Physicists from MIT and the University of Belgrade have developed a new technique that can successfully entangle 3,000 atoms using only a single photon. The results, published in the journal Nature, represent the largest number of particles that have ever been mutually entangled experimentally. The researchers say the technique provides a realistic method to generate large ensembles of entangled atoms, which are key components for realizing more-precise atomic clocks.
"You can make the argument that a single photon cannot possibly change the state of 3,000 atoms, but this one photon does - it builds up correlations that you didn't have before," says Vladan Vuletic, the Lester Wolfe Professor in MIT's Department of Physics, and the paper's senior author. "We have basically opened up a new class of entangled states we can make, but there are many more new classes to be explored."
Vuletic's co-authors on the paper are Robert McConnell, Hao Zhang, and Jiazhong Hu of MIT, as well as Senka Cuk of the University of Belgrade.
Atomic entanglement and timekeeping
Entanglement is a curious phenomenon: As the theory goes, two or more particles may be correlated in such a way that any change to one will simultaneously change the other, no matter how far apart they may be. For instance, if one atom in an entangled pair were somehow made to spin clockwise, the other atom would instantly be known to spin counterclockwise, even though the two may be physically separated by thousands of miles.
The phenomenon of entanglement, which physicist Albert Einstein once famously dismissed as "spooky action at a distance," is described not by the laws of classical physics, but by quantum mechanics, which explains the interactions of particles at the nanoscale. At such minuscule scales, particles such as atoms are known to behave differently from matter at the macroscale.
Scientists have been searching for ways to entangle not just pairs, but large numbers of atoms; such ensembles could be the basis for powerful quantum computers and more-precise atomic clocks. The latter is a motivation for Vuletic's group.
Today's best atomic clocks are based on the natural oscillations within a cloud of trapped atoms. As the atoms oscillate, they act as a pendulum, keeping steady time. A laser beam within the clock, directed through the cloud of atoms, can detect the atoms' vibrations, which ultimately determine the length of a single second.
"Today's clocks are really amazing," Vuletic says. "They would be less than a minute off if they ran since the Big Bang -- that's the stability of the best clocks that exist today. We're hoping to get even further."
The accuracy of atomic clocks improves as more and more atoms oscillate in a cloud. Conventional atomic clocks' precision is proportional to the square root of the number of atoms: For example, a clock with nine times more atoms would only be three times as accurate. If these same atoms were entangled, a clock's precision could be directly proportional to the number of atoms - in this case, nine times as accurate. The larger the number of entangled particles, then, the better an atomic clock's timekeeping.