Stefan Weis

Optomechanically induced transparency

Mechanical Transparency In atomic gases and other solid-state systems with appropriate energy levels, manipulation of the optical properties can be induced with a control pulse, allowing the system to transmit light of specific wavelengths that would otherwise have been absorbed. Weis et al. (p. 1520, published online 11 November) now report electromagnetically induced transparency in an optomechanical system whereby the coupling of a cavity to a light pulse is used to control the transmission of light through the cavity. This approach may help to allow the engineering of light storage and routing on an optical chip. Radiation pressure is used to manipulate the optical properties of an optomechanical system. Electromagnetically induced transparency is a quantum interference effect observed in atoms and molecules, in which the optical response of an atomic medium is controlled by an electromagnetic field. We demonstrated a form of induced transparency enabled by radiation-pressure coupling of an optical and a mechanical mode. A control optical beam tuned to a sideband transition of a micro-optomechanical system leads to destructive interference for the excitation of an intracavity probe field, inducing a tunable transparency window for the probe beam. Optomechanically induced transparency may be used for slowing and on-chip storage of light pulses via microfabricated optomechanical arrays.

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

Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions, 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. 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. Optical experiments have not attained this regime owing to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation. 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.

Optomechanically Induced Transparency

In analogy to electromagnetically induced transparency observed in atomic systems, we demonstrate that the transmission of a probe laser beam through an optomechanical device can be modulated using a second, “control” laser beam.

Optomechanical sideband cooling of a micromechanical oscillator close to the quantum ground state

Cooling a mesoscopic mechanical oscillator to its quantum ground state is elementary for the preparation and control of quantum states of mechanical objects. Here, we pre-cool a 70-MHz micromechanical silica oscillator to an occupancy below 200 quanta by thermalizing it with a 600-mK cold He-3 gas. Two-level-system induced damping via structural defect states is shown to be strongly reduced and simultaneously serves as a thermometry method to independently quantify excess heating due to the cooling laser. We demonstrate that dynamical back action optical sideband cooling can reduce the average occupancy to 9 +/- 1 quanta, implying that the mechanical oscillator can be found (10 +/- 1)% of the time in its quantum ground state.

Cavity quantum optomechanics: Coupling light and micromechanical oscillators

Cavity optomechanics1 is a new research field that has seen spectacular advances in recent years. Optomechanics combines advances in nano- and electromechanical systems with radiation pressure enabled control. The radiation pressure backaction enables to readout mechanical motion of micro- and nanoscale mechanical oscillators with an imprecision at the standard quantum limit, enables to amplify2 mechanical motion - enabling coherent mechanical oscillators. Likewise the cooling3,4 of mechanical oscillators has enabled to access the quantum regime of optomechanical systems. Likewise mechanical degrees of freedom provide new ways to control the propagation of light via the phenomenon of optomechanically induced transparency5, which can e.g. enable switching, slowing or advancing of electromagnetic pulses6. Cavity optomechanical systems also have reached the quantum regime of mechanical oscillators, which has been long anticipated. As one example of the possible range of optomechanical phenomena, we review an optomechanical microresonator in which optical and mechanical degrees of freedom exchange energy at a rate exceeding the relevant decoherence rates in the system, enabling quantum control of a mechanical oscillator with light. Such quantum coherent coupling provided a quantum coherent link7 between engineered microscale oscillators and the light field.

High-sensitivity monitoring of nanomechanical motion using optical heterodyne detection

Summary form only given. Due to the prospect of transferring the optical quantum control achieved with ions and atoms also to macroscopic mechanical oscillators, cavity optomechanics [1,2] has been an increasingly active research field commencing with the first demonstration of radiation pressure dynamical backaction cooling more than half a decade ago. By optimizing the optical and mechanical quality factors and operating at cryogenic temperatures, quantum-coherent coupling could be achieved [3]. Simultaneously, it has been shown that nanomechanical resonators based on photonic crystals can be advantageous, as they offer low phonon occupancies (n̅ = k<;sub>B<;/sub>T/ħΩ) due to the high vacuum optomechanical coupling rates and high mechanical frequencies in the GHz domain [4]. To fully exploit the potential of these systems, sensitive measurement techniques need to be developed.Here we present a novel route using a heterodyne measurement technique to achieve significantly improved sensitivity. Based on an optical design presented in [5], we fabricate a suspended 1 D photonic crystal cavity surrounded by 2 D photonic crystals with a band gap at 1550 nm in silicon-on-insulator (Fig. 1 (a)). Together with a mesa structure defined by photolithography, it is possible to couple the cavity with a straight tapered fiber. The loaded optical Q-factor is measured as > 104. In direct detection, we observe the mechanical breathing mode around 5.7 GHz using the 12 GHz detector 1544-A by Newport, thus placing the optical cavity in the resolved sideband limit. The low quality factor of the mechanical oscillator <; 103 is explained by the domination of anchor and surface losses but can be improved by appropriate engineering.In the heterodyne experiment, we branch the light of a tunable diode laser before the device under test (DUT). One branch is coupled in and out of the cavity using a straight tapered fiber. A shifted local oscillator is created in the second branch, offset by a tunable frequency close to the mechanical oscillator mode. The shifted LO is then combined using free space optics with the signal coming from the device and carrying the mechanically induced sideband. Afterwards, the beam is split again and both branches are sent onto a balanced heterodyne detector. Fig. 1 (c) shows the resulting measurement. The local oscillator is detuned to proof that indeed the mixed signal of the nano-mechanical mode is measured and not any other components. The down-mixed signal exhibits a SNR of nearly 20 dB, greatly exceeding the SNR in direct detection. The shown data was carried out with an unfiltered signal. Further improvement is expected from suppressing the carrier by using a fiber loop cavity or Fabry-Perot filter. The elemental demonstration of this measurement scheme paves the way towards quantum limited mechanical measurements in the GHz domain.

Cavity optomechanics: Cooling of a micromechanical oscillator into the quantum regime

Using optical sideband cooling, a micromechanical oscillator is cooled to a phonon occupancy below 10 phonons, corresponding to a probability of finding it in its quantum ground state more than 10% of the time.

Cooling of a mechanical oscillator close to the quantum regime

The control of low-entropy quantum states of a micro-oscillator could not only allow researchers to probe quantum phenomena—such as entanglement and decoherence—at an unprecedentedly large scale, but also enable their use as interfaces in hybrid quantum systems. Preparing and probing an oscillator in the conceptually simplest low-entropy state, its quantum ground state, has now become a major goal in Cavity Optomechanics [1]. However, to experimentally achieve this goal, its effective temperature T<inf>eff</inf> has to be reduced sufficiently so that ħΩ<inf>m</inf> >>s k<inf>B</inf>T<inf>eff</inf> (ħ is the reduced Planck constant, k<inf>B</inf> the Boltzman constant, and Ω<inf>m</inf> the mechanical resonance pulsation). Using conventional cryogenic refrigeration, a nanomechanical oscillator has recently been cooled to the quantum regime and probed by a superconducting qubit to which it was piezoelectrically coupled [2].

Cooling of a micromechanical oscillator into the quantum regime

Using optical sideband cooling, a micromechanical oscillator is cooled to a phonon occupancy below 10 phonons, corresponding to a probability of finding it in its quantum ground state more than 10% of the time.