Basic Q C One moose two moose Red
Basic Q. C. One moose, two moose Red moose, blue moose Live moose, dead moose
Superposition States • A ‘qubit’ can be in an infinite number of states • |Ψ> = a|0> + b|1> • Probability of 0: |a|² • Probability of 1: |b|² • |a|² + |b|² = 1
More on superposition states • “A full system of m qubits has a basis of 2 m states. ”* • A classical system of m bits can be set to any of these states. • A quantum system can be set to all of those states at once. *Introduction to Quantum Computation and Information (Lo, Popescu, Spiller 2000)
Entanglement • The states of qubits in a closed system are ‘entangled’. • Consider a system of two qubits, A and B. • |Ψ>AB = 2 -1/2 (|0>A|0>B + |1>A|1>B) • Cannot be written in factored form. • The two qubits don’t have states of their own - they are ‘entangled. ’
Reversible Unitary Evolution • A. K. A. Reversibility • For any truly closed quantum system, you can reverse the system and get back to the original state • Works on paper, but not usually in theory.
Irreversibility, Measurement, Decoherence • “[Irreversibility] has to be stopped from biting before some desired unitary quantum evolution of the system has been completed. ”* • In short, it has to work right or it won’t work right. *Introduction to Quantum Computation and Information (Lo, Popescu, Spiller 2000)
No Cloning • No matter how hard you try, you can’t copy the state of a superpositioned quantum system. • If you observe it to copy it, it collapses into a base state. • This makes absolutely secure communication possible using a quantum media and the One. Time Pad, or Vernam’s Cipher
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