Dynamic Circuit -- qif, qwhile

API reference for dynamic circuit control flow in pyqpanda3. Dynamic circuits allow mid-circuit measurement and classical feed-forward, enabling conditional execution and loop constructs within a quantum program.

Overview

pyqpanda3 provides a fluent builder API for constructing dynamic quantum programs. The two entry-point functions are qif() and qwhile(), which produce builder objects that are appended to a QProg via the << operator.

Builder chain summary:

Entry	Next method	Purpose
qif(cbits)	.then(prog)	Start an if-branch
QIfThen	.qendif()	End with no else
QIfThen	.qelse(prog)	Add an else-branch and end
QIfThen	.qelseif(cbits)	Add another condition branch
QElseif	.then(prog)	Bind the elseif body
QElseifThen	.qendif()	End with no further branches
QElseifThen	.qelse(prog)	Add an else-branch and end
QElseifThen	.qelseif(cbits)	Chain another elseif
qwhile(cbits)	.loop(prog)	Bind the loop body

---

qif

Creates a conditional branch builder that checks whether all specified classical bits are 1.

Signature

qif(cbits: list[int]) -> QIf

Parameters

Parameter	Type	Description
cbits	list[int]	Classical bit addresses whose values are evaluated as the condition. The condition is true when all specified cbits are 1.

Returns

A QIf builder object. Call .then() on it to specify the if-branch body.

---

QIf

Builder returned by qif(). Use .then() to bind a branch sub-program.

Signature

class QIf:
    def then(branch_qprog: QProg) -> QIfThen

Parameters

Parameter	Type	Description
branch_qprog	QProg	The quantum program to execute when the condition is true.

Returns

A QIfThen builder for chaining further branches or finalizing.

---

QIfThen

Builder returned by QIf.then(). Supports adding else-branches, elseif-branches, or finalizing.

Methods

QIfThen.qelse

QIfThen.qelse(branch_qprog: QProg) -> QProg

Adds an else-branch and returns the completed QProg.

Parameter	Type	Description
branch_qprog	QProg	The quantum program to execute when no prior condition matched.

QIfThen.qelseif

QIfThen.qelseif(cbits: list[int]) -> QElseif

Adds an additional conditional branch after the if-branch.

Parameter	Type	Description
cbits	list[int]	Classical bit addresses for the new condition.

QIfThen.qendif

QIfThen.qendif() -> QProg

Finalizes the if construct without an else-branch and returns the completed QProg.

---

QElseif

Builder for an additional conditional branch within a qif chain.

Signature

class QElseif:
    def then(branch_qprog: QProg) -> QElseifThen

Parameters

Parameter	Type	Description
branch_qprog	QProg	The quantum program to execute when this elseif condition is true.

Returns

A QElseifThen builder for chaining further branches or finalizing.

---

QElseifThen

Builder returned by QElseif.then(). Supports adding further else, elseif, or ending the construct.

Methods

QElseifThen.qelse

QElseifThen.qelse(branch_qprog: QProg) -> QProg

Adds an else-branch and returns the completed QProg.

Parameter	Type	Description
branch_qprog	QProg	The quantum program to execute when no prior condition matched.

QElseifThen.qelseif

QElseifThen.qelseif(cbits: list[int]) -> QElseif

Chains another elseif condition.

Parameter	Type	Description
cbits	list[int]	Classical bit addresses for the new condition.

QElseifThen.qendif

QElseifThen.qendif() -> QProg

Finalizes the construct without an else-branch and returns the completed QProg.

---

qwhile

Creates a loop builder that repeatedly executes a body while all specified classical bits are 1.

Signature

qwhile(cbits: list[int]) -> QWhile

Parameters

Parameter	Type	Description
cbits	list[int]	Classical bit addresses evaluated before each iteration. The loop continues while all specified cbits are 1.

Returns

A QWhile builder object. Call .loop() on it to bind the loop body.

---

QWhile

Builder returned by qwhile(). Use .loop() to bind the loop body.

Signature

class QWhile:
    def loop(loop_body: QProg) -> QProg

Parameters

Parameter	Type	Description
loop_body	QProg	The quantum program to execute on each iteration.

Returns

The completed QProg containing the while loop construct.

---

Examples

Simple if-branch with mid-circuit measurement

Measure qubit 0 into classical bit 0, then conditionally apply an X gate to qubit 1.

from pyqpanda3.core import QProg, X, measure, qif

prog = QProg()
prog << X(0) << measure(0, 0)

if_branch = QProg()
if_branch << X(1)

prog << qif([0]).then(if_branch).qendif()

If-else branch

from pyqpanda3.core import QProg, X, measure, qif

prog = QProg()
prog << X(0) << measure(0, 0)

if_branch = QProg()
if_branch << X(1)

else_branch = QProg()
else_branch << X(2)

prog << qif([0]).then(if_branch).qelse(else_branch)

Multi-branch with elseif

from pyqpanda3.core import QProg, X, I, CNOT, measure, qif

prog = QProg()
prog << X(0) << I(1) << I(2) << I(3) << measure(0, 0) << measure(3, 3)

branch1 = QProg()
branch1 << X(1)

branch2 = QProg()
branch2 << X(0)

branch3 = QProg()
branch3 << X(2)

else_branch = QProg()
else_branch << CNOT(0, 1)

(prog << qif([0]).then(branch1)
        .qelseif([3]).then(branch2)
        .qelseif([0, 3]).then(branch3)
        .qelse(else_branch)
)

While loop with mid-circuit measurement

from pyqpanda3.core import QProg, X, measure, qwhile

prog = QProg()
prog << X(0) << measure(0, 0)

loop_body = QProg()
loop_body << X(1) << X(0) << measure(0, 0)

prog << qwhile([0]).loop(loop_body)

See Also

• QProg
• Measurement