Observation of Quantum Motion of a Nanomechanical Resonator

Abstract

In this Letter we use resolved sideband laser cooling to cool a mesoscopic mechanical resonator to near its quantum ground state (phonon occupancy 2:6 AE 0:2), and observe the motional sidebands generated on a second probe laser. Asymmetry in the sideband amplitudes provides a direct measure of the displacement noise power associated with quantum zero-point fluctuations of the nanomechanical resonator, and allows for an intrinsic calibration of the phonon occupation number. DOI: 10.1103/PhysRevLett.108.033602 PACS numbers: 42.50.Wk, 42.65.Àk, 62.25.Àg Experiments with trapped ions and neutral atoms [1-3], dating back several decades, utilized techniques such as resolved sideband laser cooling and motional sideband absorption and fluorescence spectroscopy to cool and measure a single trapped particle in its vibrational quantum ground state. These experiments generated significant interest in the coherent control of motion and the quantum optics of trapped atoms and ions [4], and were important stepping stones towards the development of ion-trap based quantum computing [5,6]. Larger scale mechanical objects, such as fabricated nanomechanical resonators, have only recently been cooled close to their quantum mechanical ground state of motion [7][8][9][10][11][12][13][14]. In a pioneering experiment by O'Connell, et al.[11], a piezoelectric nanomechanical resonator has been cryogenically cooled (T b $ 25 mK) to its vibrational ground state and strongly coupled to a superconducting circuit qubit allowing for quantum state preparation and readout of the mechanics. An alternate line of research has been pursued in circuit and cavity optomechanics [15], where the position of a mechanical oscillator is coupled to the frequency of a high-Q electromagnetic resonance allowing for backaction cooling [16,17] and continuous position readout of the oscillator. Such optomechanical resonators have long been pursued as quantum-limited sensors of weak classical forces [9,15,[18][19][20], with more recent studies exploring optomechanical systems as quantum optical memories and amplifiers [21][22][23][24], quantum nonlinear dynamical elements [25], and quantum interfaces in hybrid quantum systems [26][27][28][29].Despite the major advances in circuit and cavity optomechanical systems made in the last few years, all experiments to date involving the cooling of mesoscopic mechanical oscillators have relied on careful measurement and calibration of the motion-induced scattering of light to obtain the average phonon occupancy of the oscillator, hni. Approach towards the quantum ground state in such experiments is manifest only as a weaker measured signal, with no evident demarcation between the classical and quantum regimes of the oscillator. A crucial aspect of zero-point fluctuations (zpfs) of the quantum ground state is that they cannot supply energy, but can only contribute to processes where energy is absorbed by the mechanics. This is different from classical noise, and techniques that attempt to measure zero-point motion withou...

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