Francesco De Martini

Realization and Characterization of a Two-Photon Four-Qubit Linear Cluster State

Cluster states, recently introduced as a fundamental resource for one-way quantum computation, represent genuine multiqubits entangled states. The one-way model is based on the initial preparation of entangled qubits in the cluster state, followed by single qubits measurements and feed-forwards. All the difficulties present in the standard computation model, for instance the implementation of two qubits gates, are transferred in the one-way model to the state preparation.

Hyperentanglement of two photons in three degrees of freedom

A six-qubit hyperentangled state has been realized by entangling two photons in three degrees of freedom. These correspond to the polarization, the longitudinal momentum, and the indistinguishable emission produced by a two-crystal system operating with type I phase matching in the spontaneous parametric down conversion regime. The state has been characterized by a chained interferometric apparatus and its complete entangled nature has been tested by a witness criterion specifically presented for hyperentangled states. The experiment represents a realization of a genuine hyperentangled state with the maximum entanglement between the two particles allowed in the given

Experimental Entanglement and Nonlocality of a Two-Photon Six-Qubit Cluster State

{2}^{3}\ifmmode\times\else\texttimes\fi{}{2}^{3}

Multipath entanglement of two photons.

Hilbert space.

Entanglement Test on a Microscopic-Macroscopic System

We create a six-qubit linear cluster state by transforming a two-photon hyperentangled state in which three qubits are encoded in each particle, one in the polarization and two in the linear momentum degrees of freedom. For this state, we demonstrate genuine six-qubit entanglement, persistency of entanglement against the loss of qubits, and higher violation than in previous experiments on Bell inequalities of the Mermin type.

Active One-Way Quantum Computation with Two-Photon Four-Qubit Cluster States

We present a novel optical device based on an integrated system of microlenses and single-mode optical fibers. It allows us to collect and direct into many modes two photons generated by spontaneous parametric down-conversion. By this device multiqubit entangled states and/or multilevel qudit states of two photons, encoded in the longitudinal momentum degree of freedom, are created. The multipath photon entanglement realized by this device is expected to find important applications in modern quantum information technology.

Proposed bell experiment with genuine energy-time entanglement

A Macro-state consisting of N ≈ 3.5 × 10 photons in a quantum superposition and entangled with a far apart single-photon state (Micro-state) is generated. Precisely, an entangled photon pair is created by a nonlinear optical process, then one photon of the pair is injected into an optical parametric amplifier (OPA) operating for any input polarization state, i.e. into a phase-covariant cloning machine. Such transformation establishes a connection between the single photon and the multi particle fields. We then demonstrate the non-separability of the bipartite system by adopting a local filtering technique within a positive operator valued measurement.

Enhancing the violation of the Einstein-Podolsky-Rosen local realism by quantum hyperentanglement

By using 2-photon 4-qubit cluster states we demonstrate deterministic one-way quantum computation in a single qubit rotation algorithm. In this operation feed-forward measurements are automatically implemented by properly choosing the measurement basis of the qubits, while Pauli error corrections are realized by using two fast driven Pockels cells. We realized also a C-NOT gate for equatorial qubits and a C-PHASE gate for a generic target qubit. Our results demonstrate that 2-photon cluster states can be used for rapid and efficient deterministic one-way quantum computing.

Exploiting quantum parallelism of entanglement for a complete experimental quantum characterization of a single-qubit device

Fransons Bell experiment with energy-time entanglement [Phys. Rev. Lett. 62, 2205 (1989)10.1103/PhysRevLett.62.2205] does not rule out all local hidden variable models. This defect can be exploited to compromise the security of Bell inequality-based quantum cryptography. We introduce a novel Bell experiment using genuine energy-time entanglement, based on a novel interferometer, which rules out all local hidden variable models. The scheme is feasible with actual technology.

Delayed-choice entanglement swapping with vacuum–one-photon quantum states

Mermins observation [Phys. Rev. Lett. 65, 1838 (1990)] that the magnitude of the violation of local realism, defined as the ratio between the quantum prediction and the classical bound, can grow exponentially with the size of the system is demonstrated using two-photon hyperentangled states entangled in polarization and path degrees of freedom, and local measurements of polarization and path simultaneously.