Quantum computers or cryptography networks would take advantage of entanglement, in which two distant particles share a complementary quantum state. In some conceptions of these devices, quantum states that act as units of information would have to be transferred from one group of atoms to another in the form of light. Because measuring any quantum state destroys it, that information cannot simply be measured and copied. Researchers have long known that this obstacle can be finessed by a process called teleportation, but they had only demonstrated this method between light beams or between atoms.
In taking the next step, Eugene Polzik and his colleagues at the Niels Bohr Institute in Copenhagen shined a strong laser beam onto a cloud of room-temperature cesium atoms whose spins were all pointing in the same direction and fluctuating according to their given quantum state. The laser became entangled with the collective spin of the cloud, meaning that the quantum states of laser and gas shared the same amplitude but had opposite phases. The goal was to transfer, or teleport, the quantum state of a second light beam onto the cloud.
To do so, the group mixed a second, weaker laser pulse with the strong laser and split the superimposed beams into two arms. A detector in one arm measured the sum of the beams' amplitudes and a detector in the second arm measured the difference between their phases. Neither measurement disturbed the delicate entangled state between the light and cesium. But the researchers could use the results to apply a precise magnetic field to the cesium vapor that effectively canceled out the ensemble's original spin state and replaced it with one that corresponded to the polarization of the weak pulse, as they report in the 5 October Nature.
"The key point was to be able to generate efficient entanglement, so that every time I push the button I get this entangled state," says Polzik. He says his team excluded most sources of noise, which would have corrupted the entanglement process, by forcing laser and atoms to interact only at a high frequency. The resulting efficiency allowed them to run the experiment at room temperature, Polzik explains. "That's what makes this very attractive. This is sufficiently cheap" compared with ultracold reservoirs of gas, he says.