Witlef Wieczorek

Pulsed Laser Cooling for Cavity Optomechanical Resonators

A pulsed cooling scheme for optomechanical systems is presented that is capable of cooling at much faster rates, shorter overall cooling times, and for a wider set of experimental scenarios than is possible by conventional methods. The proposed scheme can be implemented for both strongly and weakly coupled optomechanical systems in both weakly and highly dissipative cavities. We study analytically its underlying working mechanism, which is based on interferometric control of optomechanical interactions, and we demonstrate its efficiency with pulse sequences that are obtained by using methods from optimal control. The short time in which our scheme approaches the optomechanical ground state allows for a significant relaxation of current experimental constraints. Finally, the framework presented here can be used to create a rich variety of optomechanical interactions and hence offers a novel, readily available toolbox for fast optomechanical quantum control.

Highly efficient heralding of entangled single photons

Single photons are an important prerequisite for a broad spectrum of quantum optical applications. We experimentally demonstrate a heralded single-photon source based on spontaneous parametric down-conversion in collinear bulk optics, and fiber-coupled bolometric transition-edge sensors. Without correcting for background, losses, or detection inefficiencies, we measure an overall heralding efficiency of 83%. By violating a Bell inequality, we confirm the single-photon character and high-quality entanglement of our heralded single photons which, in combination with the high heralding efficiency, are a necessary ingredient for advanced quantum communication protocols such as one-sided device-independent quantum key distribution.

Optimal state estimation for cavity optomechanical systems

We demonstrate optimal state estimation for a cavity optomechanical system through Kalman filtering. By taking into account nontrivial experimental noise sources, such as colored laser noise and spurious mechanical modes, we implement a realistic state-space model. This allows us to obtain the conditional system state, i.e., conditioned on previous measurements, with a minimal least-squares estimation error. We apply this method to estimate the mechanical state, as well as optomechanical correlations both in the weak and strong coupling regime. The application of the Kalman filter is an important next step for achieving real-time optimal (classical and quantum) control of cavity optomechanical systems.

Revealing Anyonic Statistics with Multiphoton Entanglement

Anyons, manifested as quasiparticles in two-dimensional systems, exhibit fractional statistics that ranges continuously from bosonic to fermionic behavior. Here, we reveal anyonic features in a quantum simulation using multi-partite entangled states of polarized photons.

Multiphoton entanglement engineering via projective measurements

We present strategies to obtain different classes of three and four photon entangled symmetric states from a single experimental setup. The basic idea originates from the property of the symmetric Dicke state with two excitations to connect the two inequivalent types of genuine tripartite entanglement. We experimentally confirm the distinct types of entanglement of the observed states. We further propose an extension of the applied scheme that allows one to obtain different classes of four-photon entanglement by adding a fifth photon. The requirement of a single fifth photon is currently a technical challenge, and thus we consider the approach of using a strongly attenuated weak coherent beam instead.

Electro-mechanical Casimir effect

The dynamical Casimir effect is an intriguing phenomenon in which photons are generated from vacuum due to a non-adiabatic change in some boundary conditions. In particular, it connects the motion of an accelerated mechanical mirror to the generation of photons. While pioneering experiments demonstrating this effect exist, a conclusive measurement involving a mechanical resonator is still missing. We show that a hybrid system consisting of a piezoelectric mechanical resonator coupled to a superconducting cavity may allow to electro-mechanically generate measurable photons from vacuum, intrinsically associated to the dynamical Casimir effect. Such an experiment may be achieved with current technology, based on film bulk acoustic resonators directly coupled to a superconducting cavity. Our results predict a measurable photon generation, which can be further increased through additional improvements such as using superconducting metamaterials.

Multi-Photon Entanglement for Sub Shot-Noise Sensitivity

We experimentally demonstrate a general criterion to identify multi-photon entangled states useful for quantum metrology and prove their applicability for phase estimation with a sensitivity higher than the shot-noise limit.

Permutationally invariant tomography of a four-qubit symmetric Dicke state

Multi-partite entangled quantum states play an important role in quantum information processing with applications, for example, in quantum enhanced metrology or quantum communication. Therefore, efficient measurement schemes to fully characterize these states are needed. However, conventional quantum state tomography which reveals all properties of a quantum state suffers from an exponentially increasing measurement effort with the number of qubits.

Tunable Setup for an Entire Family of Four-Photon Entangled States

We report on the experimental observation and analysis of an entire family of four-photon entangled states. We demonstrate how these states can be obtained with a single linear optics set-up and analyze particular entanglement properties.

Multi-particle Correlations and Characteristic Bell Inequalities

The correlations between local measurement results are characteristic for multi-partite quantum systems. Under the condition of local realism, which assumes that properties of physical systems exist independent of being measured, and that space-like separated actions on the system cannot influence each other, these correlations are constrained by Bell-inequalities. Quantum mechanics predicts correlations which are not explainable under these two assumptions. The resulting violation of Bell inequalities was originally studied for pairs of entangled particles (Einstein et al.,1935). Meanwhile these ideas were extended to three-and more partite quantum states in particular with respect to the different kinds of multi-particle entanglement and their implications on the obtainable correlations (Mermin, 1990). Here we present a method to construct characteristic Bell inequalities for particular four-qubit states. These inequalities are a versatile tool, not only to provide evidence for non-locality, but also for the characterization of the states, with respect to detection of genuine multipartite entanglement and to state discrimination. Exemplarily we apply our approach to the symmetric four qubit Dicke-state (Dicke, 1954) with two excitations. (A symmetric N-qubit Dicke state with M excitations is the equal superposition of N-qubit pure product states having M logical 1 s and (N-M) logical 0s.) Experimentally the state was observed with photons as carrier of the qubits, using polarization encoding.