We find the EIT coherence plays an essential role in determining the coherence time for paired photons. These can be sensitive to phase shifts induced by fundamental physics processes such as interactions with ultralight dark matter or the passage of gravitational waves. A number of factors limiting the coherence time are analyzed in detail. Atom interferometers measure quantum interference patterns in the wave functions of cold atoms that follow superpositions of different space-time trajectories. Most labs need home-made stabilization of the field according to the actual setup, as commercially available power supply cannot directly satisfy their requirements. In our knowledge, this is the best record of coherence time for paired photons achieved so far. Cold atom experiments usually need a controllable and low-noise bias magnetic field to provide a quantization axis. 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 particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. Such strong coupling is desirable in quantum information processing, quantum storage and communication. Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. For theexcited states 3 and 4 of 85 Rb atoms, the populationdecay rates are 3 (4) 2 × 6 MHz.The dephasing rates of the excited states are 13 (14) (1/2) 3 (4) for cold atomic. This thesis describes quantum measurements of an ensemble of cold rubidium-87 atoms.
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