title: Coherence time limit of the biphotons generated in a dense cold atom cloud.
abstract: Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. Such strong coupling is desirable in quantum information processing, quantum storage and communication. In particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. In this paper, we report the narrow-band biphotons with a coherence time of 2.34 μs generated from spontaneous four-wave mixing (SFWM) in a dense cold atom cloud, in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency (EIT) window. In our knowledge, this is the best record of coherence time for paired photons achieved so far. A number of factors limiting the coherence time are analyzed in detail. We find the EIT coherence plays an essential role in determining the coherence time for paired photons. The EIT dephasing rate is the ultimate limit to the coherence time, and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.
authors.affiliation: Quantum Institute for Light and Atoms, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, No. 500 Dongchuan Road, Shanghai, 200241, China.
authors.forename: Zhiguang
authors.initials: Z
authors.lastname: Han
authors.affiliation: Quantum Institute for Light and Atoms, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, No. 500 Dongchuan Road, Shanghai, 200241, China.
authors.forename: Peng
authors.initials: P
authors.lastname: Qian
authors.affiliation: Quantum Institute for Light and Atoms, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, No. 500 Dongchuan Road, Shanghai, 200241, China.
authors.forename: L
authors.initials: L
authors.lastname: Zhou
authors.affiliation: Quantum Institute for Light and Atoms, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, No. 500 Dongchuan Road, Shanghai, 200241, China.
authors.forename: J F
authors.initials: JF
authors.lastname: Chen
authors.affiliation: Quantum Institute for Light and Atoms, State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, No. 500 Dongchuan Road, Shanghai, 200241, China.
authors.forename: Weiping
authors.initials: W
authors.lastname: Zhang