Built-in quantum instructions

This section describes built-in non-unitary operations.

Initialization

The statement reset qubit|qubit[]; resets a qubit or quantum register to the state \(|0\rangle\). This corresponds to a partial trace over those qubits (i.e. discarding them) before replacing them with \(|0\rangle\langle 0|\). Reset is shown in Fig. 2.

// Initialize and reset a register of 10 qubits
qubit[10] qubits;
reset qubits;

Fig. 2 The reset statement prepares a qubit or quantum register in the state \(|0\rangle\).

Measurement

The statement bit|bit[] = measure qubit|qreg; measures the qubit(s) in the \(Z\)-basis and assigns the measurement outcome(s) to the target bit(s). For backwards compatibility this is equivalent to measure qubit|qubit[] -> bit|bit[]; which is also supported. Measurement corresponds to a projection onto one of the eigenstates of \(Z\), and qubit(s) are immediately available for further quantum computation. Both arguments must be register-type, or both must be bit-type. If both arguments are register-type and have the same size, the statement b = measure a; broadcasts to b[j] = measure a[j]; for each index j into register a. Measurement is shown in Fig. 3.

// Initialize, flip and measure a register of 10 qubits
qubit[10] qubits;
bit[10] bits;
x qubits;
bits = measure qubits;

Fig. 3 The measure statement projectively measures a qubit or each qubit of a quantum register. The measurement projects onto the \(Z\)-basis and leaves qubits available for further operations. The top row of circuits depicts single-qubit measurement using the statement c[0] = measure q[0]; while the bottom depicts measurement of an entire register using the statement c = measure q;. The center circuit of the top row depicts measurement as the final operation on q[0].