Built-in quantum instructions

This section describes built-in operations on qubits, including non-unitary ones.

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].

Explicit no-operation

Added in version 3.2: The entire nop directive.

The statement nop <qubits>; is an explicit no-operation on all mentioned qubits, but marks them as being “used” within a scope. <qubits> is a comma-separated list of qubit or qubit-register operands.

In the global scope this has no effect on the program operation, though a compiler must still validate the arguments. This has a particularly important effect on boxed scopes; a nop counts as a “use” of the qubit within the box, causing the qubit to be synchronized with the rest of the box at entry and exit.

A nop statement is valid in exactly the places that a gate-call statement is valid. The operands of a nop statement must all be in scope and be of type qubit or qubit[n] for any non-negative integer n.

For example:

include "stdgates.inc";

stretch s;
box [s] {
   // Qubits $0 and $1 are explicitly used by the `cx`.
   cx $0, $1;

   // Qubit $2 must be synchronized to $0 and $1 at the start
   // and end of the box, but has no defined operations within
   // the box.
   nop $2;

   // More than one qubit can be specified in the list.  It is
   // not an error to duplicate qubits, nor to ``nop`` a qubit
   // with explicit operations in this scope.
   nop $3, $0;

   // The ``nop`` statement can have zero operands, and this
   // has no observable effect at all.
   nop;
}