Experimental Demonstration of Four-Photon Entanglement and High-Fidelity Teleportation

Abstract

We experimentally demonstrate observation of highly pure four-photon GHZ entanglement produced by parametric down-conversion and a projective measurement. At the same time this also demonstrates teleportation of entanglement with very high purity. Not only does the achieved high visibility enable various novel tests of quantum nonlocality, it also opens the possibility to experimentally investigate various quantum computation and communication schemes with linear optics. Our technique can in principle be used to produce entanglement of arbitrarily high order or, equivalently, teleportation and entanglement swapping over multiple stages.Entanglement is not only the essence of quantum mechanics as suggested by Erwin Schrödinger [1], but is also at the basis of nearly all quantum information protocols such as quantum cryptography, quantum teleportation and quantum computation [2]. While entanglement of two qubits is routine in the laboratory, entanglement of three photons [3] with high quality has only recently been experimentally realized [4] and used to experimentally demonstrate the extreme contradiction between local realism and quantum mechanics [5] in so-called GHZ states. In a parallel development entanglement of the quantum states of three atoms [6] or four qubits in ions [7] has been demonstrated, yet in all these cases the quality of the entangled states still needs to be significantly improved in order to be useful for tests of quantum mechanics or in quantum information schemes.A similar situation is found in the recent teleportation experiments [8][9][10][11]. To verify the nonlocal character of teleportation, two conditions must be satisfied in any experiment. On the one hand, one has to demonstrate that a genuinely unknown state (in the optimal case, a qubit which itself is still entangled to another one) is teleported [12], on the other hand a high experimental visibility is necessary in order to exclude local hidden variable models (LHV) [13][14][15][16]. The so-called entanglement swapping experiment [10] is the only one to date that demonstrates the teleportation of a genuinely unknown state. However, since its observed visibility was lower than 71%, one could in principle still doubt the nonlocal feature of teleportation [13].In this letter we report on an experiment that not only demonstrates the observation of four-photon entanglement but also shows high-fidelity entanglement swapping, thus proving the nonlocal character of quantum teleportation. Both features are not only important for performing novel fundamental experiments to test quantum mechanics or to demonstrate its counter-intuitive features, but also to expand our toolbox for quantum computation and quantum communication.Our technique of observing four-photon GHZ entanglement uses two independently created photon pairs (Fig. 1). Suppose that the two pairs are in the statewhich is a tensor product of two polarization entangled photon pairs. Here |H (|V ) indicates the state of a horizontally (vertically) polarized photon. Principle fo...

Entanglement of the orbital angular momentum states of photons

“…two-state quantum systems including recent realization of three- [3,4] and four-qubit [5,6] entanglements. Yet, an ever increasing body of theoretical work calls for entanglement in quantum system of higher dimensions [7,8].…”

Experimental generation of an eight-photon Greenberger–Horne–Zeilinger state

“…In particular, polarization-entangled Einstein-Podolsky-Rosen (EPR) pairs are often used as the basic blocks for generating multi-photon entangled states or as resources for performing quantum teleportation tasks. A number of experiments involving two photon pairs [1][2][3][4][5][6][7][8] and a few experiments involving five photons (two photon pairs plus one single photon) 9 or three photon pairs [10][11][12][13] have been carried out to demonstrate various protocols in quantum communication and linear optical quantum computing. Typical systems for two-photon-pair experiments use a frequency-doubled Ti:sapphire mode-locked laser as a pump, with a repetition rate of ~80 MHz, a pulse duration of ~150 fs, and an operating wavelength of ~400 nm.…”

The Correlations of Greenberger-Horne-Zeilinger States Described by Hilbert-Schmidt Decomposition

“…Greenberger-Horne-Zeilinger (GHZ) states have attracted much attention in recent literature [1][2][3][4][5][6][7][8][9][10][11] as experiments which are done with these states disprove local-realism, without the use of Bell's inequalities. In many articles it was claimed that GHZ states violate locality.…”