Tweezer arrays of alkaline-earth atoms have recently shown potential capabilities in quantum computing and precision timekeeping. They have the ability to encode new kinds of qubits, and long coherence times and serve as state-of-the-art atomic clocks. The optical tweezer arrays can enable powerful quantum processors and optical clocks using trapped neutral atoms with two valence electrons.
Over the years, neutral atoms were seen to be a potential candidate in the race to build a quantum computer, a machine that would exploit the laws of quantum physics to solve certain important computational problems far more efficiently than any conventional computer.
Scaling up quantum computers from hundreds to millions of qubits has always been a challenge, however, when it comes to neutral atoms, ‘space’ will never be an issue. A millimeter-scale array can already accommodate up to a million qubits.
In a neutral-atom quantum processor, an array of tightly focused laser beams called optical tweezers is used to hold the atoms in an ultrahigh vacuum. Researchers have scaled up arrays of more than 100 alkali atoms, each of which has one valence electron, and used smaller arrays to run quantum algorithms. They are currently looking into how arrays of two-valence atoms, like those found in alkaline earth elements, can process and measure new quantum information.
In other aspects, the rich electronic energy-level structure of atoms that resemble alkaline earth quite benefits the quantum computing industry. Because it can offer a method for enhancing entanglement fidelity and a novel quantum error correction mechanism, both of which depend on a long-lived metastable state unique to alkaline-earth atoms and lacking in alkali atoms.
From Atoms to Qubits
There are an endless number of discrete energy levels for each atomic species that correspond to various quantum states. In theory, any pair of states might function as a qubit. In real-world applications, scientists choose a pair of long-lived low-energy states that allow for a large number of successive quantum logic operations before decoherence, or the leakage of quantum information from the qubit into its surroundings.
In building a neutral-atom quantum processor, researchers divide an incident laser beam into several beams that are then concentrated inside a glass vacuum cell by a potent microscope objective lens. The tweezer array is loaded with clouds of cold atoms, which are then shuffled to create a filled array with one atom in each tweezer. Researchers can precisely control each atom’s position, quickly reorganize the array, and individually adjust the trapping potential of each tweezer thanks to optical devices outside the vacuum cell.
Low-energy neutral atoms interact with one another weakly, making it possible to arrange them into dense arrays. Researchers use a laser pulse to target a pair of nearby atoms and excite one of them to a high-energy state known as a Rydberg state. Rydberg blockade is the result of the Rydberg atom’s strong electric dipole interactions preventing its neighbor from also being excited by the laser, although it is impossible to determine which of the atoms was excited.
The outcome is a single excitation that is shared by two qubits and that cannot be represented individually. Entanglement is the main phenomenon that distinguishes quantum computers from their classical counterparts. In the last five years, Rydberg entanglement fidelity has significantly increased, although it still falls short of that of trapped ions and superconducting qubits.
One advantage of alkaline-earth atoms is that their valence electrons pair off in the electronic ground state, thus avoiding hyperfine interaction. Qubits stored in alkaline-earth atom nuclear spin states are consequently projected to have substantially longer coherence periods than hyperfine qubits.