Sideband cooling beyond the quantum backaction limit with squeezed light

Squeezed light is used to sideband cool the motion of a macroscopic mechanical object below the limit imposed by quantum fluctuations. Squeezed light cools a mechanical system below its quantum limit Using techniques developed in quantum optomechanics, which studies the interaction between light and mechanical objects, researchers have been able to cool massive mechanical objects to temperature regimes close to the limit imposed by quantum fluctuations. These quantum fluctuations are a consequence of the uncertainty principle, because the position and momentum of a quantum-mechanical particle are never fixed, but rather fluctuate constantly. John Teufel and colleagues show how they can cool these massive mechanical objects even further by using 'squeezed' light—light in which the quantum noise, also arising from the uncertainty principle, has been reduced by redistributing the underlying uncertainty. This technique may allow for cooling larger and larger mechanical objects to lower and lower temperatures for metrology applications and fundamental tests of quantum mechanics. Quantum fluctuations of the electromagnetic vacuum produce measurable physical effects such as Casimir forces and the Lamb shift 1 . They also impose an observable limit—known as the quantum backaction limit—on the lowest temperatures that can be reached using conventional laser cooling techniques 2 , 3 . As laser cooling experiments continue to bring massive mechanical systems to unprecedentedly low temperatures 4 , 5 , this seemingly fundamental limit is increasingly important in the laboratory 6 . Fortunately, vacuum fluctuations are not immutable and can be ‘squeezed’, reducing amplitude fluctuations at the expense of phase fluctuations. Here we propose and experimentally demonstrate that squeezed light can be used to cool the motion of a macroscopic mechanical object below the quantum backaction limit. We first cool a microwave cavity optomechanical system using a coherent state of light to within 15 per cent of this limit. We then cool the system to more than two decibels below the quantum backaction limit using a squeezed microwave field generated by a Josephson parametric amplifier. From heterodyne spectroscopy of the mechanical sidebands, we measure a minimum thermal occupancy of 0.19 ± 0.01 phonons. With our technique, even low-frequency mechanical oscillators can in principle be cooled arbitrarily close to the motional ground state, enabling the exploration of quantum physics