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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Micro-macro entangled photon systems: results and perspectives

Chiara Vitelli, Nicolò Spagnolo, Fabio Sciarrino, F. De Martini

Published Apr 23, 2009• 0 citations
Publisher

Abstract

In this work we present the realization of multiphoton quantum states, obtained by optical parametric amplification, and we investigate their perspectives and possible applications. The multiphoton quantum states are generated by a quantum-injected optical parametric amplifier (QI-OPA) seeded by a single-photon belonging to an EPR entangled pair. The entanglement between the micro-macroscopic photon system is experimentally demonstrated, and the possible applications of the macro-qubits states are presented and discussed.

DOI
10.1117/12.821092

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In this work we present the realization of multiphoton quantum states, obtained by optical parametric amplification, and we investigate their perspectives and possible applications. The multiphoton quantum states are generated by a quantum-injected optical parametric amplifier (QI-OPA) seeded by a single-photon belonging to an EPR entangled pair. The entanglement between the micro-macroscopic photon system is experimentally demonstrated, and the possible applications of the macro-qubits states are presented and discussed.
In the present work we propose to realize a macroscopic light-matter entangled state, obtained by the interaction of a multiphoton quantum superposition with a BEC system. The multiphoton quantum state is generated by a quantum-injected optical parametric amplifier (QI-OPA) seeded by a single-photon belonging to an EPR entangled pair and interacts with a Mirror-BEC shaped as a Bragg interference structure. The overall process will realize an entangled macroscopic quantum superposition involving a "microscopic" single-photon state of polarization and the coherent "macroscopic" displacement of the BEC structure acting in space-like separated distant places. This hybrid photonic-atomic system could open new perspectives on the possibility of coupling the amplified radiation with an atomic ensemble, a Bose-Einstein condensate, in order to implement innovative quantum interface between light and matter.
The multiphoton quantum superposition generated by a quantum-injected optical parametric amplifier (QI-OPA) seeded by a single-photon belonging to an EPR entangled pair is made to interact with a Mirror-BEC shaped as a Bragg interference structure. The effect can be greatly enhanced if the mirror is part of a cavity structure or is used within an optical interferometer. The overall process will realize an Entangled Macroscopic Quantum Superposition involving a ''microscopic'' singe-photon state of polarization and the coherent ''macroscopic'' displacement of the BEC structure acting in space-like separated distant places.
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.
Macro-micro entanglement originates from the Schrodinger's Cat paradox. The paradox has been attracting the interest of the physicists since it was proposed. Schrodinger's Cat paradox is a thought experiment that entangles a cat with some decay atoms, in which the entanglement between the macroscopic object and the microscopic atoms is established. Mac-micro entanglement relates to some important problems in quantum physics. It is more likely to interact with the surroundings for the quantum system as its size increases, which is the reason why we hardly observe the macroscopic superposition state. Can the superposition state theory of quantum physics be used in macro domain? Is there a limitation to the scale for the objects in the superposition states? These questions need studying and verifying in experiment. In addition, the preparation of the macro-micro entanglement state provides a new possibility to study the decoherence model. Macro-micro entanglement can be realized in many physical systems, such as atomic ensembles, superconducting circuits, electro-mechanical and opto-mechanical systems. Here in this paper we will introduce the development of macro-micro entanglement in optical system. The initial approach to creating the macro-micro entanglement in the context of optical system is quantum cloning by simulating the emission. Then the quantum-injected optical parametric amplification is used to amplify single photon to a macroscopic level. Afterwards, the displacement in phase space is proposed to create the macro-micro entanglement. Since the photon number of the macro-micro entanglement with the optical parametric amplification approach can be about 104, the studies towards the detection of this type of entanglement with human eyes have been extensively conducted. But it is realized that the coarse-grained measurements, such as those with the human eye, generally cannot judge whether macro-micro entanglement exists, and hence cannot be used to prove the considered type of micro-macro entanglement. A way of overcoming this difficulty is to invert the amplification process, bringing the macro system back to the micro level. The entanglement can then be verified by using single-photon detectors. Because local operation and classical communication cannot create entanglement, the de-amplification process will not increase the entanglement and the presence of the entanglement in the end shows that entanglement is present between the amplification and de-amplification process. Inspired by this thought, two groups create and verify mac-micro entanglement between one photon and 108 photons. What they used to amplify the micro states is the displacement operation in phase space, which can be realized by combining a single photon state and a coherent state with a highly asymmetric beam splitter. Because the entanglement is a precondition for a secure quantum key distribution, and the macro-micro entanglement has more photons than the traditional micro entanglement, we will discuss the possibility whether the macro-micro entanglement can be used in quantum key distribution and improve the distance of the quantum key distribution. We point out that the mac-micro entanglement and the binary reverse reconciliation continuous variable quantum key distribution protocol are the same in physics essence. We will introduce a quantum key distribution scheme with two phase entangled coherent states. Although the security proof of the scheme is not complete, it still provides us with the possibility to use the macro-micro entanglement in quantum key distribution.
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.
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.