One of the potential applications of a quantum computer would be as a coprocessor to a classical computing system, much in the same way as a graphics processing unit (GPU) performs specialized operations that the traditional central processing unit (CPU) cannot do.
I've looked at the main books on quantum computing and Googled around for research papers, but I'm struggling to find mechanisms that describe the interchange of data between quantum and classical computers.
I imagine it is relatively easy for a classical computer to receive information from a quantum circuit after measurement, but is it possible for a classical computer to send in classical bits through quantum gates for algorithms that mix classical bits and quantum bits?
This question has two parts:
- Theoretical: What is different from a mathematical perspective? Are classical bits mathematically identical to qubits in states $|0\rangle$ or $|1\rangle$?
- Physical: How would the various potential physical implementations of a quantum computer (e.g. trapped ions) have an interface connecting to traditional classical circuits?
Best Answer
I'm no expert on quantum computing, but I had the same question, and found the following to help, a little bit.
http://tph.tuwien.ac.at/~oemer/doc/quprog/node8.html#SECTION00323300000000000000
The gist of it is, first you 'cool' your qbit to the zero state, and then apply a unitary transformation which has the result $U|0\rangle = |S\rangle$, where $|S\rangle$ is your initial string.
Now, as far as I understand,$|S\rangle$ really is a recording of the probabilities of all possible bit strings, so I would assume that your initial $|S\rangle$ value would be something with:
A high probability for your desired, classical string: $$0.9 \cdot |0101010111\rangle$$
and a very low probability for all other possibilities: $$\frac{0.1}{2^{10}-1} \cdot |others\rangle$$
I really don't know if this is a valid assumption to make, though, so please, tell me if I'm wrong.