Читать книгу Principles of Superconducting Quantum Computers - Daniel D. Stancil - Страница 16

In a classical logic circuit we can measure the state of a bit at any time, and make as many copies of the state as we want. We can also do this for a qubit if it is known to be in one of the basis states; as described above, we can measure the |0⟩ or |1⟩ state without disturbing it, and we can make as many copies of |0⟩ or |1⟩ as needed.

However, it turns out that it is not possible to create a precise, independent copy of an arbitrary quantum state. This is known as the no-cloning theorem. We’ll show a proof in Section 1.7, but for now let us consider the challenges this poses to the quantum programmer.

For example, we can’t get estimates of α or β from running a circuit once. Cloning would allow me to run the circuit, make lots of copies of the result, and then measure each copy to estimate |α|² and |β|² by the probability of measuring |0⟩ or |1⟩. Without cloning, we can only measure the result once. To get many measurements, we must run the same computation many times to produce (hopefully!) the same result over and over.

We cannot make copies of states for breakpoints, or to help with debugging or error recovery. It is also challenging to apply different operators to a single state during the course of a computation. Classical programmers take the ability to copy a state for granted, and this limitation requires some adjustment.

It turns out that it is possible to copy a quantum state using entanglement (Section 1.1.5), but only by destroying the state being copied. This represents a transfer of state rather than a copy, and is referred to as teleportation.