DIVE INTO THE QUANTUM
WORLD
WITH NEUTRAL
ATOM QUANTUM
COMPUTING
website created by Rohma Khan, Thanjila Thahsin,
and Shakibul Alam
Pumping atoms
in to glass cell
Rubidium, symbol Rb, is the 37th element in the periodic table. Discovered in 1861, it is the 16th most abundant element in the earth’s crust. Rubidium atoms hosted in glass cells form the basis of rubidium atomic clocks, an integral tool in global positioning systems (GPS). Neutral atom quantum computing using rubidium atoms is the latest cutting edge approach for quantum computing. Let’s follow the 87Rb atoms as they go from atoms to qubits in a Neutral Atom Quantum Computer. Turning solid rubidium into a gas cloud is the first step in the process. Because Rubidium has a melting point of 39.3 °C and a boiling point of 688 °C, it is solid at room temperature. In a high vacuum a solid stick of Rubidium is heated to expel atoms and produce a vapor. The vaporized rubidium atoms are pumped into our glass cell in the form of an atomic beam. They are at room temperature, or hotter at this point, with high kinetic energy!
Trapping atoms into the MOT
The second step is to cool the Rb gas cloud down and trap the atoms in the center of the cell, using a Magneto-Optical Trap, MOT. The MOT is comprised of coils in and three slightly red-detuned lasers going through the center and reflected back through. A magnetic field is created by the coils around the cell to keep the cloud in the center of the cell. The three laser beams are arranged to go through the cell in the x,y,z,-x,-y,-z direction to form standing waves. The laser beams are slightly red-detuned, meaning they have less energy required for the atom to absorb a photon. When the atom moves however, it sees the laser as a slightly higher energy due to the doppler affect and can absorb photons. This way only atoms that are moving can absorb photons heading in the direction opposite to the atom’s movement, slowing them down overall. The Rubidium gas cloud is now cooled to temperatures as low as 10-6 K and effectively trapped in the center of the cell.
Making a laser array pattern
The third step is to organize the atoms into patterns so they can later be manipulated individually. To do this a spatial light modulator, SLM, is used to change the shape the laser into a pattern. SLMs use programable liquid crystals to change the intensity, phase, or polarization of light. This allows the beam to be programmed to be shaped into an array. This patterned beam goes through an objective, which can be thought of as a telescope. The beam enters the back of the objective and comes out of the front. After leaving the front of the objective there is a point where the beam is most focused, sometimes called the beam waist. At the beam waist atoms can be trapped due to small electrostatic forces. This is the basic concept behind optical tweezers. Using the optical tweezers with the SLM, atoms from the dense cold Rubidium gas cloud in the center of the cell are trapped into the desired pattern. Not all atoms will be trapped perfectly, so extra slots are programmed into the pattern, should they be needed.
Moving atoms into the array
At this point the MOT is turned off, and any atoms not laser trapped fly away! Then using the camera a picture is taken to “see” where the atoms are, and where they are not. Using an acousto-optic deflector (AOD), the laser can be used as moving optical tweezers to move the atoms from the “extra” slots to the desired location. An AOD is a device that uses sound waves to deflect or redirect a laser beam, and using diffraction can also create multiple optical tweezers. A final picture is taken to ensure the pattern is completely filled!
Turning atoms into qubits
The next step is to use the atoms as quibits! This step is initialization and manipulation of the atom using a different set of lasers and microwave setup. Microwave excitation can be used to manipulate the two |0> and |1> states of the hyperfine ground states of the atom. The atoms are also excited to their Rydberg state, which is when an electron in an atom has been excited to a very high energy level. These interactions are done in multi-photon process, meaning different frequencies of laser lights are needed. The qubits are used and manipulated around using lasers to excite them and then moving tweezers to move them around. Different gate operations can be done at this point!
Making calculations and taking pictures
The final step is to do a readout. This is done through fluorescence imaging. The quantum state readouts are done through a camera “seeing” the light scattered off of the atoms. If the atom is in one state it will scatter light, but not if it is in another state. In this way we can see what state the atom is in!
Work Cited
Bernien, H., Schwartz, S., Keesling, A. et al. Probing many-body dynamics on a 51-atom quantum simulator. Nature 551, 579–584 (2017). https://doi.org/10.1038/nature24622
Bluvstein, D., Evered, S.J., Geim, A.A. et al. Logical quantum processor based on reconfigurable atom arrays. Nature 626, 58–65 (2024). https://doi.org/10.1038/s41586-023-06927-3
Notes, powerpoints, and discussions provided by Pedro and the Quera team.
For information on Rubidium: Los Alamos National LAb https://periodic.lanl.gov/37.shtml
https://doi.org/10.1063/1.2812121