 11, 389–392 (2015) doi:10.1038/nphys3280

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