Читать книгу Principles of Superconducting Quantum Computers - Daniel D. Stancil - Страница 18

The fact that we can only calculate the probability of measurement results—not the precise results—on qubits in superposition states allows for a phenomenon that has no classical analog: entanglement. If two qubits are entangled, then the states are correlated even though the outcome of a measurement on either of the qubits can only be predicted by its probability. For example, if two qubits are entangled, then a measurement on one of the qubits will give a result with probability determined by its superposition state, just as would happen with an isolated qubit. However, once the state of one of the qubits has been collapsed by a measurement, the entanglement means that we know exactly what we will get if we measure the second qubit.

It is a bit like flipping two coins at the same time, and having them always come up the same, i.e., both heads or both tails. Or alternatively, having them always come up opposite—one heads and the other tails. Prior to making a measurement, the states of both of the qubits have not yet been determined. However, when the state of one is collapsed by a measurement, the second qubit somehow instantaneously “knows” the result. This is true regardless of how far apart the qubits are. For example, suppose we entangle two qubits, send one to New York and the other to Tokyo, then make prior arrangements to measure both at the same time. We will discover that the results of the measurements will be correlated even though there was not enough time for a signal traveling the speed of light to travel between the qubits. Einstein called this “spooky action at a distance.”

If it were possible to control the result of the first measurement, then it would appear that we could communicate faster than the speed of light—violating the principle of special relativity. However, the fact that we cannot control the result of the first measurement means that we cannot actually send any information using this mechanism.³ So we can rest comfortably knowing that relativity has not been violated.

We will see that the phenomena of superposition and entanglement give quantum computing its unusual and powerful capabilities.