Experimental quantum networking protocols via four-qubit hyperentangled Dicke states.

We report the experimental demonstration of two quantum networking protocols, namely quantum 1→3 telecloning and open-destination teleportation, implemented using a four-qubit register whose state is encoded in a high-quality two-photon hyperentangled Dicke state. The state resource is characterized using criteria based on multipartite entanglement witnesses. We explore the characteristic entanglement-sharing structure of a Dicke state by implementing high-fidelity projections of the four-qubit resource onto lower-dimensional states. Our work demonstrates for the first time the usefulness of Dicke states for quantum information processing.

Experimental realization of optimal noise estimation for a general Pauli channel.

We present the experimental realization of the optimal estimation protocol for a Pauli noisy channel. The method is based on the generation of 2-qubit Bell states and the introduction of quantum noise in a controlled way on one of the state subsystems. The efficiency of the optimal estimation, achieved by a Bell measurement, is shown to outperform quantum process tomography.

Hyperentangled mixed phased Dicke states: optical design and detection.

A recently introduced family of multipartite entangled states, the 4-qubit phased Dicke states, has been created by 2-photon hyperentanglement. Our experimental method allows high state fidelity and generation rate. By introducing quantum noise in the multipartite system in a controlled way, we have tested the robustness of these states. To this purpose the entanglement of the resulting multipartite entangled mixed states has been verified by using a new kind of structural witness.

Bose-einstein partition statistics in superradiant spontaneous emission

We report the realization of the spatial counterpart of the Dicke superradiance. The new process is revealed by the realization of the spatial quantum partition statistics within the detection of photons emitted in sub-Poissonian regime by an active microcavity excited by ultrashort pulses. The superradiant enhancement of the time decay of the dipole excitation has also been investigated.

Long-distance distribution of genuine energy-time entanglement.

Any practical realization of entanglement-based quantum communication must be intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell's inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energy-time entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energy-time entanglement distributed over 1 km of optical fibres that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fibre-based long interferometer. Our results represent an important step towards long-distance secure quantum communication in optical fibres.

All-versus-nothing nonlocality test of quantum mechanics by two-photon hyperentanglement.

We report the experimental realization and the characterization of polarization and momentum hyperentangled two-photon states, generated by a new parametric source of correlated photon pairs. By adoption of these states an "all-versus-nothing" test of quantum mechanics was performed. The two-photon hyperentangled states are expected to find at an increasing rate a widespread application in state engineering and quantum information.

Microcavity transverse coherence length and microlaser threshold.

The concept of transverse coherence length in an active microscopic cavity is investigated with regard to the threshold of the microlaser action. This physical parameter is related to the linear spatial extent, or the effective radius, of the transverse lasing mode operating in a plane Fabry-Perot microcavity. This problem is analyzed by means of a full semiclassical theory, by which the microlaser threshold is expressed as a function of the relevant parameters of the thresholdless microlaser and of the extension of the active pumped zone. Experimental results are in satisfactory agreement with the theoretical predictions.

Frequency hopping in quantum interferometry: efficient up-down conversion for qubits and ebits.

A novel Mach-Zehnder interferometer terminated at two different frequencies realizes in a quantum regime the nonlinear frequency conversion of optical quantum superposition states. The information-preserving character of the relevant unitary transformation has been experimentally demonstrated for input qubits and ebits. Besides its own intrinsic fundamental interest, the new scheme is expected to find important applications in modern quantum information technology.

Polarization temporal dynamics in a dye microlaser.

We have investigated the temporal dynamics of the output polarization of a dye microlaser operating in a transient regime, i.e., under femtosecond excitation. In these conditions the dipole angular diffusion has an important role in the formation of the microlaser pulse, as is shown in a theoretical model. By performing the experiment for different values of the microcavity length and varying the polarization angle, we measured threshold and buildup time of the microlaser and compared the results obtained with two dye solutions of different viscosity. The agreement between theory and experimental results is adequate.

Generation and characterization of Werner states and maximally entangled mixed states by a universal source of entanglement.

We present a novel technique for generating two-photon polarization mixed states of any structure, which is based on the peculiar spatial characteristics of a high brilliance source of entangled pairs. Werner states and maximally entangled mixed states, two well-known families of mixed states important for quantum information, have been created and fully characterized by this technique. We have also investigated and tested the nonlocal properties of these states.

Detection of entanglement with polarized photons: experimental realization of an entanglement witness.

We report on the first experimental realization of an entanglement witness, a method to detect entanglement with few local measurements. The present demonstration has been performed with polarized photons in Werner states, a well-known family of mixed states that can be either separable or nonseparable. The Werner states are generated by a novel high brilliance source of bipartite entangled states by which the state mixedness can be easily adjusted.