Steven Steinke

Mechanically detecting and avoiding the quantum fluctuations of a microwave field

Avoiding back-action in quantum measurements The very process of measuring a quantum system has an influence on the system through the process of back-action. Suh et al. used a back-action evasion scheme to monitor the motion of a miniature oscillator without influencing its motion (see the Perspective by Bouwmeester). The scheme should help in the understanding of the fundamental limits associated with measurement and will have practical implications in providing a low-temperature thermometer and a probe of extremely weak forces. Science, this issue p. 1262 The measurement-induced back-action effects on a quantum system can be avoided. [Also see Perspective by Bouwmeester] Quantum fluctuations of the light field used for continuous position detection produce stochastic back-action forces and ultimately limit the sensitivity. To overcome this limit, the back-action forces can be avoided by giving up complete knowledge of the motion, and these types of measurements are called “back-action evading” or “quantum nondemolition” detection. We present continuous two-tone back-action evading measurements with a superconducting electromechanical device, realizing three long-standing goals: detection of back-action forces due to the quantum noise of a microwave field, reduction of this quantum back-action noise by 8.5 ± 0.4 decibels (dB), and measurement imprecision of a single quadrature of motion 2.4 ± 0.7 dB below the mechanical zero-point fluctuations. Measurements of this type will find utility in ultrasensitive measurements of weak forces and nonclassical states of motion.

Role of quantum fluctuations in the optomechanical properties of a Bose-Einstein condensate in a ring cavity

We analyze a detailed model of a Bose-Einstein condensate (BEC) trapped in a ring optical resonator and contrast its classical and quantum properties to those of a Fabry-Perot geometry. The inclusion of two counterpropagating light fields and three matter field modes leads to important differences between the two situations. Specifically, we identify an experimentally realizable region where the systems behavior differs strongly from that of a BEC in a Fabry-Perot cavity, and also where quantum corrections become significant. The classical dynamics are rich, and near bifurcation points in the mean-field classical system, the quantum fluctuations have a major impact on the systems dynamics.

Quantum-measurement backaction from a Bose-Einstein condensate coupled to a mechanical oscillator

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 Schrodinger-cat states, via such repeated measurements.

Quantum backaction in spinor-condensate magnetometry

We provide a theoretical treatment of the quantum backaction of Larmor frequency measurements on a spinor Bose-Einstein condensate by an off-resonant light field. Two main results are presented; the first is a “quantum jump” operator description that reflects the abrupt change in the spin state of the atoms when a single photon is counted at a photodiode. The second is the derivation of a conditional stochastic master equation relating the evolution of the condensate density matrix to the measurement record. We provide a few examples of the application of this formalism and comment on its application to metrology.

Generation of mechanical squeezing via magnetic dipoles on cantilevers

A scheme to squeeze the center-of-mass motional quadratures of a quantum mechanical oscillator below its standard quantum limit is proposed and analyzed theoretically. It relies on the dipole-dipole coupling between a magnetic dipole mounted on the tip of a cantilever to equally oriented dipoles located on a mesoscopic tuning fork. We also investigate the influence of several sources of noise on the achievable squeezing, including classical noise in the driving fork and the clamping noise in the oscillator. A detection of the state of the cantilever based on state transfer to a light field is considered. We investigate possible limitations of that scheme.

Optomechanical backaction-evading measurement without parametric instability

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.