For example, the properties of entangled photons are inextricably linked to each other, even if the photons are located on opposite sides of the galaxy. To study this “correlation at a distance,” Kwiat and graduate students Joseph Altepeter and Evan Jeffrey produce pairs of polarization-entangled photons by passing a laser pulse through two adjacent nonlinear crystals.
“You can think of polarization as the `wiggle’ direction of the photon – either horizontal, vertical or diagonal,” Kwiat said. “As soon as you determine the wiggle direction of one photon in an entangled pair, you immediately know the wiggle direction of the other photon, no matter how far apart they are.”
A major production problem, however, is that entangled photons are emitted in many directions and with a wide range of polarization phase relationships, each acting like an individual singer in a large choir.
“Instead of hearing a soloist hit one note, we were hearing many choir members, some of whom were singing off-key,” Kwiat said.
The trick was to come up with a way of tuning the system. “We found that we could pass the photons through another crystal – one that has a different phase profile – to compensate for the different phase relationships,” Kwiat said. “The dissonance is corrected and the system becomes harmonized.”
In the same manner as a corrector lens in a telescope removes chromatic aberration and improves image quality, the researchers’ special birefringent crystal removes distortions in the quality of the entanglement. “After the compensator crystal, the photons are all entangled in exactly the same way,” Altepeter said. “We can open the iris and get more than 1 million useful pairs per second.”
Ultrabright, ultrapure sources of entangled photons are essential for pursuing quantum computing and quantum networks, as a resource for teleportation in quantum communication, and for sending more information faster by means of quantum cryptography. High fidelity quantum states can also provide researchers with a clearer picture of how the universe works on a very fundamental level.
“Using a low-brightness source is like looking into the quantum world through a foggy window,” Altepeter said. “With a bright, pure source, we have a very clear window that allows us to see phenomena we couldn’t see before.”
The work was funded by the National Science Foundation, the Army Research Office, and the Advanced Research and Development Activity.