Squeezed light from a silicon micromechanical resonator

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Monitoring a mechanical object’s motion, even with the gentle touch of light, fundamentally alters its dynamics. The experimental manifestation of this basic principle of quantum mechanics, its link to the quantum nature of light and the extension of quantum measurement to the macroscopic realm have all received extensive attention over the past half-century1, 2. The use of squeezed light, with quantum fluctuations below that of the vacuum field, was proposed nearly three decades ago3 as a means of reducing the optical read-out noise in precision force measurements. Conversely, it has also been proposed that a continuous measurement of a mirror’s position with light may itself give rise to squeezed light4, 5. Such squeezed-light generation has recently been demonstrated in a system of ultracold gas-phase atoms6 whose centre-of-mass motion is analogous to the motion of a mirror. Here we describe the continuous position measurement of a solid-state, optomechanical system fabricated from a silicon microchip and comprising a micromechanical resonator coupled to a nanophotonic cavity. Laser light sent into the cavity is used to measure the fluctuations in the position of the mechanical resonator at a measurement rate comparable to its resonance frequency and greater than its thermal decoherence rate. Despite the mechanical resonator’s highly excited thermal state (104 phonons), we observe, through homodyne detection, squeezing of the reflected light’s fluctuation spectrum at a level 4.5±0.2 per cent below that of vacuum noise over a bandwidth of a few megahertz around the mechanical resonance frequency of 28megahertz. With further device improvements, on-chip squeezing at significant levels should be possible, making such integrated microscale devices well suited for precision metrology applications.

At a glance


  1. Optomechanical device.
    Figure 1: Optomechanical device.

    a, Scanning electron microscope image of a waveguide-coupled zipper optomechanical cavity. The waveguide width is adiabatically tapered along its length and terminates with a photonic crystal mirror next to the cavity. The tapering of the waveguide allows for efficient input–output coupling and the photonic crystal termination makes the coupling to the cavity single sided. Two zipper cavities are coupled above and below the waveguide, each with a slightly different optical resonance frequency, allowing them to be separately addressed. b, Left: close-up of the coupling region between one of the cavities and the waveguide. Right: finite-element method (FEM) simulation of the cavity field leaking into the waveguide (log scale). Note that the field does not leak into the mirror region of the waveguide. c, Top: FEM simulation showing the in-plane electrical field of the fundamental optical cavity mode. Bottom: FEM simulation of the displacement of the fundamental in-plane differential mode of the structure with frequency ωm/2π = 28MHz. The mechanical motion, modifying the gap between the beams, shifts the optical cavity frequency, leading to optomechanical coupling.

  2. Experimental set-up and device characterization.
    Figure 2: Experimental set-up and device characterization.

    a, The optical signal is derived from an external-cavity diode laser and is sent into a tapered optical fibre inside a liquid-helium cryostat where the silicon sample is cooled to T16K. The fibre taper is used to couple light evanescently into the silicon optomechanical device. The optical reflection from the device is collected by the same fibre taper and sent to a balanced homodyne detector (BHD) for characterization. For further details of the experimental set-up, see Methods Summary. The efficiencies of the circulator, switch and BHD are respectively denoted η23, η3H and ηHD. AOM, acousto-optic modulator; EDFA, erbium-doped fibre amplifier; ENA, electronic network analyser; FPC, fibre polarization controller; FS, fibre stretcher; IM, intensity modulator; λ-meter, wavemeter; LPF, low-pass filter; PD, photodetector; PM, power meter; RSA, real-time spectrum analyser; VC, variable coupler; VOA, variable optical attenuator. b, Top: reflected signal from the optical cavity at low optical power (left fencencright fence10; linewidth, κ/2π = 3.42GHz). Bottom: high-power (left fencencright fence = 790) reflected signal, showing the cavity-laser detuning (dashed line) locked to during squeezing measurements. c, Homodyne noise PSD of the reflected signal showing the transduced thermal Brownian motion of the zipper cavity at Tb = 16K (green curve; left fencencright fence = 80). The red curve is the shot-noise level and the black curve is the detector’s dark noise (in the absence of light input). Inset, close-up of the fundamental in-plane differential mechanical mode of the zipper cavity (fitted by blue curve; linewidth, γi/2π = 172Hz). d, Mean value of the PSD of the BHD as a function of the local-oscillator (LO) power (signal blocked). The filled data point indicates the local-oscillator power used in the squeezing measurements. The red and dashed black curves correspond to a linear fit to the data and the level of the detector dark current, respectively. e, Noise PSD as a function of θlock (ranging from 0 (green) to π (red)) with the signal detuned far off resonance at Δ/κ30, referenced to the noise level with the signal blocked (blue).

  3. Optomechanical squeezing of light.
    Figure 3: Optomechanical squeezing of light.

    a, Theoretical model. Density plot of the predicted reflected-signal noise PSD, as measured on a balanced homodyne detector and normalized to shot-noise, for a simplified model of the optomechanical system (Supplementary Information). Areas with noise below shot-noise are shown in blue shades on a linear scale. Areas with noise above shot-noise are shown in orange shades on a log scale. The solid white line is a contour delineating noise above and below shot-noise. b, Experimental data. Density plot of the measured reflected-signal noise PSD for left fencencright fence = 790, normalized to the measured shot-noise level. c, Slice of the measured density plot in b taken at θlock/π = 0.23. d, Slice of the measured density plot in b taken at θlock/π = 0.16. In c and d, the black curve corresponds to the measured data slice extracted from b. The dark blue traces are several measurements of the shot-noise level (average shown in light blue). Also shown is a model of the squeezing in the absence of thermal noise (orange curve), the same model with ohmic thermal noise of the mechanical mode included (green) and a full noise model including additional phenomenological noise sources (red curve). The vertical white dashed lines in a and b indicate the data slices shown in c and d.

  4. Spectral and power dependence of noise.
    Figure 4: Spectral and power dependence of noise.

    a, Measured balanced homodyne noise power of the reflected signal at ω/2π = 27.9MHz (filled circles) versus quadrature angle (Δlock/κ = 0.044 and left fencencright fence = 790). The green and red curves correspond to the single-mode and full noise models, respectively. The orange curve represents a model including the response of the mechanical mode in the absence of thermal noise, that is, when driven by RPSN only, and the dashed blue curve shows the thermal noise component. Inset, close-up of boxed region. b, Measured minimum noise PSD normalized to shot-noise (filled circles) versus left fencencright fence. Left: maximum squeezing for ω<ωm; right: maximum squeezing for ω>ωm. Also shown are the single-mode noise model (green curve) and the full noise model (red curve). c, Balanced homodyne noise PSD of the reflected cavity signal for Δlock/κ = 0.052 and left fencencright fence = 3,153. Left: phase quadrature corresponding to maximum transduction of mechanical motion; right: phase quadrature corresponding to minimum transduction of mechanical motion. In each plot, the black curve is the measured data with the shot-noise level subtracted. Also shown are the modelled laser phase noise (dashed brown curve), the noise contribution from a single mechanical mode (dashed blue curve) and the full noise model (red curve).


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Author information

  1. These authors contributed equally to this work.

    • Amir H. Safavi-Naeini,
    • Simon Gröblacher &
    • Jeff T. Hill


  1. Kavli Nanoscience Institute and Thomas J. Watson, Sr, Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA

    • Amir H. Safavi-Naeini,
    • Simon Gröblacher,
    • Jeff T. Hill,
    • Jasper Chan &
    • Oskar Painter
  2. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA

    • Amir H. Safavi-Naeini,
    • Simon Gröblacher,
    • Jeff T. Hill &
    • Oskar Painter
  3. Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, A-1090 Wien, Austria

    • Markus Aspelmeyer
  4. Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Bldg 24, D-91058 Erlangen, Germany

    • Oskar Painter


A.H.S.-N., S.G. and M.A. designed the experiment. A.H.S.-N., S.G., J.C. and J.T.H. designed and fabricated the device, and performed the measurements. A.H.S.-N., S.G., J.T.H. and O.P. performed the analysis and modelling of the data. All authors were involved in writing and editing the paper.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (1.9 MB)

    This file contains Supplementary Text and Data 1-6, Supplementary Figures 1-15, Supplementary Table 1 and additional references.

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