Figure 1

Proposed setup for an “entanglement laser.” An intense pump pulse propagates back and forth between the mirrors $M_1$ and $M_2$. Whenever it traverses the nonlinear crystal $C$ it creates polarization entangled photon pairs into the modes $a$ and $b$, which are counterpropagating pulses inside the cavity formed by the mirrors $M_3$ to $M_6$. The interferometrically stable cavities are adjusted such that the three pulses (pump, $a$ and $b$) always overlap in the crystal. The number of photons in $a$ and $b$ increases exponentially with the number of round-trips. They can be switched out of the cavity by electro-optic switches $S_a$ and $S_b$. The polarization of each pulse is then analyzed with the help of polarizing beam splitters (PBS) followed by photodiodes that give a signal proportional to the number of photons. Taking the difference between the photon numbers for the two polarizations behind each PBS corresponds to a spin measurement. The axis of spin analysis is changed by appropriate wave plates in front of the PBS.Reuse & Permissions

Figure 2

Time development of the ratio $⟨{\mathbf{J}}^2⟩/⟨N⟩$ and of the mean photon number $⟨N⟩$. The units are chosen such that $t=1$ corresponds to a single pass through the crystal. The initial photon creation rate ${\kappa }_0=1$, the mean down-converted photon loss rate $\overline{\lambda }=0.03$, and the pump loss rate $\Lambda =0.01$. After eight passes $⟨N⟩$ reaches the range of millions. The ratio $⟨{\mathbf{J}}^2⟩/⟨N⟩$ is shown for three different values of the loss rate imbalance $\Delta \lambda$, namely, 0, 0.001, and 0.002.Reuse & Permissions