Final Report Summary - QM (Quantum measurements and ground state cooling of mechanical oscillators)
Optomechanics, the study of the mutual coupling between optical and mechanical degrees of freedom via radiation pressure has given rise to a quickly growing and highly exciting field of research, with implications ranging from fundamental science to technological issues, in particular in very sensitive force and motion sensing. The Quantum measurements and ground-state cooling of a micro-mechanical oscillator project, carried out in the laboratory of Tobias Kippenberg between March 2010 and December 2011 consisted in studying various effects resulting from the optomechanical coupling in whispering gallery mode silica micro-resonators. These on-chip micro-fabricated devices host at the same time high-Q optical and mechanical modes.
The results obtained during the post-doctorate project are listed below. They represent a significant contribution to the field of optomechanics. The impact of the project can be witnessed, for instance by the number of publications in highly renowned journals (1 publication in Nature and 1 in Science).
Sideband cooling of a micro-mechanical oscillator close to its quantum ground state
The use of laser light to trap and control the motion of atoms and molecules has led to a true revolution in modern experimental physics, enabling, for instance, to realise clocks with precisions beyond imagination. Laser light can also be used to cool the motion of trapped particles and even control their motional degree of freedom at the quantum level. In a similar fashion, in an optomechanical set-up, the motional degree of freedom can be cooled when the cavity is coupled to a red-detuned light field. In the resolved sideband regime, where the cavity lifetime exceeds the mechanical period, one can even reach the quantum ground state by tuning the laser on the lower optomechanical sideband. Cooling a micromechanical oscillator close to its quantum ground state is a longstanding goal and an enabling step for the realisation of quantum experiments with engineered quantum mechanical systems. In this work (PRA 2011), with a combination of passive cryogenic cooling in a 3He cryostat and active laser sideband cooling, we could demonstrate the preparation of the mechanical degree of freedom close to its quantum ground state, with a remaining occupation of only 9±1 quanta. This was the lowest occupation achieved at the time with optical means.
Definition and determination of the vacuum optomechanical coupling rate
With the growing number of experiments exploring the optomechanical coupling, and the possibility to control the mechanical motion at the quantum level, we found it necessary to introduce a new convention to characterise the magnitude of the optomechanical coupling. In close analogy with the field of cavity quantum electrodynamics, we defined the vacuum optomechanical coupling rate as the optical frequency shift induced by a displacement equivalent to the vacuum fluctuations of the mechanical oscillator. This definition is extremely well suited to the formalism used in the Hamiltonian description of the interaction and is becoming a widely accepted convention in the community. We also described in the corresponding publication (Optics express 2010) a method to determine optically the vacuum coupling rate based on the comparison of a known phase calibration and thermal noise measurement at a given temperature.
Optomechanically induced transparency (OMIT)
Atoms (and molecules) with a well-suited energy level structure can be rendered transparent to a light beam they would usually absorb by the presence of a 'control' laser beam. This 'electromagnetically induced transparency' (EIT) can be understood as a consequence of quantum interference of different excitation pathways of the atom by the probing beam. Strong optical dispersion concomitant with the narrow transparency window can induce large - and tunable - group delay for the probe laser. This effect has been used to slow and even store light pulses in atomic vapours. It has been pointed out that optomechanical systems, can exhibit a similar phenomenon. This effect, referred to as OMIT, arises from destructive interference of excitation pathways for an intracavity probe field induced by the presence of an additional control field driving red sideband transitions in the system.
We reported the first experimental demonstration of the phenomenon (science 2011). This result has attracted significant attention from various scientific communities. It opens the way towards all optical switching of light pulses, as well as storage of optical information in an array of optomechanical systems.
Quantum coherent coupling of a mechanical resonator to an optical cavity mode
Controlling the quantum states of macroscopic mechanical oscillators is one of the key goals in experimental quantum science. Laser light can be used to achieve such control, in analogy with the cooling and manipulation of atoms, ions and molecules. Coupling mechanical motion to light at the quantum level is particularly appealing as it establishes a quantum interface between mechanical motion and optical photons, which can provide decoherence-free transport of quantum states at room temperature. In order to enable effective quantum state transfer between light in an optical cavity and a mechanical oscillator, the two must be coupled with a rate that exceeds the decoherence rates of the individual systems. Until recently, this regime of 'quantum-coherent' coupling was not obtained due to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation.
By developing a new form of resonators, miniaturised and anchored to the substrate using thin spokes in order to isolate the mechanical motion from the support, we could increase the vacuum coupling rate of our structures and operate for the first time in the quantum-coherent coupling regime (Nature 2012). With a coupling rate exceeding both the optical and mechanical decoherence rates this experiment satisfies the conditions for full control of the mechanical oscillator with optical fields. As a classical proof of principle experiment, we have observed how a weak pulsed coherent excitation is transferred from the optical to the mechanical mode and back. Moreover, in doing so, the mechanical oscillator mode was cooled to an average occupancy of 1.7 motional quanta, meaning that it spends 37 % of its time in the quantum mechanical ground state.
See http://k-lab.epfl.ch/ online
The results obtained during the post-doctorate project are listed below. They represent a significant contribution to the field of optomechanics. The impact of the project can be witnessed, for instance by the number of publications in highly renowned journals (1 publication in Nature and 1 in Science).
Sideband cooling of a micro-mechanical oscillator close to its quantum ground state
The use of laser light to trap and control the motion of atoms and molecules has led to a true revolution in modern experimental physics, enabling, for instance, to realise clocks with precisions beyond imagination. Laser light can also be used to cool the motion of trapped particles and even control their motional degree of freedom at the quantum level. In a similar fashion, in an optomechanical set-up, the motional degree of freedom can be cooled when the cavity is coupled to a red-detuned light field. In the resolved sideband regime, where the cavity lifetime exceeds the mechanical period, one can even reach the quantum ground state by tuning the laser on the lower optomechanical sideband. Cooling a micromechanical oscillator close to its quantum ground state is a longstanding goal and an enabling step for the realisation of quantum experiments with engineered quantum mechanical systems. In this work (PRA 2011), with a combination of passive cryogenic cooling in a 3He cryostat and active laser sideband cooling, we could demonstrate the preparation of the mechanical degree of freedom close to its quantum ground state, with a remaining occupation of only 9±1 quanta. This was the lowest occupation achieved at the time with optical means.
Definition and determination of the vacuum optomechanical coupling rate
With the growing number of experiments exploring the optomechanical coupling, and the possibility to control the mechanical motion at the quantum level, we found it necessary to introduce a new convention to characterise the magnitude of the optomechanical coupling. In close analogy with the field of cavity quantum electrodynamics, we defined the vacuum optomechanical coupling rate as the optical frequency shift induced by a displacement equivalent to the vacuum fluctuations of the mechanical oscillator. This definition is extremely well suited to the formalism used in the Hamiltonian description of the interaction and is becoming a widely accepted convention in the community. We also described in the corresponding publication (Optics express 2010) a method to determine optically the vacuum coupling rate based on the comparison of a known phase calibration and thermal noise measurement at a given temperature.
Optomechanically induced transparency (OMIT)
Atoms (and molecules) with a well-suited energy level structure can be rendered transparent to a light beam they would usually absorb by the presence of a 'control' laser beam. This 'electromagnetically induced transparency' (EIT) can be understood as a consequence of quantum interference of different excitation pathways of the atom by the probing beam. Strong optical dispersion concomitant with the narrow transparency window can induce large - and tunable - group delay for the probe laser. This effect has been used to slow and even store light pulses in atomic vapours. It has been pointed out that optomechanical systems, can exhibit a similar phenomenon. This effect, referred to as OMIT, arises from destructive interference of excitation pathways for an intracavity probe field induced by the presence of an additional control field driving red sideband transitions in the system.
We reported the first experimental demonstration of the phenomenon (science 2011). This result has attracted significant attention from various scientific communities. It opens the way towards all optical switching of light pulses, as well as storage of optical information in an array of optomechanical systems.
Quantum coherent coupling of a mechanical resonator to an optical cavity mode
Controlling the quantum states of macroscopic mechanical oscillators is one of the key goals in experimental quantum science. Laser light can be used to achieve such control, in analogy with the cooling and manipulation of atoms, ions and molecules. Coupling mechanical motion to light at the quantum level is particularly appealing as it establishes a quantum interface between mechanical motion and optical photons, which can provide decoherence-free transport of quantum states at room temperature. In order to enable effective quantum state transfer between light in an optical cavity and a mechanical oscillator, the two must be coupled with a rate that exceeds the decoherence rates of the individual systems. Until recently, this regime of 'quantum-coherent' coupling was not obtained due to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation.
By developing a new form of resonators, miniaturised and anchored to the substrate using thin spokes in order to isolate the mechanical motion from the support, we could increase the vacuum coupling rate of our structures and operate for the first time in the quantum-coherent coupling regime (Nature 2012). With a coupling rate exceeding both the optical and mechanical decoherence rates this experiment satisfies the conditions for full control of the mechanical oscillator with optical fields. As a classical proof of principle experiment, we have observed how a weak pulsed coherent excitation is transferred from the optical to the mechanical mode and back. Moreover, in doing so, the mechanical oscillator mode was cooled to an average occupancy of 1.7 motional quanta, meaning that it spends 37 % of its time in the quantum mechanical ground state.
See http://k-lab.epfl.ch/ online