Coupling Laser

The coupling laser is tuned to the energy difference between states 2 and 3, so it doesn't interact at all with the electrons in the ground state. Nothing happens. Then a second laser, called the "probe laser" that contains the information they want to capture is sent into the condensate. The probe laser is tuned to the energy difference between energy level 1 and 3. In the absence of the coupling laser beam, the probe laser pulse would be completely absorbed by the electrons in the ground state promoting them to the higher energy level and destroying the condensate. All information would be lost. Instead, the coupling laser prevents this absorption. The two laser beams shift the electrons into a quantum superposition of states 1 and 2, meaning that each individual electron is in both states at once. In a way, the two laser beams cancel each other out, like evenly matched competitors in a tug of war. This superposition state allows the light pulse to be imprinted in the atoms.