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The Japan Society of Applied Physics

Two-color Polarization-Entangled States in a Collective Depolarizing Channel

Youhei Okawa, Fuminori Omura, Yuhsuke Yasutake, S. Fukatsu

Published Feb 3, 2017• 0 citations
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We demonstrate decoherence-tolerant transmission of a Bell state through a single-mode fiber (SMF) using the photon frequency degree of freedom. To this end, a slightly non-degenerate polarization-entangled singlet, |Ψ−⟩=(|HV⟩−|VH⟩)/2, is localized at the SMF as the depolarization channel subject to random noise due to incessant fiber bending. Two-photon beats and quantum state tomography jointly verify the absence of collective decoherence, showing that |Ψ−⟩ is the sought-after one-dimensional decoherence-free subspace (DFS) pertaining to SMF. Efficient splitting and combining of photon streams in our DFS help outperform the DFS in time domain. This motivates us to attempt DFS-enabled fault-tolerant fiber transmission of biphoton qubits. Two-photon BB84 protocol is implemented in a polarization-maintaining fiber to which dephasing noise is relevant so that a two-dimensional DFS is appropriate. A low bit error rate 5.4% is achieved by encoding one-qubit information onto the biphoton state in spite of significant polarization fluctuation. Our scalable frequency-based DFS has a natural affinity for wavelength division multiplexing in fiber communication by design and as such is extensible to multi-particle entanglement.
Entangled photons are important resources for quantum information. Especially, in quantum cryptography, the polarization entangled photons are highly secured. However, the entanglement of photon can hold in isolated system. In realistic system, the entangled photons inevitably interact with environment. In this work, we study the effect of depolarizing noise on the two general entangled qubits. The entangled photon are prepared with different entangled states and send to the depolarizing channel. We measure the entanglement quality by using concurrence. When the entangled photons interact to the depolarizing noise, the entanglement quality of photons will be degraded, and the concurrence of entangled photons reduces to 0. Moreover, we also apply Hadamard gate to the entangled qubits and send to the depolarizing channel. The Hadamard gate can filter the maximally entangled qubits in the depolarizing channel.
The characteristics of quantum channel play important roles on designing a high performance quantum communication system. In this paper, we analyze the fidelity of polarization state travelling through a non-Markovian optical fiber depolarizing quantum channel with color noise environment. The quantum bit error rate (QBER) of a polarization coding BB84 quantum key distribution (QKD) system under non-Markovian depolarizing channel is also discussed. Analysis results show that there are several security regions caused by the fluctuation of QBER in such a non-Markovian channel. These will provide references for designing polarization compensation scheme and estimating of secure distance.