Scientists lay groundwork for an ac-driven quantum motor

The smallest electric motor could be built from just two ultracold atoms moving in a ring by lasers, according to researchers at University of Augsburg, Germany.

In their latest paper, Alexey Ponomarev, Sergey Denisov, and Peter Hänggi detail the working principles of a theoretical idea for the smallest possible quantum machine and demonstrate that it is able to perform useful work.

The motor consists of one neutral (the starter) atom and one charged (the carrier) atom trapped in a ring-shaped optical lattice, or “bracelet”. Once a magnetic field is applied, the charged atom feels the magnetic flux and moves--but yields no net motion. When placed in the same well of the lattice, like two eggs in the same compartment of a carton, the neutral atom provides a kick to the carrier atom. This kick causes the carrier atom to jump from one site in the lattice to the next, resulting in net movement around the ring.

The use of a starter atom helped the research team overcome a fundamental problem with a quantum motor, which is that rotation would occur both clockwise and counterclockwise, canceling out movement and resulting in no work output. (This aspect is examined in another paper published in June '09).

The presence of the neutral atom helps kick-start the motor, but that process also requires some kind of asymmetry which is provided by a combination of a symmetry-breaking driving field. The team found that the motor velocity can suitably be controlled by that asymmetry and manipulation of the external magnetic ?ux.

Technology Review claims that while the Augsburg team is the first to come up with a detailed analysis of how such a motor works and under what conditions it best works, they were not the first to build a quantum motor. The credit for that goes to a team from the University of Glasgow in the UK who built one in 2007 that they called an optical ferris wheel for ultracold atoms.

In any case, Ponomarev and his team have outlined their next steps. They may introduce more atoms into their motor setup to include several interacting carries or starters.

"A particular interesting objective to pursue is the problem of whether the motor velocity can be optimally tuned with the number of participating atoms," they wrote.

They also plan to connect the motor to a nanomechanical resonator that, among many things, can provide ultra-sensitive detection of tiny displacements and forces.  A hybrid system like that, they said, would allow you to power a "classical" object (electrical/nano-sized) using a quantum motor.