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arXiv:0806.2218 · Physical Review Letters

Entanglement Test on a Microscopic-Macroscopic System

F. De Martini, Fabio Sciarrino, Chiara Vitelli

Published Jun 26, 2008• 114 citations
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Abstract

A macrostate consisting of N approximately 3.5x10{4} photons in a quantum superposition and entangled with a far apart single-photon state (microstate) 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 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 multiparticle fields. We then demonstrate the nonseparability of the bipartite system by adopting a local filtering technique within a positive operator valued measurement.

DOI
10.1103/physrevlett.100.253601

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A macrostate consisting of N approximately 3.5x10{4} photons in a quantum superposition and entangled with a far apart single-photon state (microstate) 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 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 multiparticle fields. We then demonstrate the nonseparability of the bipartite system by adopting a local filtering technique within a positive operator valued measurement.
In recent years two fundamental aspects of quantum mechanics have attracted a great deal of interest, namely the investigation on the irreducible nonlocal properties of Nature implied by quantum entanglement and the physical realization of the 'Schroedinger Cat'. In the present work a macro - state consisting of about 3.5 x 10^4 photons in a quantum superposition and entangled with a far apart single - photon state is generated. Then, the non-separability of the overall micro-macro system is demonstrated and the corresponding Bell's inequalities are found to be violated. 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 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.
The present work investigates quantum phenomena with large size systems generated in the microscopic world and then transferred in the macro one through an amplification process. Precisely, a microscopic entangled photon state is created by a nonlinear optical process, then one subsystem is injected into an optical parametric amplifier and then converted into a macro — state consisting of N ≈ 3.5 × 104 photons in a quantum superposition. By adopting a local filtering technique, we then demonstrate the entanglement of the bipartite system. The new conceptual framework paves the way toward the generation of light-matter entangled states by coupling the output radiation with a Bose-Einstein condensate. The multiphoton superposition generated by a quantum-injected optical parametric amplifier (QI-OPA) can be made to interact with a Mirror-BEC shaped as a Bragg interference structure.
We investigate the multiphoton states generated by high-gain optical parametric amplification of a single injected photon, polarization encoded as a "qubit". The experiment configuration exploits the optimal phase-covariant cloning in the high gain regime. The interference fringe pattern showing the non local transfer of coherence between the injected qubit and the mesoscopic amplified output field involving up to 4000 photons has been investigated. A probabilistic new method to extract full information about the multiparticle output wavefunction has been implemented. This technique can be adopted to test the entanglement between a microscopic system and a macro one.
A Bell test is performed on a Micro‐Macroscopic photon system obtained by parametric amplification of a spin‐1 entangled photon state. The optical parametric amplification leads to the creation of a Micro‐Macroscopic entangled system consisting of N≃3.5×104. The performed Bell’s inequality non locality test involves the adoption of a positive operator valued measurement (POVM) strategy for a sharper identification of the amplified states. This is obtained through a local filtering technique and the experimental data are found to violate the Bell’s inequality albeit with a loophole similar to the detection efficiency one in photon counting experiments.
We discuss the conclusions that can be drawn on a recent experimental micro-macro entanglement test [De Martini, Sciarrino, and Vitelli, Phys. Rev. Lett. 100, 253601 (2008)]. The system under investigation is generated through optical parametric amplification of one photon belonging to an entangled pair. The adopted entanglement criterion makes it possible to infer the presence of entanglement before losses that occur on the macrostate under a specific assumption. In particular, an a priori knowledge of the system that generates the micro-macro pair is necessary to exclude a class of separable states that can reproduce the obtained experimental results. Finally, we discuss the feasibility of a micro-macro ``genuine'' entanglement test on the analyzed system by considering different strategies, which show that in principle a fraction $\ensuremath{\varepsilon}$, proportional to the number of photons that survive the lossy process, of the original entanglement persists in any loss regime.
The new process of quantum-injection into an optical parametric amplifier operating in entangled configuration is adopted to amplify into a large dimensionality spin-½ Hilbert space the quantum entanglement and superposition properties of the photon-couples generated by parametric down-conversion. The invariance properties of the OPA interaction hamiltonian show that, under certain conditions, the device may act as a Universal Quantum Cloning Machine (UQCM) of the input qubits. Preliminary results here reported show the first experimental realization of such a device. The structure of the Wigner function and of the field's correlation functions shows a multiphoton Schrödinger-cat behaviour of the emitted field which is largely detectable against the squeezed-vacuum noise.