Generation of Narrow-Band Polarization-Entangled Photon Pairs for Atomic Quantum Memories

We report an experimental realization of a narrow band polarization-entangled photon source with a linewidth of 9.6 MHz through cavity-enhanced spontaneous parametric down-conversion. This linewidth is comparable to the typical linewidth of atomic ensemble-based quantum memories. Single-mode output is realized by setting a reasonable cavity length difference between different polarizations, using of temperature controlled etalons and actively stabilizing the cavity. The entangled property is characterized with quantum state tomography, giving a fidelity of 94% between our state and a maximally entangled state. The coherence length is directly measured to be 32 m through two-photon interference.

Figure 1

Layout of the experiment. The generation of polarization entanglement is realized by interfering the two down-converted photons on PBS2. An electro-optic modulator (EOM) phase modulated at 70 MHz is used to generate sidebands for the locking beam. A PBS, a Faraday rotator (FR) and a half-wave plate (HWP) is used to extract the reflection beam and to generate the error signal for the locking system.

Figure 2

(a) Polarization correlations for the entangled photon pair. Polarization angle for photon $A$ is fixed to 90° for the red (dotted curve), and $-45°$ for the blue (solid curve). Error bars represent statistical errors. (b) Tomography measurement result, with the left for the real part, the right for the imaginary part.

Figure 3

(a) Time correlation measurement. The data are fitted with a function of $c_0{e}^{-2\pi \Delta \nu |t|}$. The FWHM correlation time is 23.0 ns. (b) Visibility in $|+⟩/|-⟩$ basis as a function of the relative delay. The data are fitted with a function of $v_0{e}^{-x/x0}$. Error bars represent statistical errors.

Figure 4

Pair generation rate as a function of the UV pump power.