Qubit (register) declaration

A qubit (register) must be declared before it can be used. The general form is given as follows:

 qubit[size]identifier

Grammar for bit (register) declaration

qubit-declaration:
  qubit array-size-declarationopt identifier

array-size-declaration:
  [ integer-literal ]

Its form is similar to the declaration of an arbitrary variable, whereby the type of the variable is specified first, i.e. qubit denotes that the declared variable is of type Qubit. and qubit[size] denotes that the declared variable is of type QubitArray. The size of a qubit register is declared by an integer value between square brackets [size]. The name of the qubit (register) is defined through an identifier.

The declaration of a qubit (register) is optional.

Note

The qubit (register) declaration replaces the qubit statement, i.e.,

 qubits size

of previous versions of the cQASM language.

Find below examples, respectively, of a single qubit declaration and qubit register declaration.

Example

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qubit q  // Single qubit declaration of a qubit named 'q'.
qubit b

H q

b = measure q

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qubit[5] q  // Qubit register declaration of a register containing 5 qubits, named 'q'.
bit[2] b

H q[0]
CNOT q[0], q[1]

b[0, 1] = measure q[0, 1]

The individual qubits of a qubit register can be referred to by their register index, e.g., in the example of the Qubit register declaration, the statement H q[0] indicates the application of a Hadamard gate H on the qubit located at index 0 of the qubit register q. Note that in the case of a single qubit, the qubit is referred to through its identifier, not through a register index.

Qubits, either declared as a single qubit or as elements of a register, are presumed to be initialized in the state \(|0\rangle\) of the standard/computational basis.