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Cavity Quantum Optomechanics: Exploring mechanical oscillators in the quantum regime

Final Report Summary - QUREM (Cavity Quantum Optomechanics: Exploring mechanical oscillators in the quantum regime)

Micro- and nanomechanical oscillators are integral part of our modern information society:

Piezo-mechanical oscillator in mobile phones provides by virtue of their high Q filtering functions for microwave signals and vibrating mechanical oscillators integrated on silicon chips are used for measuring acceleration or rotation and are virtually in any modern cell phone, car or airplane. In recent years, a new frontier in quantum science has been reached: to control such micro- and nanomechanical oscillators at the quantum level. Following quantum control of atoms, molecules and ions, and electrical circuits, it is now-a-days possible to prepare mechanical oscillators close to their quantum mechanical ground state, and manipulate their mechanical degree of freedom at the quantum level. These advances have been enabled by the developed of cavity optomechanical systems and techniques, in particular “backaction” effects of radiation pressure, first conceived in the field of gravity wave detection. The present proposal builds on these developments and demonstrates mechanical oscillators’ ability as a quantum technology.

Since the start, this ERC Grant has advanced this notion in several regards. First, by developing a novel nano-optomechanical system that couples a lightweight, nanomechanical oscillator to a high Q optical microcavity made from glass (the material for fiber optics), a system has been conceived that enables to make shot-noise-limited optical measurements with a noise floor that is 40 dB (i.e. 4 orders of magnitude) below the displacement spectral density of the zero point motion of the mechanical oscillator. This represents to date the most sensitive measurements of motion relative to the standard quantum limit, and extends precision measurements of nanomechanical oscillators into a new realm, where quantum feedback techniques become viable. Indeed, it has been possible using feedback to cool the oscillator close to the ground state, and observe backaction imprecision quantum correlations, a manifestly quantum mechanical effect of the radiation pressure optomechanical interaction. The ability to achieve such imprecision represents a new frontier in precision measurements of mechanical oscillators, and may have applications in both fundamental and applied science.

A second new frontier this ERC has recently accessed is the new regime of dissipative optomechanics in circuit optomechanics, where a mechanical energy relaxation rate exceeds that of the electromagnetic mode. In this regime, the roles of optical and mechanical modes are reversed: the microwave “backaction” allows to cool or amplify the microwave field, rather than the mechanical oscillator, a situation that can be regarded as “reservoir” engineering of the microwave field. This regime among others enables to create a low noise phase preserving microwave amplifier that is based uniquely on micromechanical elements and has demonstrated a performance superior to the best commercial microwave amplifiers based on High Electron Mobility Transistors (HEMTs). In the future, this dissipative interaction can enable to create non-reciprocal devices on-chip such as microwave isolators, without using any magnetic fields.

Finally, this ERC AdG has revealed that backaction can also be discovered in a seemingly disparate field: that of surface and tip enhanced Raman scattering (SERS and TERS). SERS and TERS describe experiments in which the vibrational mode of molecules is enhanced via the presence of a plasmonic modes of metallic particles or tips. Recent work has demonstrated that cavity optomechanics can provide a proper theoretical framework to describe the interaction of molecular coupled to plasmon resonances at the quantum level. This theoretical framework leads to a natural interpretation of the unusually large Raman enhancement factors, by invoking the concept of “dynamical backaction” amplification, as routinely observed in optomechanics. This insight can prove very useful in guiding the design of future SERS and TERS experiments for maximal enhancement, and provide a route to quantitative simulations of the interaction, taking into account backaction.