During the theoretical investigation of the ultimate sensitivity of gravitational wave detectors through the 1970's and '80's, it was debated whether quantum fluctuations of the light field used for detection, also known as photon shot noise, would ultimately produce a force noise which would disturb the detector and limit the sensitivity. Carlton Caves famously answered this question with "They do." [1] With this understanding came ideas how to avoid this limitation by giving up complete knowledge of the detector's motion [2][3][4]. In these back-action evading (BAE) or quantum non-demolition (QND) schemes, one manipulates the required quantum measurement back-action by placing it into a component of the motion which is unobserved and dynamically isolated. Using a superconducting, electro-mechanical device, we realize a sensitive measurement of a single motional quadrature with imprecision below the zero-point fluctuations of motion, detect both the classical and quantum measurement back-action, and demonstrate BAE avoiding the quantum back-action from the microwave photons by 9 dB. Further improvements of these techniques are expected to provide a practical route to manipulate and prepare a squeezed state of motion with mechanical fluctuations below the quantum zero-point level, which is of interest both fundamentally [5] and for the detection of very weak forces [6].Since the discovery of Shor's Algorithm [7] almost 20 years ago, a major theme in physics has been about the untapped power and benefits of quantum phenomena, largely stemming from the resource of quantum entanglement. However much earlier, it was understood how quantum physics places limits on our knowledge [8,9]. This limitation can be useful, as in the case of quantum cryptography schemes where the required quantum measurement back-action of an eavesdropper leaves its trace on the transmitted information, providing proof of their snooping. For measurements of position, this limitation, called the Standard Quantum Limit (SQL) [9] is not beneficial: back-action due to the quantum nature of the measurement field, ultimately obscures our vision for a sufficiently sensitive measurement.Quantum limitations on the detection of position are no longer academic issues; in recent years, the detection of motion has now advanced to the point that quantum back-action engineering is now required to improve the sensitivity. Detections of motion have been realized with imprecision below that at SQL [10,11]. Back-action forces from the quantum noise of the detection field have been demonstrated to drive the motion of mechanical oscillators, first with electrons in an electro-mechanical structure [12] and then with photons in opto-mechanical systems [13,14]. In this work, we demonstrate the backaction forces due to the shot noise of microwave photons, which are 10 4 times lower in energy than optical photons.Strategies to manipulate the quantum measurement back-action have included modifying the quantum fluctuations of the measurement field [15,16], and modulati...

We study theoretically the dynamics of a hybrid optomechanical system consisting of a macroscopic mechanical membrane magnetically coupled to a spinor Bose-Einstein condensate via a nanomagnet attached at the membrane center. We demonstrate that this coupling permits us to monitor indirectly the center-of-mass position of the membrane via measurements of the spin of the condensed atoms. These measurements normally induce a significant backaction on the membrane motion, which we quantify for the cases of thermal and coherent initial states of the membrane. We discuss the possibility of measuring this quantum backaction via repeated measurements. We also investigate the potential to generate nonclassical states of the membrane, in particular Schrödinger-cat states, via such repeated measurements.

We review a scheme for performing a backaction-evading measurement of one mechanical quadrature in an optomechanical setup. The experimental application of this scheme has been limited by parametric instabilities caused in general by a slight dependence of the mechanical frequency on the electromagnetic energy in the cavity. We find that a simple modification to the optical drive can effectively eliminate the parametric instability even at high intracavity power, allowing realistic devices to achieve sub-zero-point uncertainties in the measured quadrature.