Читать книгу Principles of Superconducting Quantum Computers - Daniel D. Stancil - Страница 14

A simple classical logic circuit is represented by the NOT gate shown in Figure 1.1(a). The NOT gate turns a “0” into “1” and vice versa. In this circuit diagram the horizontal direction represents space, i.e., the input and output of the circuit are physically accessible from different points in the circuit, and they can be measured simultaneously.

Figure 1.1 Interpretation of classical versus quantum NOT gates. (a) Classical NOT Circuit diagram. The horizontal direction represents space, i.e., the input and output of the circuit are physically accessible from different points in the circuit, and they can be measured simultaneously. (b) Quantum X gate circuit (quantum version of the NOT gate). The horizontal direction represents time, i.e., the input and output of the circuit represent the state of the same qubit after performing the X gate operation. The lower part of the Figure shows an alternate symbol for the quantum NOT gate.

The quantum version of the NOT gate is the X gate shown in Figure 1.1(b). For qubits, the “0” and “1” states are normally written |0⟩, and |1⟩, respectively. We will discuss the meaning of this notation in more detail in a future section, but for now just consider them to be labels for the two states. In this case, the horizontal direction represents time, i.e., the input and output of the circuit represent the state of the same qubit after performing the X gate operation. In other words, unlike the usual structure of classical logic, a quantum gate represents an operation that you perform on a single qubit or set of qubits. The output effectively overwrites the input, and every time a gate is applied it changes the state of the qubit.