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IEEE International Conference on Space Optical Systems and Applications (ICSOS)

Hybrid entanglement photon pair source for fiber-space flexible QKD network

Mikio Fujiwara, Morio Toyoshima, Masahide Sasaki, K. Yoshino, Yoshihiro Nambu, Akihisa Tomita

Published May 1, 2011• 0 citations
Publisher

Abstract

We report on the experimental demonstration of a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The hybrid entanglement source is constructed of a non-degenerate photon pair source and a format transformer from time encoding to polarization encoding. This entanglement photon source will provide a fiber- space flexible QKD system.

DOI
10.1109/icsos.2011.5783681

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We report on the experimental demonstration of a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The hybrid entanglement source is constructed of a non-degenerate photon pair source and a format transformer from time encoding to polarization encoding. This entanglement photon source will provide a fiber- space flexible QKD system.
We report on the experimental demonstration of a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The polarization qubit at 810 nm is transformed by an asymmetric Mach–Zehnder interferometer consisted of a Glan laser prism and a polarization-maintaining fiber. We obtained visibilities of 95.8% and 88% with tolerance ±0.2% and ±1% along Z-Z and X-X axes on the Poincare sphere, respectively, with a coincidence count rate of more than 800 c/s after entanglement format transformation. These values are well above the threshold of 70.7% needed to violate a Bell inequality and allow distilling a secure key in the quantum key distribution.
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We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). We use a fibered birefingent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ+⟩ = 1/√2 [|s,l⟩ − |l,s⟩], where s and l represent the short and long time-bins, respectively.
We show that the quantum disentanglement eraser implemented on a two-photon system from parametric downconversion is a general method to create hybrid photonic entanglement, namely the entanglement between different degrees of freedom of the photon pair. To demonstrate this, we generate and characterize a source with tunable degree of hybrid entanglement between two qubits, one encoded in the transverse momentum and position of a photon, and the other in the polarization of its partner. In addition, we show that a simple extension of our setup enables the generation of two-photon qubit-qudit hybrid entangled states. Finally, we discuss the advantages that this type of entanglement can bring for an optical quantum network.