Quantum Computer
A quantum computer is said to solve problems in minutes that would take millions of year for a classical computer to solve.
Although many billions of dollars have been invested in quantum computers, and perhaps even more in China, quantum computing may be farther away than nuclear fusion.
The memory system of a quantum computer uses qubits ("quantum bits"). They are the quantum counterpart of the classical "bit" of a digital computer. The "bit" is an abbreviation of "binary digit."
Where classical bits can have the value "0" or "1," qubits are in a coherent
superposition or
linear combination of |0> or |1> quantum states, as
Paul Dirac described in his 1930 text
Principles of Quantum Mechanics.
Measurement of a qubit
projects the qubit
randomly into either
single-particle state |0> or |1>, with probability 1/2 following Dirac's "
projection postulate."
Ψ = (1/√2) |0> +/- (1/√2) |1>
The 1/2 probability of each state is the square of the "probability amplitude" 1/√2.
When two qubits a and b are
entangled, their wave function Ψ is a linear combination (or
superposition of
two-particle "product" states |01> and |10>,
Ψab = (1/√2) |0a1b> +/- (1/√2) |1a0b>
The two-particle wave function Ψ
ab describes the behavior of two entangled particles (electrons, photons, or atoms) with spin angular momentum up |↑> or down |↓> that have traveled far apart.
When either a or b is measured, the two-particle wave function Ψ
ab collapses, both particles are
individually projected into random states up (|↑> or down |↓>), but
jointly they always appear
perfectly correlated in one of the two product states up-down |↑↓> or down-up |↓↑>.
Either of these product states
conserves the total spin angular momentum of the initial prepared entangled state, although particular spin directions are not defined.
We describe this conserved total angular momentum as a
hidden constant of the motion.
See the
Wikipedia article.
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