Physicists at the University of Bonn have successfully demonstrated a five-qubit quantum register, using neutral atoms.
Registers are the central memory of a computer, in which information is stored in 1s and 0s. Neutral atoms are considered natural candidates for building a register because they can exist in an abundance of quantum states, and these individual states can be manipulated relatively simply. (Not to mention the fact that they can be counted – quite a useful property, when building a register.)
In Physical Review Letters, the researchers explain how they set up the register experimentally. You can access their paper here.
The team cooled five caesium atoms until they were almost stationary, and then loaded them into an optical lattice. An optical lattice is a light grid created by the interference of two or more laser beams. More poetically, it can be thought of as an "artificial crystal of light". In this particular case, the researchers loaded the atoms on to a so-called standing wave trap.
This lattice can trap the neutral atoms in potential wells because the electric fields of the lasers induce a dipole moment in the atom. Depending on the frequency of this dipole moment, and the frequency of the electric field, an atom will either be pushed into the areas of maximum light intensity, or into the areas of minimum light intensity.
Once the atoms were loaded onto the grid, the team initialised the register, that is, they set all the atoms to the state corresponding with 0 (zero). The team took photographs of the atoms in their potential wells using an intensified CCD camera. Then, using a polarised laser, the team performed an operation known as a spin-flip on two of the atoms, switching them to the state corresponding to 1.
Next, the team bombarded the array with a laser tuned to the state-0 atoms, to check that the information had genuinely been transferred to the register. The laser knocked the state-0 atoms off the carrier wave, leaving the state-1 atoms behind. Another picture from their CCD imaging system shows the state-1 atoms are exactly where they were at the beginning of the experiment.
The team is now working to create a quantum gate in which two or more qubits of the register will interact in a controlled way. Dominik Schrader, the lead scientist, he hopes to get there in two years. ®