An optomechanical device has allowed quanta, or 'grains', of mechanical vibration to be counted by optical means. The system may open up new possibilities in acoustics and thermal engineering. See Letter p.522
According to quantum theory, all forms of energy come in tiny 'grains' called quanta. This quantum granularity may or may not be discernible, depending on the instruments available and the nature of the energy. The quanta are photons in the case of electromagnetic waves such as light, and phonons for mechanical vibrations in solids. Phonons are ubiquitous in condensed-matter systems, in which they underlie the transport of sound and heat. They also govern the performance of electronic and optical devices, and play a central part in conventional superconductivity. However, whereas photons can be detected at the single-quantum level using today's optical receivers, there is a lack of versatile instruments and techniques to measure phonons at the same level of precision. The scientific and technological stakes are high, given the many potential applications of such systems. On page 522 of this issue, Cohen et al.1 describe a promising step towards building an optical single-phonon detector.
The researchers used an optomechanical device in which photons from an external light source are scattered by phonons associated with the system's mechanical vibration. The phonons are detected by collecting the scattered photons in single-photon detectors. The experiment builds on developments in the field of optomechanics, in which the force of light, circulating in a type of light trap called an optical cavity, is used to control and monitor the mechanical motion of tiny objects such as mirrors, membranes or flexible wires2,3. Although light generally has a feeble mechanical effect on large bodies, it dictates the dynamics of such minute optomechanical systems. Progress in the field over the past five years has involved confining both photons and phonons in a submicrometre-sized volume to achieve strong coupling between light and mechanical motion4,5.
Cohen and colleagues' optomechanical device is a nanostructured semiconductor crystal that yields such strong coupling. As a result, laser light that is incident on, and scattered from, the optomechanical crystal is efficiently modulated by the crystal's mechanical vibration. The scattered light acquires blue and red spectral sidebands, respectively above and below the laser's frequency, that are analogous to the 'satellite lines' observed in the Raman scattering of photons from a material. If the material absorbs energy during the scattering process, the scattered photon has a lower energy than the incident photon (red Stokes line); if the material loses energy, the scattered photon has a higher energy (blue anti-Stokes line). The shift in energy provides information about the vibrational or rotational modes of motion of the material's constituents. In a sense, optomechanical devices are simply highly engineered Raman systems.
By appropriately tuning the laser's frequency to the optical resonance of the optomechanical crystal, the system's natural optical vibration frequency, Cohen et al. could finely adjust the amplitude of the sidebands. For example, they suppressed the amplitude of the red sideband and selectively enhanced the generation of scattered blue photons (Fig. 1). By using optical filtering, the researchers then suppressed scattered photons that had the same energy as the incident laser photons. In this way, only the blue photons produced 'clicks' on the detector, revealing the presence of phonons in the device. Owing to the strong optomechanical interaction, the optical measurement attained sufficient sensitivity to resolve individual phonons, and enabled their precise counting in the system's vibrational state.
Although the present experiments still fall short of actually generating and measuring vibrational states containing a single phonon, the authors' phonon-counting technique already allows measurement of the statistical properties of the device's vibrational motion. The authors applied the concept of intensity correlations to their experimental set-up. Such correlations were used by Robert Hanbury Brown and Richard Twiss nearly 60 years ago to detect correlations between photons emitted by distant stars6. Cohen et al. measured these correlations in the arrival of photons at the detectors, thereby probing phonon correlations in the optomechanical device. Using this technique, they could directly observe a transition in the statistical behaviour of phonons as the system underwent a change from a purely thermal, random state of vibrational motion to a coherent, more-ordered one, which was reached above a certain threshold of power of the incident laser light7,8. In other words, Cohen and colleagues observed the phononic analogue of the 'lasing' transition that enables lasers to emit coherent light — light that is made up of waves that have the same wavelength and are in step with each other.
Finally, and looking ahead, it should be noted that, in the realm of quantum physics, measuring is also acting. The very act of measuring a system may alter its state. Therefore, future optomechanical experiments operating in the single-phonon counting regime could be used to generate complex quantum states of phonons. Given the rapid pace at which the field of optomechanics is advancing, this point might be reached in the not-too-distant future. Such quantum optomechanical control of matter could allow researchers both to test the fundamental principles of quantum mechanics and to venture into new applications in acoustics, thermal management and electrical-conductivity engineering.Footnote 1
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