Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode

Tobias Kippenberg, Albert Schliesser, Stefan Alexander Weis
2012
Article

Résumé

Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions(1,2), molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities(3-6). If the optomechanical coupling is 'quantum coherent'-that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate-quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures(7,8). Optical experiments have not attained this regime owing to the large mechanical decoherence rates(9) and the difficulty of overcoming optical dissipation(10). Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7 +/- 0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links(11-15).

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https://infoscience.epfl.ch/record/175168?ln=fr

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Tobias Kippenberg, Albert Schliesser, Stefan Alexander Weis
2012
Article

Résumé

Official source

https://infoscience.epfl.ch/record/175168?ln=fr

À propos de ce résultat

Concepts associés (30)

Concepts associés

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Publications associées (45)

Publications associées

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Cavity-Optomechanics with Silica Microtoroids

Stefan Alexander Weis

Here, I report on a cryogenic cavity optomechanics experiment that has been set up with the goal to cool a mechanical degree of freedom of a fused silica microtoroidal resonator into the quantum regime by means of a combination of cryogenic and laser cooling. Based on the experience with a Helium-4 exchange gas cryostat obtained during a previous cryogenic optomechanics experiment, a novel setup with a Helium-3 cryostat at its heart has been set up. Cooling of a mechanical degree of freedom of a microtoroid close to its motional quantum ground state could be achieved and a regime, where full quantum control becomes possible, has come into reach. Silica microtoroids sustain at the same time ultra-high finesse optical whispering gallery modes (WGM) as well as radial mechanical modes ("radial breathing modes", RBM). The two degrees of freedom are mutually coupled, since mechanical motion changes the optical resonance frequency, and the mechanical motion is affected by the radiation pressure forces of an optical field contained in the optical mode. As the optical cavity lifetime is finite, the intracavity optical field amplitude is not adjusting instantaneously to the changed boundary conditions as induced by a mechanical displacement, but in a retarded manner, which gives rise to an effect known as dynamical backaction, that for example can be used to laser cool a mechanical mode. Using a 1550 nm laser important insight has been gained on the dependency of mechanical decay rate and frequency as a function of temperature, which is dominated by two level systems within amorphous fused silica. The different temperature regimes have been explored, including experiments at the lowest accessible temperatures, where evidence of resonant saturable absorption of TLS has been found. Using 780 nm light instead, cooling below ten quanta could be achieved and "optomechanically induced transparency", the optomechanical equivalent of electromagnetically induced transparency as found in atomic vapors, could be demonstrated, enabling all-optical switching of a laser beam and storage of pulses. Novel, optimized spokes-supported toroids then enabled us to push up the optomechanical coupling sufficiently, such that cooling to below two thermal quanta could be achieved and —for the first time in the optical domain— the quantum-coherent coupling regime could be accessed. Here, the optomechanical coupling rate exceeds the optical and mechanical decay rates (i.e. "strong coupling"), but also the mechanical decoherence rate, such that quantum-state transfer between optics and mechanics comes into reach. In addition, this thesis contains the technological steps taken and experimental hurdles overcome towards these experiments.

EPFL2012

Chargement

In quantum mechanics, the Heisenberg uncertainty principle places a fundamental limit in the measurement precision for certain pairs of physical quantities, such as position and momentum, time and energy or amplitude and phase. Due to the Heisenberg uncertainty principle, any attempt to extract certain information from a quantum object would inevitably perturbitinanunpredictableway. This raises one question,"What is the precision limit in such quantum measurements?" The answer, standard quantum limit (SQL), has been obtained by Braginsky to figure out the fundamental quantum limits of displacement measurement in the context of gravitational wave detection. To circumvent the unavoidable quantum back-action from the priori measurement, quantum non-demolition measurement (QND) methods were introduced by Braginsky and Thorne. To surpass the SQL of the displacement measurement in an interferometer, one can measure only one quadrature of the mechanical motion while give up the information about the other canonically conjugated quadrature. Such measurements can be performed by periodic driving the mechanical oscillator, i.e. the back-action evading (BAE) measurement. Cavity optomechanics provides an ideal table-top platform for the testing of the quantum measurement theory. Mechanical oscillator is coupled to electromagnetic field via radiation pressure, which is enhanced by an optical micro-cavity. Over the last decade, laser cooling has enabled the preparation of mechanical oscillator in the ground state in both optical and microwave systems. BAE measurements of mechanical motion have been allowed in the microwave electromechanical systems, which led to the observations of mechanical squeezing and entanglement. However, despite the theoretical proposal almost 40 years ago, the sub-SQL measurements still remain elusive. This thesis reports our efforts approach the sub-SQL with a highly sideband-resolved silicon optomechanical crystal (OMC) in a 3He buffer gas environment at 2K. The OMC couples an optical mode at telecommunication wavelengths and a colocalized mechanical mode at GHz frequencies. The Helium3 buffer gas environment allows sufficient thermalization of the OMC despite the drastically decreased silicon thermal conductivity. We observe Floquet dynamics in motional sideband asymmetry measurement when employing multiple probe tones. The Floquet dynamics arises due to presence of Kerr-type nonlinearities and gives rise to an artificially modified motional sideband asymmetry, resulting from a synthetic gauge field among the Fourier modes. We demonstrate the first optical continuous two-tone backaction-evading measurement of a localized GHz frequency mechanical mode of silicon OMC close to the ground state by showing the transition from conventional sideband asymmetry to backaction-evading measurement. We discover a fundamental two-tone optomechanical instability and demonstrate its implications on the back-action evading measurement. Such instability imposes a fundamental limitation on other two-tone schemes, such as dissipative quantum mechanical squeezing. We demonstrate state-of-art laser sideband cooling of the mechanical motion to a mean thermal occupancy of 0.09 quantum, which is 7.4dB of the oscillator's zero-point energy and corresponds to 92% ground state probability. This also enables us to observe the dissipative mechanical squeezing below the zero-point motion for the first time with laser light.

EPFL2020

Chargement

Parametric coupling of optical and mechanical degrees of freedom forms the basis of many ultra-sensitive measurements of both force and mechanical displacement. An optical cavity with a mechanically compliant boundary enhances the optomechanical interaction, which gives rise to qualitatively new behavior which can modify the dynamics of the mechanical motion. As early as 1967, in a pioneering work, V. Braginsky analyzed theoretically the role of radiation pressure in the interferometric measurement process, but it has remained experimentally unexplored for many decades. Here, we use whispering-gallery mode optical microresonators to study these radiation pressure phenomena. Optical microresonators simultaneously host optical and mechanical modes, which are systematically analyzed and optimized to feature ultra-low mechanical dissipation, photon storage times exceeding the mechanical oscillation period (i.e. the "resolved-sideband regime") and large optomechanical coupling. In this manner, it is demonstrated for the first time that dynamical backaction can be employed to cool mechanical modes, i.e., to reduce their thermally excited random motion. Utilizing this novel technique together with cryogenic pre-cooling of the mechanical oscillator, the phonon occupation of mechanical radial-breathing modes could be reduced to (n) = 63 +/- 20 excitation quanta. The corresponding displacement fluctuations are monitored interferometrically with a sensitivity at the level of 1.10(-18) m/root Hz, which is below the standard quantum limit (SQL). This implies that the readout is already in principle sufficient to measure the quantum mechanical zero-point position fluctuations of the mechanical mode. Moreover, it is shown that optical measurement techniques employed here are operating in a near-ideal manner according to the principles of quantum measurement, displaying a backaction-imprecision product close to the quantum limit.

2010

Chargement

Cavity-Optomechanics with Silica Microtoroids

Stefan Alexander Weis

EPFL2012

EPFL2020