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“coherence times”, i.e. the time-scales of information retention, into the list of
requirements. Once coherence times reach a system-dependent threshold, it becomes
possible to employ error correction techniques that allow for information to
be stored indefinitely [Shor, 1995]. Any feasible system must eventually be able
to reach this threshold.
1.3.4 Universal Set of Quantum Gates
To be considered a generally useful quantum computer, the architecture needs
to support a basic universal set of operations that suffices as a basis set for constructing
any arbitrary computation [Barenco et al., 1995]. For a classical computer,
the corresponding set commonly consists of just the NAND gate. Neither
in classical nor in quantum computation this set of operations is unique.
For quantum systems, the common example-set used by theoretical physicists
contains the Clifford Gates H and cNOT augmented by the P π/8 gate. The H,
or Hadamard, gate translates the | 0 〉 state of the qubit into the | 0 〉 + | 1 〉 state
and the | 1 〉 state into the | 0 〉 − | 1 〉 state. The cNOT , or controlled NOT, gate
performs a NOT operation on one of two qubits if and only if the other qubit is in
the | 1 〉 state and corresponds to the classical XOR gate. The P π/8 , or π/8 phase,
gate changes the phase of the qubit’s | 1 〉 state relative to the | 0 〉 state by π/8.
The actual set of gates provided by any given implementation may deviate from
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