Ordinary computers process data as 0's or 1's. But in the fuzzy world of quantum mechanics, a quantum bit or "qubit" can be both 0 and 1 at the same time. Now scientists have upped the ante with a new way to create "qutrits" that can be 0, 1, and 2--and send them over longer distances than was previously possible. Researchers hope this new technique can help lead to better communications and solve problems too complex for any computer now known.
Qubits can become "entangled," each qubit mirroring the behavior of the other even across vast distances. Scientists hope to use entanglement to beam data securely from one place to another. Theory suggests that qutrits can become more strongly entangled than qubits, says physicist Nicolas Gisin of the University of Geneva in Switzerland. This could mean that signals sent with qutrits are more resistant to disturbances that ruin entanglement, enabling communication over longer distances.
Doctoral student Rob Thew of Gisin's group developed a new way to generate, control, and analyze entangled qutrits. First they fired photons into crystals of lithium niobate. Once inside, photons can spontaneously split into entangled pairs of photons. These pairs are themselves split up, with each photon sent down a separate optical fiber leading to an interferometer. What's new about this setup is that once inside the interferometer, a photon has an equal chance of going down one of three fiber-optic paths, either short, medium, or long. Due to their entanglement, each member of a photon pair has to travel the same distance. Once measured, both photons are detected in the same length path. Each photon thus acts as an entangled qutrit, with photons taking the short path representing a 0, those taking the medium path a 1, and those taking the long path a 2. This demonstration of entangled qutrits opens up new possibilities for quantum communication, the researchers say.
With the new technique, photon pairs could be separated by 10 kilometers or possibly more, Gisin predicts. When entangled qutrits were first created last year, using quartz crystals instead of fiber-optics, they could only be separated by a meter or two, limiting their potential use for communication, Gisin says. His group has posted its findings on the online preprint archive Arxiv.org at Cornell University. "Now that we, the field, can actually start creating these states, we can start thinking more seriously about the implications" for quantum communication, says quantum physicist Paul Kwiat of the University of Illinois, Urbana-Champaign.