Spin state transferred between atomic gases
The macroscopic quantum spin state of caesium atoms held in a vessel has been teleported to a second vessel 50 cm away – according physicists in Denmark, Spain and the UK, who have performed the feat. Although this distance is far smaller than the 143 km record for the quantum teleportation of relatively simple states, the experiment achieves a different type of teleportation that had previously been achieved only across microscopic distances. The technique can teleport complex quantum states and could therefore have a range of technological applications – including quantum computing, long-distance quantum communication and remote sensing.
Quantum teleportation was first proposed in 1993 by Charles Bennett, of the IBM Thomas J Watson Research Center in New York, and colleagues. It allows one person (Alice) to send information about an unknown quantum state to another person (Bob) by exchanging purely classical information. It utilizes the quantum entanglement between two particles; one with Alice and one with Bob. Alice interacts the unknown quantum state with her half of the entangled state, measures the combined quantum state and sends the result through a classical channel to Bob. The act of measurement alters the state of Bob's half of the entangled pair and this, combined with the result of Alice's measurement, allows Bob to reconstruct the unknown quantum state.
This deterministic continuous-variable teleportation was proposed and realized in the lab by Eugene Polzik and colleagues at the Niels Bohr Institute in Copenhagen, together with researchers at the Institute of Photonic Sciences (ICFO) in Barcelona and the University of Nottingham. Their experimental set-up involves two room-temperature samples of caesium-133 gas held in glass containers and separated by about 50 cm. The aim of the experiment is to use light to teleport the collective quantum spin state of 10E12atoms from one container to the other. The team extended the life of the state by coating the insides of the containers with a special material that does not absorb angular momentum from the atoms.
Precise control over the spin states of the system was done using constant and oscillating magnetic fields. They also collaborated with theorists Christine Muschik at the ICFO and Ignacio Cirac of the Max Planck Institute for Quantum Optics, near Munich, to develop a new model of the interaction between the atoms and the light. Using these advances, they teleported multiple collective spin states between the two canisters and looked at the variance in their measurements. When they compared this with the theoretical minimum variance that could be achieved by sending the spin state information in a purely classical manner, they found that the variance from their process was lower. "We have achieved the first deterministic, atomic-to-atomic teleportation over a macroscopic distance," says Polzik.
Hugues de Riedmatten, a quantum-optics expert at the ICFO – who was not involved with the experiment – says that the research is "very significant", describing the results as "convincing". He cautions, however, that it is "a proof of principle", saying "I think it's a first step. If you would like to use it for doing useful things in quantum-information science, for example, you would need to transport much more complicated quantum states. It remains to be seen whether this will be possible or not."