# Qubit Connectivity and Reconfigurability **Connectivity** describes which [[Qubit|qubits]] can directly interact. **Reconfigurability** is the ability to *change* that pattern during a computation. Together they decide which algorithms and which [[Quantum Error Correction|error-correcting codes]] are cheap on a given machine. ## First principles Every two-qubit gate needs the two qubits to be coupled somehow. Hardware falls on a spectrum: - **Fixed nearest-neighbour:** a qubit interacts only with its immediate neighbours on a static layout. Simple, but distant qubits must communicate via long chains of intermediate operations — costly. - **All-to-all:** any qubit can interact with any other. Powerful, but harder to engineer at scale. A distinctive middle path is **physical reconfigurability**: if the qubits themselves can be *moved*, the connectivity is not baked into wiring. Whoever needs to interact is brought together on demand. In [[Neutral Atom Qubits|neutral-atom]] hardware, atoms held in [[Optical Tweezers]] are literally transported mid-computation, so the array is "rewired" by rearranging atoms. > [!intuition] Movable desks versus a fixed floor plan > In a fixed office, talking to someone far away means relaying messages desk to desk. If desks roll, you wheel the right people together whenever you need them. Reconfigurable qubits roll their desks. ## Why it matters for error correction > [!note] Connectivity decides which codes are cheap > Codes are increasingly **matched to hardware**, not chosen in the abstract. Local connectivity favours the [[Surface Code]]; richer connectivity makes low-overhead [[qLDPC Codes]] and symmetric [[Permutation-Invariant Codes]] feasible. Because movable atoms aren't locked to a fixed layout, the *same* machine can host whichever code is most efficient — a strategic advantage as code design evolves. ## Related - [[Neutral Atom Qubits]] - [[Optical Tweezers]] - [[Surface Code]] - [[qLDPC Codes]] - [[Permutation-Invariant Codes]] - [[Bosonic Bus]]