How machines are built.
WHAT APPROACHES EXIST
Superconducting circuits, cooled to near absolute zero Trapped ions, held and manipulated by lasers Photonic systems, using light Neutral atoms Semiconductor spin qubits
WHAT THEY TRADE OFF
Speed of operations How long coherence lasts How well qubits connect to each other How easily the system scales
WHAT DECOHERENCE IS
The loss of quantum state through interaction with the environment.
WHY IT IS THE CENTRAL PROBLEM
It limits how long a computation can run before the information is destroyed.
WHAT ERROR RATES LOOK LIKE
Substantially higher than classical computing, per operation.
WHAT ERROR CORRECTION REQUIRES
Many physical qubits encoding one reliable logical qubit.
WHAT THE RATIO IS
Large. Estimates range from hundreds to thousands, depending on the scheme.
WHAT THAT MEANS PRACTICALLY
Machines with thousands of physical qubits may provide very few reliable logical ones.
WHY THAT MATTERS WHEN READING ANNOUNCEMENTS
Qubit counts alone say little. Error rates and logical qubits say more.
