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Declarations

Declaring is naming — it gives a label to a qubit, a classical bit, a list of qubits, or a gate pipeline so you can reference it later.

Comments​

Lines starting with # are comments and are ignored by the compiler. Inline comments are supported too.

# This is a comment
q 0 -> H  # inline comment

Qubit declaration​

The standard way to declare a qubit uses the q keyword followed by an index:

alice : q 0
bob   : q 1

This maps alice to qubit index 0 and bob to qubit index 1.

You can also give a qubit a custom register name for clarity:

alice : q ancilla 0

This maps to Qubit("ancilla", 0) in pytket instead of the default Qubit("q", 0).

The shorthand without q is also valid:

alice : 0

Classical bit declaration​

Declare a classical bit (used to store measurement results) with the b keyword:

result : b 0

Custom register names are supported:

result : b outcomes 0

Classical bits are essential for conditional actions and classical bit operations.

Qubit reference without a name (q N)​

You can refer to a qubit by its index directly in actions, lists, and gate arguments using the q prefix — no declaration needed:

q 0 -> H
q 0 -> CX(q 1)

This is identical to declaring alice : q 0 and then writing alice -> H or alice -> CX(bob).

Qubit list declaration​

Group qubits together under one name:

pair : [alice, bob]

Mix named qubits with direct indices:

all3 : [alice, q 1, q 2]

Once declared, the list acts as a multi-qubit target in actions:

pair -> H   # applies H to both alice and bob

Gate pipeline declaration​

A pipeline is a named sequence of gates that can be applied to a qubit and also traversed in reverse:

bell_prep : H  FCX(q 1)

Pipelines can reference named qubits as gate arguments:

alice    : q 0
bob      : q 1
entangle : H | FCX(bob)

Pipelines can also reference other pipelines by name:

diffuse_in : H | X

When used in an action, write diffuse_in <- to traverse it in reverse.

Using declared objects together​

Named qubit in a gate argument​

alice : q 0
bob   : q 1

alice -> CX(bob)

Named qubit in an action​

alice -> bell_prep

Named qubit added to a list​

alice : q 0
group : [alice, q 1, q 2]

Named pipeline inside another pipeline​

prep   : H | S
expand : prep | X | prep <-