Abstract
Quantum electro-mechanical systems offer a unique opportunity to probe quantum noise properties in macroscopic devices, properties which ultimately stem from the Heisenberg Uncertainty Principle. A simple example of this is expected to occur in a microwave parametric transducer, where mechanical motion generates motional sidebands corresponding to the up and down frequency-conversion of microwave photons. Due to quantum vacuum noise, the rates of these processes are expected to be unequal. We measure this fundamental imbalance in a microwave transducer coupled to a radio-frequency mechanical mode, cooled near the ground state of motion. We also discuss the subtle origin of this imbalance: depending on the measurement scheme, the imbalance is most naturally attributed to the quantum fluctuations of either the mechanical mode or of the electromagnetic field.
Highlights
We discuss the subtle origin of this imbalance: depending on the measurement scheme, the imbalance is most naturally attributed to the quantum fluctuations of either the mechanical mode or of the electromagnetic field
A fascinating aspect of quantum measurement is that the outcome of experiments and the apparent nature of the object under study depend critically on the properties of both the system and the measurement scheme [1]
Our system is composed of a superconducting microwave resonator, referred to as a “cavity,” where the resonance frequency is modulated by the motion of a compliant membrane [13]. This frequency modulation leads to the desired parametric coupling between microwave field and mechanical motion [Fig. 2(a)]
Summary
A fascinating aspect of quantum measurement is that the outcome of experiments and the apparent nature of the object under study depend critically on the properties of both the system and the measurement scheme [1]. For a mechanical oscillator in a thermal state with an occupation factor nm, the spectral densities follow Sxxð−ωmÞ 1⁄4 ð4=γmÞx2ZPnm and SxxðþωmÞ 1⁄4 ð4=γmÞx2ZPðnm þ 1Þ [3] This asymmetricin-frequency motional noise spectrum was first measured in atomic systems prepared in quantum ground states of motion [8,9,10], where the motional sideband absorption and fluorescence spectra were detected via photodetection. Analogous quantum noise effects can be studied in macroscopic mechanical systems, using electromechanical and optomechanical devices prepared and probed at quantum limits [5,11,12,13] These systems exhibit the Ramanlike processes of the up- and down-conversion of photons, resulting from the parametric coupling between mechanical motion and electromagnetic modes of a resonant cavity; the rates of these processes should naturally mirror the asymmetry in the mechanical quantum noise spectral density SxxðÆωmÞ. After a brief discussion of these theoretical issues, we present measurements of the imbalance in a microwave-frequency electromechanical device
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