Observation of strong coupling between a micromechanical resonator and an optical cavity field (0903.5293v2)

Abstract: Achieving coherent quantum control over massive mechanical resonators is a current research goal. Nano- and micromechanical devices can be coupled to a variety of systems, for example to single electrons by electrostatic or magnetic coupling, and to photons by radiation pressure or optical dipole forces. So far, all such experiments have operated in a regime of weak coupling, in which reversible energy exchange between the mechanical device and its coupled partner is suppressed by fast decoherence of the individual systems to their local environments. Controlled quantum experiments are in principle not possible in such a regime, but instead require strong coupling. So far, this has been demonstrated only between microscopic quantum systems, such as atoms and photons (in the context of cavity quantum electrodynamics) or solid state qubits and photons. Strong coupling is an essential requirement for the preparation of mechanical quantum states, such as squeezed or entangled states, and also for using mechanical resonators in the context of quantum information processing, for example, as quantum transducers. Here we report the observation of optomechanical normal mode splitting, which provides unambiguous evidence for strong coupling of cavity photons to a mechanical resonator. This paves the way towards full quantum optical control of nano- and micromechanical devices.

• The paper reports the experimental observation of reversible energy exchange via optomechanical normal mode splitting when the enhanced coupling rate exceeds both cavity and mechanical damping rates.
• The study employs a micromechanical resonator integrated with a Fabry-Pérot cavity and a continuous-wave laser to achieve strong coupling with significant optical drive power.
• The demonstrated strong coupling paves the way for coherent quantum control and advances in quantum state transfer and entanglement generation with mechanical systems.

Strong Coupling between Micromechanical Resonators and Optical Cavity Fields

The paper "Observation of strong coupling between a micromechanical resonator and an optical cavity field" presents a significant advancement in the field of optomechanics by demonstrating strong coupling between micromechanical resonators and optical cavity fields. This research moves beyond the conventional weak coupling regime to achieve a critical threshold where reversible energy exchange between the mechanical systems and their optical counterparts becomes experimentally feasible.

Research Background and Significance

The transition from weak to strong coupling is vital for achieving coherent quantum control over massive mechanical devices. In weak coupling scenarios, fast decoherence rates compromise reversible energy interchange, thereby limiting experimental realizations of quantum phenomena. This paper addresses the stringent requirements for strong coupling articulated in previous theoretical frameworks, notably where the coupling rate $g$ exceeds both the optical cavity amplitude decay rate $\kappa$ and the mechanical damping rate $\gamma_m$.

Experimental Setup and Methodology

The experimental configuration involves a micromechanical resonator integrated at the terminal face of a Fabry-Pérot cavity, driven by a continuous-wave laser. The mechanical mode resonates at a frequency $\omega_m = 2\pi \times 947$ kHz with a natural linewidth of $\gamma_m = 2\pi \times 140$ Hz and a mechanical quality factor $Q \approx 6,700$. The cavity system is characterized by an optical finesse $F \approx 14,000$ and an amplitude decay rate $\kappa = 2\pi \times 215$ kHz, positioning the system in the resolved sideband regime.

To achieve the required strong coupling, an external optical field enhances the coupling factor $g = g_0 \alpha$, where $g_0 = 2\pi \times 2.7$ Hz is the single-photon coupling rate. The experimental design employs significant optical driving powers to boost this rate to $g = 2\pi \times 325$ kHz, successfully surpassing both cavity and mechanical damping rates.

Key Findings

The core observation presented is the optomechanical normal mode splitting within the frequency spectrum of the system. The emergence of distinct hybrid modes $\omega_{\pm}$ substantiates the presence of strong coupling, with spectral separations aligning closely with the enhanced coupling strength $g$. The research confronts a notable experimental challenge by resolving normal mode spectral peaks amidst thermal decoherence, thus confirming the strong optomechanical coupling regime.

Theoretical and Practical Implications

Achieving strong coupling facilitates a multitude of optomechanical experiments, ranging from quantum state preparation and transfer to entanglement generation. The demonstration advocates for future avenues in full quantum optical control over mechanical systems, which depend critically on minimizing thermal decoherence. The work suggests that constructing experiments at cryogenic temperatures could efficiently reconcile strong coupling with ground-state cooling.

The paper posits a future direction towards realizing strong coupling at the single-photon level, thereby opening possibilities for exploring vacuum Rabi splitting characteristic of nonlinear interactions. This trajectory implies substantial enhancements in the utility of nanomechanical and micromechanical resonators within the quantum information science domain as quantum transducers and processors.

In summary, the research delineated in this paper serves as an instrumental step toward optomechanical systems' operational viability in the quantum field, hinging on the corroborated demonstration of strong coupling between mechanical resonators and optical cavity fields.