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

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(1-3). Here, we report the generation of a squeezed state of an oscillator by a stroboscopic back-action-evadingmeasurement. 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 10(8) atoms with an angular spin variance of 8 +/- 10(10) rad(2) is promising for magnetic field sensing.