G. Di Giuseppe

Optomechanically induced transparency in a membrane-in-the-middle setup at room temperature

Summary form only given. In cavity optomechanics one can manipulate the dynamics of a nanomechanical resonator with light, and at the same time one can control light by tayloring its interaction with one (or more) mechanical resonances. A notable example of this kind of light beam control is provided by the optomechanical analogue of electromagnetically induced transparency (EIT), the so called optomechanically induced transparency (OMIT), which has been recently demonstrated [1-3]. In OMIT, the internal resonance of the medium is replaced by a dipole-like interaction of optical and mechanical degrees of freedom which occurs when the pump is tuned to the lower motional sideband of the cavity resonance. OMIT may offer various advantages with respect to standard atomic EIT: i) it does not rely on naturally occurring resonances and could therefore be applied to previously inaccessible wavelength regions; ii) a single optomechanical element can already achieve unity contrast, which in the atomic case is only possible within the setting of cavity quantum electrodynamics; iii) one can achieve significant optical delay times, since they are limited only by the mechanical resonance lifetime 1/γm. Previous OMIT demonstrations have been carried out in a cryogenic setup [1,2]; here we show OMIT in a room temperature optomechanical setup consisting of a thin semitransparent membrane within a high-finesse optical Fabry-Perot cavity [3]. Fig. 1 (left upper panel) shows the phase shift acquired by the probe beam during its transmission through the optomechanical cavity. The derivative of such a phase shift gives the group advance due to causality-preserving superluminal effects which a probe pulse spectrally contained within the transparency window would accumulate in its transmission through the cavity. From the fitting curve we infer a maximum signal time advance τT ≈ -108 ms, which is very close to the theoretical achievable maximum τTmax = -2C/[γm(1 +C)], which is -109 ms in our case where the optomechanical cooperativity is C = 160. The reflected field is instead delayed, and from the corresponding expression for the maximum time delay τRmax = 2/[γm(1 +C)], we can also infer a group delay of the reflected probe field τR ≈ 670 μs [3]. In the left lower panel the transparency frequency window in which the probe is completely reflected by the interference associated with the optomechanical interaction is evident. The width of the transparency window is related to the effective mechanical dampingγeffm ≈ γm(1 +C). Therefore both delay and width can be tuned by changing C which in our case is achieved by shifting the membrane along the cavity axis. This is illustrated in the right panel, where the modulus of the beat amplitude vs Δ is plotted for different positions shifts z0 of the membrane from a field node (see caption).

Optomechanical sideband cooling of a thin membrane within a cavity

We present an experimental study of dynamical back-action cooling of the fundamental vibrational mode of a thin semitransparent membrane placed within a high-finesse optical cavity. We study how the radiation?pressure interaction modifies the mechanical response of the vibrational mode, and the experimental results are in agreement with a Langevin equation description of the coupled dynamics. The experiments are carried out in the resolved sideband regime, and we have observed cooling by a factor of ?350. We have also observed the mechanical frequency shift associated with the quadratic term in the expansion of the cavity mode frequency versus the effective membrane position, which is typically negligible in other cavity optomechanical devices.

Quantum dynamics of an optical cavity coupled to a thin semitransparent membrane: Effect of membrane absorption

We study the quantum dynamics of the cavity optomechanical system formed by a Fabry-Perot cavity with a thin vibrating membrane at its center. We first derive the general multimode Hamiltonian describing the radiation pressure interaction between the cavity modes and the vibrational modes of the membrane. We then restrict the analysis to the standard case of a single cavity mode interacting with a single mechanical resonator and we determine to what extent optical absorption by the membrane hinder reaching a quantum regime for the cavity-membrane system. We show that membrane absorption does not pose serious limitations and that one can simultaneously achieve ground state cooling of a vibrational mode of the membrane and stationary optomechanical entanglement with state-of-the-art apparatuses.

Assessment of a quantum phase-gate operation based on nonlinear optics

We analyze in detail the proposal for a two-qubit gate for travelling single-photon qubits recently presented by Ottaviani et al. [Phys. Rev. A 73, 010301(R) (2006)]. The scheme is based on an ensemble of five-level atoms coupled to two quantum and two classical light fields. The two quantum fields undergo cross-phase modulation induced by electromagnetically induced transparency. The performance of this two-qubit quantum phase gate for travelling single-photon qubits is thoroughly examined in the steady-state and transient regimes, by means of a full quantum treatment of the system dynamics. In the steady-state regime, we find a general trade-off between the size of the conditional phase shift and the fidelity of the gate operation. However, this trade-off can be bypassed in the transient regime, where a satisfactory gate operation is found to be possible, significantly reducing the gate operation time.

Tunable linear and quadratic optomechanical coupling for a tilted membrane within an optical cavity: theory and experiment

We present an experimental study of an optomechanical system formed by a vibrating thin semi-transparent membrane within a high-finesse optical cavity. We show that the coupling between the optical cavity modes and the vibrational modes of the membrane can be tuned by varying the membrane position and orientation. In particular, we demonstrate a large quadratic dispersive optomechanical coupling in correspondence with avoided crossings between optical cavity modes weakly coupled by scattering at the membrane surface. The experimental results are well explained by a first order perturbation treatment of the cavity eigenmodes.

TEST OF THE VIOLATION OF LOCAL REALISM IN QUANTUM MECHANICS WITHOUT BELL INEQUALITIES

Abstract A new, versatile polarization-entanglement scheme is implemented experimentally to investigate the violation of the EPR local-realims for a non-maximally entangled two-photon system according to the recent non-locality proof by Hardy.

Microfabrication of large-area circular high-stress silicon nitride membranes for optomechanical applications

In view of the integration of membrane resonators with more complex MEMS structures, we developed a general fabrication procedure for circular shape SiNx membranes using Deep Reactive Ion Etching (DRIE). Large area and high-stress SiNx membranes were fabricated and used as optomechanical resonators in a Michelson interferometer, where Q values up to 1.3 × 106 were measured at cryogenic temperatures, and in a Fabry-Perot cavity, where an optical finesse up to 50000 has been observed.

Engineering entangled-photon states using two-dimensional PPLN crystals

We propose to use periodically poled lithium niobate crystals (PPLN) as a source of specially engineered entangled photon pairs. We demonstrate the generation of polarization-entangled states using one-dimensional PPLN, and frequency-entangled states using two-dimensional PPLN.

Entangled Non-Local Quantum Interferometry

A novel quantum optical interference experiment based on a delayed correlation technique and involving photons prepared in a polarization-entangled state is presented. The resulting phenomenology is found to be determined by the simultaneous realization of several non-classical interference processes of distinct nature albeit strictly interrelated. A formal interpretation of the results is attempted by a standard quantum analysis and by the adoption of a set of interfering Feynman diagrams. In addition, a physical interpre-tation at a deeper level is undertaken by introducing for the system the basic theoretical background of a new concept in quantum optics: entangled interferometry. As a relevant application of this new paradigm, a novel two-photon welcher-weg argument within a delayed-choice experiment is presented. It leads to an unexpected complementarity puzzle which expresses the manifestation of the EPR non-locality within the general context of interferometry. PACS numbers: 03.65.Bz, 42.50.-p, 42.50.Wm.

Quantum dynamics of a vibrational mode of a membrane within an optical cavity

Optomechanical systems are a promising candidate for the implementation of quantum interfaces for storing and redistributing quantum information. Here we focus on the case of a high‐finesse optical cavity with a thin vibrating semitransparent membrane in the middle. We show that robust and stationary optomechanical entanglement could be achieved in the system, even in the presence of nonnegligible optical absorption in the membrane. We also present some preliminary experimental data showing radiation‐pressure induced optical bistability.