Nanog News wrote:
Latest from ICANN: Quantum Computers are "Interesting"… But Don't Lose your Head
But, quantum computers are mocked up by theoretical physicists, IT amateurs who don't know basics of computational and/or information theory at all, and, as such, just do not work better than classical ones. As for adiabatic quantum computers (those of D-wave), they can be configured to form various quantum circuits. Thus, a quantum circuit, the minimum energy state of which is a solution to some optimization problem, may be constructed. In addition, according to quantum mechanical adiabatic theorem, the state can be reached within polynomial time (w.r.t. problem size), *IF* (very big if) energy difference between the lowest and the second lowest energy states is not very small. However, it is obvious that errors to construct the circuit will badly affect the result. IIRC, D-wave quantum computer has 8bit ADC to control coupling strength between qubits, which is poorly accurate. Worse, there are known classical, not quantum, approximation algorithms to compute the lowest energy state with certain accuracy (w.r.t. energy) within polynomial time. As such, "if energy difference between the lowest and the second lowest energy states is not very small", the classical algorithms can find the correct answer and quantum computing is no better than the classical algorithms. It should be noted that, with really hard problems, the energy differences are very small (become exponentially smaller as problem size increases), which can not be solved by classical approximation nor quantum algorithms in polynomial time. Though it is possible to construct quantum circuits to enhance energy differences a lot, such circuits, like high Q resonators, require extreme accuracy, which is impossible to construct for large problem size, which means such attempts do not contribute scalability. Uselessness of quantum logic gate style quantum computers will be discussed in a separate mail. Masataka Ohta