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06/11/2015, 14:00 — 15:00 — Room P9, Mathematics Building

Eugene Polzik, *University of Copenhagen*

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Measurements not restricted by the Heisenberg uncertainty principle

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 ago by Braginsky and colleagues. We have recently implemented such a measurement experimentally using a collective spin of an atomic ensemble precessing in a magnetic field as an oscillator [1]. 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 dB squeezed state of the oscillator. The demonstrated spin-squeezed state of 10^{8} atoms with an angular spin variance of 8∙10^{-10} rad^{2} allowed us to achieve 150 femtoT/√Hz magnetic field sensitivity.

An even more striking notion is the back-action evasion in *both* quadratures for the *continuous* measurement on a mechanical oscillator entangled with the atomic spin oscillator [2]. Such measurement does not violate quantum mechanics, but still provides a way to detect disturbances on the oscillator trajectory which are way below those set by the standard Heisenberg uncertainty bound by using a negative mass reference system [3]. Experimental demonstration of this ABATE (Atomic Back-action Eater) principle will be discussed.

**References**

[1] G. Vasilakis, H. Shen, K. Jensen, M. Balabas, D. Salart, B. Chen, and E. S. Polzik. Nature Physics, doi:10.1038/nphys3280 (2015).

[2] K. Hammerer, M. Aspelmeyer, E.S. Polzik, P. Zoller. Phys. Rev. Lett. **102**, 020501 (2009).

[3] E.S. Polzik and K. Hammerer. Annalen der Physik. **527**, No. 1–2, A15–A20 (2015).