Decoherence

Decoherence is the loss of a quantum system's coherent superposition through interaction with its surrounding environment. As a Qubit couples to stray fields, vibrations, or thermal noise, information about its state leaks outward and the delicate phase relationships that define Superposition and entanglement wash out, leaving behavior that looks classical. Decoherence is why quantum states cannot simply be left alone to compute.

#Why error correction is needed

Every physical qubit decoheres after a characteristic time, and each gate operation adds further error. Long computations such as cryptanalysis need far more reliable operations than any physical qubit provides. The response is quantum error correction: many physical qubits are combined into a logical qubit whose encoded state is measured and repaired faster than decoherence corrupts it, an idea developed in depth in the fault-tolerance literature. The surface code is the leading scheme for doing this. Because error correction sets the true hardware cost, estimates for a cryptographically relevant quantum computer are expressed in logical qubits rather than raw physical ones.