In recent years, the Austrian physicist Anton Zeilinger has bounced entangled photons off orbiting satellites and made 60-atom fullerene molecules exist in quantum superposition–essentially, as a smear of all their possible positions and energy states across local space-time. Now he hopes to try the same stunt with bacteria hundreds of times larger. Meanwhile, Hans Mooij of the Delft University of Technology, with Seth Lloyd, who directs MIT’s Center for Extreme Quantum Information Theory, has created quantum states (which occur when particles or systems of particles are superpositioned) on scales far above the quantum level by constructing a superconducting loop, visible to the human eye, that carries a supercurrent whose electrons run simultaneously clockwise and counterclockwise, thereby serving as a quantum computing circuit.
The physicist Richard Feynman proposed the idea of quantum computing in 1981 to exploit the information-processing potential of atoms, photons, and elementary particles. By now, the field has advanced sufficiently far that researchers not only are able to manipulate physics for unprecedented experimental effects but have proposed commercial applications.
But before technologies like quantum communications, computing, and metrology can realize their potential–a quantum Internet and uncounterfeitable money are two interesting possibilities–quantum networks must be able to transmit and store data. The quantum optics group at the California Institute of Technology has been working toward this goal. The team is headed by H. Jeff Kimble, Valentine Professor of Physics, who led the 1998 effort that achieved the first unambiguous teleportation of one photon’s quantum state–that is, the information represented by its spin, energy, and such–to another photon. Now Kimble and his team have demonstrated a way for entanglement–the nonlocal relationship that allows quantum teleportation, which Einstein skeptically dismissed as “spooky action at a distance”–to be created in networks.
Much as the motion of electrons in microprocessor circuits transmits data within today’s computers, the teleportation of quantum states between entangled particles would perform that task in quantum networks. As for data storage, says Kyung Soo Choi, a researcher in Kimble’s group, a central question that one of their recent experiments resolved was, “How do you convert entangled light into an entanglement of matter and back into light?” Entangled states are fragile, and networks of entangled light will require repeating devices–much the way long-distance fiber-optic networks require optoelectronic repeating devices to regenerate diminishing signals. Therefore, entanglement will need to be generated and stored in component subsystems within a greater quantum network. Now Kimble and his team have demonstrated a technical solution to the problem.
The Caltech team used two ensembles of cesium atoms whose states they influenced with a laser, making them either transparent or opaque as needed to manipulate incoming photons’ speeds. The researchers then split single photons, putting them in superposition–that is, they were part of the same quantum wave function and, thus, entangled–while ensuring that they propagated along two paths into the two cesium ensembles. Choi explains, “We slowed the light to a crawl and halted it inside the matter by deactivating the control laser that was making the cesium ensembles transparent, so the quantum information–the entangled light–was stored inside the atomic ensembles. By reactivating the control laser, we reaccelerated the photons to normal speed, restoring the beams of entangled light.” So far, the Caltech researchers have stored entanglement in matter for spans of one microsecond. Kimble estimates that he and his team can extend that to 10 microseconds.