# Neutral Atom Qubits A **neutral-atom qubit** uses a single electrically neutral atom — held in place by light — as the carrier of one [[Qubit|qubit]]. The two qubit states are two internal energy levels of the atom. ## First principles Start from a real atom (e.g. an alkali or alkaline-earth element). It has discrete internal energy levels set by quantum mechanics. Choose two long-lived levels to be $|0\rangle$ and $|1\rangle$. Because the atom is neutral, it barely couples to stray electric fields, so the qubit is naturally well isolated from a major source of noise. To build a processor you need three capabilities, each a distinct concept: 1. **Trap and hold** atoms in space — done with [[Optical Tweezers]]. 2. **Cool** them so they sit still enough to address — done with [[Laser Cooling]]. 3. **Make them interact** to create two-qubit gates — done with [[Rydberg Interactions]]. > [!note] The defining advantage: identical qubits > Atoms of the same isotope are *exactly* identical — nature manufactures them with zero variance. Unlike fabricated solid-state qubits, there is no chip-to-chip or qubit-to-qubit manufacturing spread to calibrate away. ## Characteristic trade-offs | Strength | Cost | |---|---| | Identical qubits, no fabrication variance | Two-qubit gates are comparatively slow | | Atoms can be physically moved → flexible [[Qubit Connectivity and Reconfigurability\|connectivity]] | [[Mid-Circuit Measurement]] without disturbing neighbours is hard | | Long [[Coherence Time]] (especially [[Nuclear-Spin Qubits\|nuclear-spin]] encodings) | Atoms can be lost and must be replaced/reloaded | The physical mobility of the atoms is unusual: because nothing is hard-wired, the array can be rearranged mid-computation, which lets the hardware adapt to whichever error-correcting code is being run. ## Related - [[Optical Tweezers]] - [[Laser Cooling]] - [[Rydberg Interactions]] - [[Nuclear-Spin Qubits]] - [[Qubit Connectivity and Reconfigurability]]