Quantum Computer

The new quantum computers rely on quantum coherence. That is, the quantum computer system contains N identical quantum subsystems for example, atoms, or optical setups, or molecules, or resonant cavities. In general, each quantum subsystem can be in many possible quantum states. Assume that the ψ_i for each quantum subsystem has only two states, which we label 1 and 0. If N = 3, then Ψ = ψ₁ + ψ₂ + ψ₃ is the QM state of the system. Therefore our quantum computer represents all eight states simultaneously: 000, 001, 010, 011, 100, 101, 110, 111.

That is, during calculations on Ψ all eight states participate in each calculation! If the quantum computer is actually a large molecule in a vacuum, then the molecule must be kept away from the walls of the container and away from other molecules. Why?

Answer

A quantum computer relies on maintaining its linear superposition of quantum states—that is, Ψ = ψ₁ + ψ₂ + ψ₃, its coherence during the calculations so that all the states participate in the calculation. Quantum decoherence is a bad thing for a quantum computer. A collision with the wall of the chamber or with another molecule will ruin the coherence because an observation has been made. By QM rule 3, we no longer sum over the amplitudes ψ_i. This decoherence then ruins the quantum computation because only one state will be participating in the computations.

Maintaining coherence in a real physical system has been progressing slowly for the past decade, with coherence times of tens of nanoseconds for three identical subsystems working as a quantum computer. No one knows what type of physical system will compose the first 18-subsystem quantum computer in the future, but this computer probably will outdo all the other classical computers combined in computing speed.