Radiation pressure

Summary

Radiation pressure (also known as light pressure) is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is absorbed, reflected, or otherwise emitted (e.g. black-body radiation) by matter on any scale (from macroscopic objects to dust particles to gas molecules). The associated force is called the radiation pressure force, or sometimes just the force of light. The forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes and technologies. This particularly includes objects in outer space, where it is usually the main force acting on objects besides gravity, and where the net effect of a tiny force may have a large cumulative effect over long periods of time. For example, had the effects of the Sun's radiation pressu

Official source

https://en.wikipedia.org/wiki/Radiation_pressure

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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

Quantum limits on measurement and control of a mechanical oscillator

Vivishek Sudhir

The precision measurement of position has a long-standing tradition in physics. Cavendish's verification of the universal law of gravitation using a torsion pendulum, Perrin's confirmation of the atomic hypothesis via the precise measurement of the Brownian motion, and, the verification of the mechanical effect of electromagnetic radiation, all belong to this classical heritage. Quantum mechanics posits that the measurement of position results in an uncertain momentum; an idea developed to full maturity within the context of interferometric searches for gravity waves. Over the past decade, standing at the confluence of quantum optics and nanomechanics, cavity optomechanics has emerged as a powerful platform to study the quantum limits of position measurements. The subject of this thesis is the precision measurement of the position of a nano-mechanical oscillator, the fundamental limits of such measurements, and its relevance to measurement-based feedback control. The nano-mechanical oscillator is coupled to light confined in an optical micro-cavity via radiation pressure. The fluctuations in the position of the oscillator are transduced onto the phase of the light, while quantum fluctuations in the amplitude of the light leads to a disturbance in the momentum of the oscillator. We perform an interferometric position measurement with a sensitivity that is 10^5 times below what is required to resolve the zero-point motion of the oscillator, constituting the most precise measurement of thermal motion yet. The resulting disturbance -- measurement back-action -- is observed to be commensurate with the uncertainty principle, leading to a 10% contribution to the total motion of the oscillator. The continuous record of the measurement (performed in a 4 K cryogenic environment) furnishes the ability to resolve the zero-point motion of the oscillator within its decoherence rate - the necessary condition for measurement-based feedback control of the state of the oscillator. Using the measurement record as error signal, the oscillator is cooled towards its ground state, resulting in a factor 10^4 suppression of its total (thermal and back-action) motion, to a final occupation of 5 phonons on average. Measurements generally proceed by establishing correlations between the system being measured and the measuring device. For the class of quantum measurements employed here - continuous linear measurements - these correlations arise due to measurement back-action. These back-action-induced correlations appear as correlations between the degrees of freedom of the measuring device. For interferometric position measurements, quantum correlations are established between the phase and amplitude of the light. In a homodyne measurement, they lead to optical squeezing, while in a heterodyne measurement, they appear as an asymmetry in the sidebands carrying information about the oscillator position. Feedback is used to enhance sideband asymmetry, a first proof-of-principle demonstration of the ability to control quantum correlations using feedback. In the regime where amplified vacuum noise dominates the feedback signal, the disappearance of sideband asymmetry visualises a fundamental limit of linear feedback control. Using a homodyne detector, we also characterise these quantum correlations manifested as optical squeezing at the 1% level.

EPFL2016

Quantum Measurement of Mechanical Motion close to the Standard Quantum Limit

Liu Qiu

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