Quote Originally Posted by Sound View Post
From Ray Kurzweils book "The Singularity is Near" (2005).
Note that this excerpt is talking not about quantum computing, but ordinary, classical computing on a really small scale.


Quantum computers are 4 states rather than two, this is what makes them so powerful. A q-bit can be on, off, neither, or both.
The following assumes that the modern standard model holds. If it doesn't, all bets are off.

IIRC, this is not exactly true. A qubit (quantum bit) doesn't simply have four distinct states of off/on/both/neither. For starters, it can't really be neither. It can be in states representing 0 or 1, or, crucially, in a quantum superposition of both states. Wikipedia probably explains this better than I can, but it means that the state of the qubit in a superposition of states 0 and 1 can be thought of as a a combination of each state multiplied by its respective probability amplitude. The value of these probability amplitudes are complex numbers with the sum of the squares of the moduli being equal to 1.

However, this doesn't mean that a qubit is like a regular bit, but with more states. Qubits are a lot more complicated to manipulate and read.

For example: Measuring a qubit will not simply reveal the probability amplitudes. You'll actually measure the state of it as either 0 or 1, with the probability of each being equal to the square of the modulus of its probability amplitude (this is why these values need to sum to 1).

Quantum computation is really interesting.