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Add multi-control functionality #1380

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2134c2e
Add multi-control functionality
Dec 8, 2025
0ae2b61
Merge branch 'main' into multi_control
keefehuang Dec 8, 2025
f8c4503
Apply minor updates to code
Dec 8, 2025
894add1
Remove old CCX test
Dec 8, 2025
fb32afb
Add tests for CCZ, CRZ, MCRZ and MCZ
Dec 8, 2025
2375b74
Rename control and target for addCrz
Dec 8, 2025
275e832
🎨 pre-commit fixes
pre-commit-ci[bot] Dec 8, 2025
c873318
:sparkles: Qiskit to IQM JSON converter
marcelwa Dec 7, 2025
068560c
:white_check_mark: Test Qiskit to IQM JSON converter
marcelwa Dec 7, 2025
53032bb
:sparkles: Support IQM JSON conversion in `QDMIBackend`
marcelwa Dec 7, 2025
5c68a2f
:white_check_mark: Tests for new backend functionality
marcelwa Dec 7, 2025
3f46cd3
:art: Apply suggestions from GitHub Copilot
marcelwa Dec 7, 2025
33b48a7
:art: Apply `pre-commit`
marcelwa Dec 7, 2025
453e00a
:memo: Add to CHANGELOG
marcelwa Dec 7, 2025
b81e3e3
:memo: Adjust documentation
marcelwa Dec 7, 2025
2505f27
:pencil2: Fix typo
marcelwa Dec 7, 2025
9ab82f3
:adhesive_bandage: Fix usage of `R` gate in docstring
marcelwa Dec 7, 2025
06a40af
:art: Apply CodeRabbit's suggestion
marcelwa Dec 7, 2025
92ed957
:art: Apply CodeRabbit's suggestion
marcelwa Dec 8, 2025
0911f7c
:white_check_mark: Add tests to cover CodeRabbit's suggestions
marcelwa Dec 8, 2025
3d74d1b
🎨 pre-commit fixes
pre-commit-ci[bot] Dec 9, 2025
9915b6a
Merge branch 'main' into multi_control
keefehuang Dec 9, 2025
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12 changes: 12 additions & 0 deletions include/mqt-core/zx/FunctionalityConstruction.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -113,6 +113,18 @@ class FunctionalityConstruction {
const std::optional<double>& unconvertedBeta = std::nullopt);
static void addCcx(ZXDiagram& diag, Qubit ctrl0, Qubit ctrl1, Qubit target,
std::vector<Vertex>& qubits);
static void addCcz(ZXDiagram& diag, Qubit ctrl0, Qubit ctrl1, Qubit target,
std::vector<Vertex>& qubits);
static void addCrz(ZXDiagram& diag, const PiExpression& phase,
const Qubit control, const Qubit target,
std::vector<Vertex>& qubits);
static void addMcrz(ZXDiagram& diag, const PiExpression& phase,
std::vector<Qubit> controls, const Qubit target,
std::vector<Vertex>& qubits);
static void addMcx(ZXDiagram& diag, std::vector<Qubit> controls,
const Qubit target, std::vector<Vertex>& qubits);
static void addMcz(ZXDiagram& diag, std::vector<Qubit> controls,
const Qubit target, std::vector<Vertex>& qubits);
static op_it parseOp(ZXDiagram& diag, op_it it, op_it end,
std::vector<Vertex>& qubits, const qc::Permutation& p);
static op_it parseCompoundOp(ZXDiagram& diag, op_it it, op_it end,
Expand Down
188 changes: 186 additions & 2 deletions src/zx/FunctionalityConstruction.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -307,6 +307,150 @@
addCnot(diag, ctrl0, ctrl1, qubits);
}

void FunctionalityConstruction::addCcz(ZXDiagram& diag, const Qubit ctrl0,
const Qubit ctrl1, const Qubit target,
std::vector<Vertex>& qubits) {

addCnot(diag, ctrl1, target, qubits);
addZSpider(diag, target, qubits, PiExpression(PiRational(-1, 4)));
addCnot(diag, ctrl0, target, qubits);
addZSpider(diag, target, qubits, PiExpression(PiRational(1, 4)));
addCnot(diag, ctrl1, target, qubits);
addZSpider(diag, ctrl1, qubits, PiExpression(PiRational(1, 4)));
addZSpider(diag, target, qubits, PiExpression(PiRational(-1, 4)));
addCnot(diag, ctrl0, target, qubits);
addZSpider(diag, target, qubits, PiExpression(PiRational(1, 4)));
addCnot(diag, ctrl0, ctrl1, qubits);
addZSpider(diag, ctrl0, qubits, PiExpression(PiRational(1, 4)));
addZSpider(diag, ctrl1, qubits, PiExpression(PiRational(-1, 4)));
addZSpider(diag, target, qubits, PiExpression(PiRational(0, 1)),
EdgeType::Hadamard);
addCnot(diag, ctrl0, ctrl1, qubits);
addZSpider(diag, target, qubits, PiExpression(), EdgeType::Hadamard);
}

void FunctionalityConstruction::addCrz(ZXDiagram& diag,
const PiExpression& phase,
const Qubit ctrl, const Qubit target,
std::vector<Vertex>& qubits) {
// CRZ decomposition uses reversed CNOT direction
addCnot(diag, target, ctrl, qubits);

Check warning on line 337 in src/zx/FunctionalityConstruction.cpp

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src/zx/FunctionalityConstruction.cpp:337:3 [readability-suspicious-call-argument] 

2nd argument 'target' (passed to 'ctrl') looks like it might be swapped with the 3rd, 'ctrl' (passed to 'target')
addZSpider(diag, ctrl, qubits, -phase / 2);
addZSpider(diag, target, qubits, phase / 2);
addCnot(diag, target, ctrl, qubits);

Check warning on line 340 in src/zx/FunctionalityConstruction.cpp

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src/zx/FunctionalityConstruction.cpp:340:3 [readability-suspicious-call-argument] 

2nd argument 'target' (passed to 'ctrl') looks like it might be swapped with the 3rd, 'ctrl' (passed to 'target')
}

void FunctionalityConstruction::addMcrz(ZXDiagram& diag,
const PiExpression& phase,
std::vector<Qubit> controls,
const Qubit target,
std::vector<Vertex>& qubits) {

switch (controls.size()) {
case 0:
addZSpider(diag, target, qubits, phase);
return;
case 1:
addCrz(diag, phase, controls.front(), target, qubits);
return;
default:
const Qubit nextControl = controls.back();
controls.pop_back();

addCrz(diag, phase / 2, nextControl, target, qubits);
addMcx(diag, controls, target, qubits);
addCrz(diag, -phase / 2, nextControl, target, qubits);
addMcx(diag, controls, target, qubits);
}
}

void FunctionalityConstruction::addMcx(ZXDiagram& diag,
std::vector<Qubit> controls,
const Qubit target,
std::vector<Vertex>& qubits) {

switch (controls.size()) {
case 0:
addXSpider(diag, target, qubits, PiExpression(PiRational(1, 1)));
return;
case 1:
addCnot(diag, controls.front(), target, qubits);
return;
case 2:
addCcx(diag, controls.front(), controls.back(), target, qubits);
return;
default:
const auto half = static_cast<std::ptrdiff_t>((controls.size() + 1) / 2);
const std::vector<Qubit> first(controls.begin(), controls.begin() + half);
std::vector<Qubit> second(controls.begin() + half, controls.end());

if (qubits.size() > controls.size() + 1) {
controls.push_back(target);
std::optional<Qubit> anc{};
for (std::size_t q = 0; q < qubits.size(); ++q) {
const auto qb = static_cast<Qubit>(q);
if (std::ranges::find(controls, qb) == controls.end()) {
anc = qb;
break;
}
}
if (!anc.has_value()) {
throw ZXException("No ancilla qubit available for MCX decomposition");
}

controls.pop_back();
second.push_back(*anc);

addMcx(diag, first, *anc, qubits);
addMcx(diag, second, target, qubits);

addMcx(diag, first, *anc, qubits);
addMcx(diag, second, target, qubits);
} else {
addRx(diag, PiExpression(PiRational(1, 4)), target, qubits);
addMcz(diag, second, target, qubits);
addRx(diag, PiExpression(-PiRational(1, 4)), target, qubits);
addMcx(diag, first, target, qubits);

addRx(diag, PiExpression(PiRational(1, 4)), target, qubits);
addMcz(diag, second, target, qubits);
addRx(diag, PiExpression(-PiRational(1, 4)), target, qubits);
addMcx(diag, first, target, qubits);
const Qubit lastControl = controls.back();
controls.pop_back();
addMcrz(diag, PiExpression(PiRational(1, 2)), controls, lastControl,
qubits);
}
}
}

void FunctionalityConstruction::addMcz(ZXDiagram& diag,
std::vector<Qubit> controls,
const Qubit target,
std::vector<Vertex>& qubits) {

switch (controls.size()) {
case 0:
addZSpider(diag, target, qubits, PiExpression(PiRational(1, 1)));
return;
case 1:
addCrz(diag, PiExpression(PiRational(1, 1)), controls.front(), target,
qubits);
return;
case 2:
addCcz(diag, controls.front(), controls.back(), target, qubits);
return;
default:
const Qubit nextControl = controls.back();
controls.pop_back();

addCrz(diag, PiExpression(PiRational(1, 2)), nextControl, target, qubits);
addMcx(diag, controls, target, qubits);
addCrz(diag, PiExpression(-PiRational(1, 2)), nextControl, target, qubits);
addMcx(diag, controls, target, qubits);
}
}

FunctionalityConstruction::op_it
FunctionalityConstruction::parseOp(ZXDiagram& diag, op_it it, op_it end,
std::vector<Vertex>& qubits,
Expand Down Expand Up @@ -538,6 +682,9 @@
qubits[static_cast<std::size_t>(target)],
EdgeType::Hadamard);
break;
case qc::OpType::RZ:
addCrz(diag, parseParam(op.get(), 0), ctrl, target, qubits);
break;

case qc::OpType::I:
break;
Expand Down Expand Up @@ -578,16 +725,42 @@
ctrl1 = static_cast<Qubit>(p.at(ctrl.qubit));
}
}
std::vector<Qubit> controls;
for (const auto& ctrl : op->getControls()) {
controls.push_back(static_cast<Qubit>(p.at(ctrl.qubit)));
}
switch (op->getType()) {
case qc::OpType::X:
addCcx(diag, ctrl0, ctrl1, target, qubits);
break;

case qc::OpType::Z:
addCcz(diag, ctrl0, ctrl1, target, qubits);
break;
case qc::OpType::RZ:
addMcrz(diag, parseParam(op.get(), 0), controls, target, qubits);
break;
default:
throw ZXException("Unsupported Multi-control operation: " +
qc::toString(op->getType()));
}
Comment on lines +728 to +745
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@coderabbitai coderabbitai bot Dec 9, 2025

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🧹 Nitpick | 🔵 Trivial

Unused controls vector in 2-control branch.

The controls vector is constructed at lines 728-731 but is only used for the RZ case at line 740. For X and Z cases, ctrl0 and ctrl1 are used directly. This is not a bug but introduces slight redundancy.

Consider constructing controls only when needed, or using it consistently for all cases:

+    // Build controls vector for multi-control dispatch
     std::vector<Qubit> controls;
     for (const auto& ctrl : op->getControls()) {
       controls.push_back(static_cast<Qubit>(p.at(ctrl.qubit)));
     }
     switch (op->getType()) {
     case qc::OpType::X:
-      addCcx(diag, ctrl0, ctrl1, target, qubits);
+      addCcx(diag, controls.front(), controls.back(), target, qubits);
       break;
     case qc::OpType::Z:
-      addCcz(diag, ctrl0, ctrl1, target, qubits);
+      addCcz(diag, controls.front(), controls.back(), target, qubits);
       break;
🤖 Prompt for AI Agents 
In src/zx/FunctionalityConstruction.cpp around lines 728-745 the vector controls
is populated unconditionally but only used in the RZ branch; avoid the
redundancy by moving the construction into the RZ case (or alternately use the
controls vector for all branches). Update the code so that controls is
declared/filled only inside the qc::OpType::RZ case before calling addMcrz(diag,
parseParam(op.get(), 0), controls, target, qubits), and remove the unused
controls construction from before the switch; ensure no other branches depend on
controls.

} else if (op->getNtargets() == 1) {
const auto target = static_cast<Qubit>(p.at(op->getTargets().front()));
std::vector<Qubit> controls;
for (const auto& ctrl : op->getControls()) {
controls.push_back(static_cast<Qubit>(p.at(ctrl.qubit)));
}
switch (op->getType()) {
case qc::OpType::X:
addMcx(diag, controls, target, qubits);
break;
case qc::OpType::Z:
addZSpider(diag, target, qubits, PiExpression(), EdgeType::Hadamard);
addCcx(diag, ctrl0, ctrl1, target, qubits);
addMcx(diag, controls, target, qubits);
addZSpider(diag, target, qubits, PiExpression(), EdgeType::Hadamard);
break;
case qc::OpType::RZ:
addMcrz(diag, parseParam(op.get(), 0), controls, target, qubits);
break;
default:
throw ZXException("Unsupported Multi-control operation: " +
qc::toString(op->getType()));
Expand Down Expand Up @@ -701,15 +874,26 @@
case qc::OpType::S:
case qc::OpType::Tdg:
case qc::OpType::Sdg:
case qc::OpType::RZ:
return true;

default:
return false;
}
} else if (op->getNcontrols() == 2) {

Check warning on line 883 in src/zx/FunctionalityConstruction.cpp

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src/zx/FunctionalityConstruction.cpp:883:39 [bugprone-branch-clone] 

repeated branch body in conditional chain
switch (op->getType()) {
case qc::OpType::X:
case qc::OpType::Z:
case qc::OpType::RZ:
return true;
default:
return false;
}
} else if (op->getNtargets() == 1) {
switch (op->getType()) {
case qc::OpType::X:
case qc::OpType::Z:
case qc::OpType::RZ:
return true;
default:
return false;
Expand Down
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