Decoherence is the process by which a qubit loses its quantum superposition and entanglement due to unwanted interactions with the surrounding environment. These interactions cause the quantum information to "leak" into the environment — effectively acting like a measurement. Decoherence is one of the primary challenges in building practical quantum computers. It is characterized by two timescales: T1 (energy relaxation time — how long it takes for a qubit to decay from |1⟩ to |0⟩) and T2 (dephasing time — how long a superposition remains coherent). Modern superconducting qubits have T1 and T2 times of 50–500 microseconds; trapped-ion qubits can maintain coherence for seconds. All quantum circuits must complete before decoherence destroys the information.
Related Terms
T1 / T2 Time
HardwareT1 is the qubit energy relaxation time; T2 is the coherence (dephasing) time. Both limit circuit duration.
Qubit
FundamentalsThe fundamental unit of quantum information — the quantum analog of a classical bit.
Quantum Error Correction
HardwareTechniques to detect and correct errors in quantum circuits without measuring (and collapsing) the qubits.
NISQ
HardwareNoisy Intermediate-Scale Quantum — devices with 50–1000 qubits without full error correction.