Neutral-Atom Quantum Computing Hardware: Optical Tweezers and Rydberg States
We are currently witnessing the use of atoms of elements such as Ytterbium, Caesium, and Rubidium being carried out by various research facilities and companies for the development of neutral atom quantum computing.
Qubits are encoded in these atoms using Optical Tweezers, which are essentially highly focused-laser beams. A Magneto-Optical Trap is where these atoms (qubits) are stored and held in place using the aforementioned optical tweezers. Measurements are done on qubits using the effect of fluorescence. What we see in these images of measurement is that fluorescence on the camera is a single spot of light that goes back and forth between a low-intensity level and a high-intensity level. The reason for this is that single qubits are entering into the optical tweezer from the magneto-optical trap and then leaving as soon as the second qubit enters. They basically kick each other out and light-assisted collisions are here to blame for it. Fortunately, this phenomenon is really helpful when we need to deterministically prepare a single atom in the tweezer without worrying about the possibility of having two or three in there. In this process actually, the time for which these atoms stay in the optical tweezers is not very long. It is limited to a few seconds only. What we can do to store them for a longer time is to get rid of the magneto-optical trap around it once the initial atoms have been loaded into the tweezers.
Another key feature of neutral atoms is that naturally, they do not interact with each other. This is beneficial to us as there are no interactions going on when we do not want them to. Although, in order to produce entanglement between them to make two-qubit gates, we can excite them into very highly-excited atomic states called the Rydberg States. In the Rydberg state, these atoms are almost-ionized and highly polarizable. Thus, dipole-dipole interaction and Van der Waal forces between them facilitate very high-speed entanglement. One of the most common approaches to this is the Rydberg Blockade. In which, the state of one atom, whether in the ground or Rydberg state can control the excitation of another atom. We cannot excite two atoms simultaneously into the Rydberg state in a given radius span, called a Blockade Radius.
References: Neutral Atom Quantum Computing lecture by Jeff Thompson. You can find it on Youtube.
