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ctrl @ tdg, sx, s, sdg, t and other standard gates without a dedicated
controlled form raised "Unsupported controlled QASM operation". When the
existing ctrl-chain lookup fails, the gate is now lowered to its
single-qubit (or swap) base gate written exactly as e^{i*gamma} U(theta,
phi, lam), and controlled on any number of qubits: a multi-controlled
phase for diagonal gates, the ABC construction otherwise, and a
recursive multi-controlled X past c4x.
Also fixes ctrl @ u, which mapped to the 4-parameter cu and raised a
TypeError, and ctrl(n) @ gphase for n >= 2, which put the phase gate's
target on its own control.
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Merge Risk: 🔵 Low · up to
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Docstring Coverage |  |
Docstring coverage is 77.42% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 6 files. (1 skipped: … | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Description Check Check skipped - CodeRabbit’s high-level summary is enabled. Title check The title clearly and concisely describes the primary change: adding support for ctrl @ on arbitrary single-qubit standard gates. It matches the main objectives and implementation changes. Linked Issues check Issue #252requires support for decomposing multi-controlled operations such asctrl(2) @ zinside a custom gate.src/pyqasm/maps/gates.pynow falls back from the direct map to `_mc_one_qubit_gate…Out of Scope Changes check The changes stay within controlled-operation support. The new decomposition code, the gphasecontrol-qubit fix, removal of the incorrectutocumapping, and the related tests support controlled…Full details: Linked Issues check
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Explanation
Issue
#252requires support for decomposing multi-controlled operations such asctrl(2) @ zinside a custom gate.src/pyqasm/maps/gates.pynow falls back from the direct map to_mc_one_qubit_gate,_mc_phase_gate, and_mcx_gate. The fallback supports arbitrary control counts for single-qubit gates.tests/qasm3/test_controlled_gates.py::test_ctrl_custom_gate_with_multi_controlled_zreproduces the issue case and compares the unrolled unitary with an independent reference. The test also validates the serialized output.
Full details: Out of Scope Changes check
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Explanation
The changes stay within controlled-operation support. The new decomposition code, the
gphasecontrol-qubit fix, removal of the incorrectutocumapping, and the related tests support controlled standard gates, multi-controlled gates, swaps, aliases, and modifier combinations. Test utilities and visualization expectations update the required verification paths. The PR retains validation errors for unsupported controlled multi-qubit gates, which matches the stated scope. No unrelated production feature is demonstrated.
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Full details: Docstring Coverage
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Explanation
Docstring coverage is 77.42% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 6 files. (1 skipped: 1 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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Review comments at @src/pyqasm/maps/gates.py:
- Around line 1392-1423: Prevent exponential expansion in _mcx_gate for large
control counts by adding a documented maximum-control guard before recursive
decomposition; when len(ctrls) exceeds the limit, raise ValidationError with the
supported maximum and received count. Preserve existing decompositions for
supported control counts and the current gate behavior otherwise.
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CHANGELOG.mdsrc/pyqasm/maps/gates.pysrc/pyqasm/visitor.pytests/qasm3/test_controlled_gates.pytests/qasm3/test_gates.pytests/utils.pytests/visualization/test_mpl_draw.py
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The ancilla-free decomposition triples in size per control past four, so ctrl(12) @ x unrolls to ~6.5M gates. Reject counts above 10 with a ValidationError instead of hanging unroll().
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Argus · 10/10 — 1 minor note
Argus reviewed 7 files and checked 2 for bugs, security, architecture, and regressions. Custom agent ponytail-review also ran. 1 minor note is collapsed below. The diff matches the stated goal.
What this PR does · 14 author checks
- Support ctrl @ on any single-qubit standard gate through general decomposition, fixing controlled u and gphase while rejecting more than 10 total controls.
- Excludes: Support ctrl @ on multi-qubit gates that are not controlled single-qubit gates or swap.
- Excludes: A linear-size MCX decomposition is deferred to a separate follow-up that could lift the control limit.
Author's checks
- CTRL_GATE_MAP lookup runs first, preserving output for gates that already worked; failed lookups use a general decomposition.
- ONE_QUBIT_U_FORM_MAP represents single-qubit gates as e^{iγ} U(θ, φ, λ), preserving relative phases under control.
- CTRL_BASE_GATE_MAP rewrites standard controlled gates into their base gates plus implicit controls.
- Remove u → cu from CTRL_GATE_MAP while retaining cu → u in REV_CTRL_GATE_MAP for unchanged drawing.
- _visit_phase_operation applies the phase on ctrls[-1], avoiding duplicate control and target qubits.
- More than 10 total controls, including implicit controls, raises ValidationError before unrolling.
- Unsupported multi-qubit gates continue raising the same ValidationError.
- Operator comparisons cover 29 single-qubit gates and aliases with 1–3 controls, plus x, z, tdg, sx, and u with 5 controls.
- Tests cover controlled standard gates, swap, the custom gate from #252, operand order, negctrl, inv @ ctrl @ t, and ctrl(2) @ gphase.
- Operator comparisons reject incorrect relative phases between control states; each unrolled test program is dumped and re-validated.
- test_ctrl_too_many_controls_raises rejects ctrl(11) @ x and ctrl(7) @ c4x.
- test_controls_beyond_c4x_realise_mcx checks ctrl @ c4x and ctrl(5) @ x against the exact 6-qubit MCX.
- The production failing program unrolls and re-validates; without reset, delay, and measure, its operator is equivalent to Qiskit's import.
- pytest tests, tox -e format-check, and sphinx-build -W pass; new code is fully covered.
Minor notes (1)
- Valid c4x control extension remains untested ·
tests/qasm3/test_controlled_gates.py:L140
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Changes requested — ctrl @ on single-qubit gates now unrolls correctly, but inv/pow(-k) combined with 2+ controls now emits a wrong unitary where main raised.
Findings
All findings are posted inline on the relevant lines. Checklist:
- H1 —
inv @ ctrl(n≥2) @ U/u3/u2/cs/cunow silently unrolls to a wrong unitary - L1 — Control-cap error masks "unsupported gate" for non-decomposable gates like
iswap
How it was tested
pytest tests: head 1161 passed / 3 skipped; main (a742796) 1015 passed / 3 skipped.- Each unrolled program was compared with Qiskit's
Gate.control(n, ctrl_state=…), up to one global phase, with the relative phase between control states checked. That covers 23 single-qubit gates and aliases at n = 1, 2, 3, 5, mixednegctrl, expression parameters, operand order, 24 controlled standard gates (cs…c4x,cswap,ctrl(5) @ cx), the #252 custom gate,pow(k) @ ctrl,ctrl @ powandinv @ ctrl @ t/sdg/h/rz/rx. All of them match. ctrl(2) @ gphase(0.3)andctrl(3) @ gphase(0.3)equal diag(1,…,1,e^{0.3i}) up to a global phase. On main both raiseValidationError.- 6 inverse-path cases are wrong; see H1.
- At the 10-control cap,
ctrl(10) @ x/sx/ugives 363k gates (1.8 s, 0.49 GB) andctrl(9) @ cswapgives 467k gates (2.3 s, 0.61 GB).
Next steps
Fix H1 and add its two regression tests before merge. L1 is optional.
| return FIVE_QUBIT_OP_MAP[ctrl_op_name], 5 | ||
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| # TODO: decompose controls if not built in | ||
| decomposed = _decomposed_ctrl_op(op_name, ctrl_count) |
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[H1] inv/pow(-k) with 2+ controls now yields a wrong unitary without an error — Implementation · High
Rationale: The defect is in unchanged code: the inverse recursion in _visit_basic_gate_operation (visitor.py:1255–1264). It first expands inv @ g (through u3_inv_gate, gates.py:118–134, or the two-qubit NO_OP branch of map_qasm_inv_op_to_callable) and then controls each resulting gate. u3_inv_gate equals U⁻¹ only up to a global phase. Once each piece is controlled, that phase becomes a relative phase between control states. On main, the controlled rz pieces with 2+ controls had no mapping, so these programs raised. The new fallback here makes them resolve, so they now return wrong output with no error. Checked against Qiskit's Gate.inverse().control(n):
| program | main | head |
|---|---|---|
inv @ ctrl(2) @ U(0.3,0.5,0.7) |
ValidationError | wrong unitary |
inv @ ctrl(3) @ u2(0.5,0.7) |
ValidationError | wrong unitary |
pow(-1) @ ctrl(2) @ U(…) |
ValidationError | wrong unitary |
inv @ ctrl(2) @ cs |
ValidationError | wrong unitary |
inv @ ctrl(2) @ mu / ctrl(2) @ inv @ mu (gate mu x { U(…) x; }) |
ValidationError | wrong unitary |
The same programs without inv match. Qiskit-exported programs use custom gates built from U, so the custom-gate case is reachable in practice. The new test_ctrl_combined_with_other_modifiers covers only inv @ ctrl @ t with one control. That path is already exact, so the test cannot catch this.
Change requested: When inverse is set and ctrls is non-empty, invert in U-form instead of controlling each gate of the inverse expansion. Note that (e^{iγ}U(θ,φ,λ))† = e^{-iγ}U(-θ,-λ,-φ). One way is to give _decomposed_ctrl_op an inverse flag, and route the visitor's inverse branch through it whenever the base gate (after CTRL_BASE_GATE_MAP resolution) has a U-form:
# gates.py, inside _decomposed_ctrl_op
def gate(*args) -> list[QuantumGate]:
params, qubits = args[:-num_qubits], args[-num_qubits:]
theta, phi, lam, gamma = u_form(*params)
if inverse:
theta, phi, lam, gamma = -theta, -lam, -phi, -gamma
return _mc_one_qubit_gate(theta, phi, lam, gamma, list(qubits[:-1]), qubits[-1])# visitor.py, _visit_basic_gate_operation: try this before the existing inverse branch
if inverse and ctrls:
decomposed = _decomposed_ctrl_op(operation.name.name, len(ctrls), inverse=True)
# if not None, use it like the non-inverse ctrl path; else keep the current recursionThis covers U/u3/u2 and the CTRL_BASE_GATE_MAP gates (cs resolves to base s, whose inverse U-form is (0, 0, -π/2, 0)). The current recursion stays for rz/rx/ry/s/t, which is already exact. If this is out of scope, raise ValidationError for inverse and ctrls on U-form or two-qubit bases instead of emitting output.
Add regression tests to tests/qasm3/test_controlled_gates.py: inv @ ctrl(2) @ U(0.3, 0.5, 0.7) and inv @ ctrl(2) @ cs, each compared with _controlled(<inverse matrix>, n).
| base, implicit_ctrls = CTRL_BASE_GATE_MAP.get(op_name, (op_name, 0)) | ||
| num_ctrls = ctrl_count + implicit_ctrls | ||
| if num_ctrls > MAX_DECOMPOSED_CTRLS: | ||
| raise ValidationError( | ||
| f"Unsupported controlled QASM operation: {op_name} with {num_ctrls} controls " | ||
| f"(at most {MAX_DECOMPOSED_CTRLS} are supported)" | ||
| ) | ||
| if base == "swap": | ||
| return (lambda *qubits: _mc_swap_gate(list(qubits[:-2]), *qubits[-2:])), num_ctrls + 2 | ||
| if base not in ONE_QUBIT_U_FORM_MAP: | ||
| return None |
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[L1] Control-cap error replaces the "unsupported gate" error — Implementation · Low
Rationale: The cap check runs before the support check. So ctrl(11) @ iswap reports iswap with 11 controls (at most 10 are supported). That message implies fewer controls would work, but iswap is never supported. Checking support first lets the caller raise the normal Unsupported controlled QASM operation: iswap with 11 controls. Applying the block below leaves the test results unchanged.
Change requested:
| base, implicit_ctrls = CTRL_BASE_GATE_MAP.get(op_name, (op_name, 0)) | |
| num_ctrls = ctrl_count + implicit_ctrls | |
| if num_ctrls > MAX_DECOMPOSED_CTRLS: | |
| raise ValidationError( | |
| f"Unsupported controlled QASM operation: {op_name} with {num_ctrls} controls " | |
| f"(at most {MAX_DECOMPOSED_CTRLS} are supported)" | |
| ) | |
| if base == "swap": | |
| return (lambda *qubits: _mc_swap_gate(list(qubits[:-2]), *qubits[-2:])), num_ctrls + 2 | |
| if base not in ONE_QUBIT_U_FORM_MAP: | |
| return None | |
| base, implicit_ctrls = CTRL_BASE_GATE_MAP.get(op_name, (op_name, 0)) | |
| if base != "swap" and base not in ONE_QUBIT_U_FORM_MAP: | |
| return None | |
| num_ctrls = ctrl_count + implicit_ctrls | |
| if num_ctrls > MAX_DECOMPOSED_CTRLS: | |
| raise ValidationError( | |
| f"Unsupported controlled QASM operation: {op_name} with {num_ctrls} controls " | |
| f"(at most {MAX_DECOMPOSED_CTRLS} are supported)" | |
| ) | |
| if base == "swap": | |
| return (lambda *qubits: _mc_swap_gate(list(qubits[:-2]), *qubits[-2:])), num_ctrls + 2 |
Closes #252.
Problem
ctrl @only worked on gates thatCTRL_GATE_MAPcould chain to a named controlled gate (x,y,z,rx/ry/rz,p,h,swap,cx,ccx,c3x). Everything else raisedValidationError: Unsupported controlled QASM operation: <gate> with <n> controls. That coveredctrl @ tdg,s,sdg,t,sx,id,U, and alsoctrl(2) @ z,ctrl @ czandctrl(5) @ x.On qBraid production, 3 jobs from one user on
openquantum:iqm:qpu:emeraldfailed this way between 2026-10-17 and 10-19 (e.g.openquantum:iqm:qpu:emerald-8504-qjob-6ad37ba138127b44c71b9969). Their OpenQASM 3 program containsctrl @ tdg q[1], q[3];,ctrl @ sx q[0], q[2];andctrl @ sdg q[2], q[1];. #252 is the same gap with more controls: a custom gate containingctrl(2) @ z, which becomeszwith 3 controls once the custom gate is itself controlled.Two related bugs came up along the way:
ctrl @ umapped tocu, which takes 4 parameters, so it raisedTypeError: cu_gate() missing 1 required positional argument.ctrl(n) @ gphasefor n >= 2 built its phase gate onctrls[0]while also usingctrls[0]as a control, so it raisedDuplicate qubit.Change
The
CTRL_GATE_MAPlookup still runs first, so output for gates that already worked does not change. If the lookup fails,map_qasm_ctrl_op_to_callablefalls back to a general decomposition inpyqasm/maps/gates.py:ONE_QUBIT_U_FORM_MAPwrites each single-qubit gate exactly ase^{iγ} U(θ, φ, λ). Underctrl, γ becomes a relative phase, so it is kept exactly (for examplesx = e^{iπ/4} rx(π/2)andrz(θ) = e^{-iθ/2} p(θ)). The matrices are the ones pyqasm already unrolls these gates to, which are also Qiskit's (xis Pauli X,u3 == U). They do not follow thegphaseterms in the spec'sstdgates.inc.CTRL_BASE_GATE_MAPrewrites standard controlled gates as their base gate plus implicit controls, e.g.cs→ (s, 1),c3sx→ (sx, 3),cu→ (Uwith γ, 1),cswap→ (swap, 1).ctrl(n) @ e^{iγ}U(θ,φ,λ)becomes:cprecursion, no ancillas);In both cases the overall phase is applied as a phase on the last control, conditioned on the others.
Multi-controlled X uses
cx/ccx/c3x/c4xup to 4 controls. Past that it usesh · C^n P(π) · h. Controlled swap iscx · MCX · cx."u": "cu"is removed fromCTRL_GATE_MAP, soctrl @ ugoes through the fallback.REV_CTRL_GATE_MAPkeepscu → u, so drawing is unchanged, and the printer test now names that one exception._visit_phase_operationputs the phase onctrls[-1].Still unsupported, and still raising the same
ValidationError:ctrl @on multi-qubit gates that are not a controlled single-qubit gate or swap (iswap,rxx,ecr,ms,rccx, …). Past 4 controls the decomposition is ancilla-free but grows about 3× per extra control (ctrl(10) @ xunrolls to about 726k gates in 5 s;ctrl(12) @ xto 6.5M in about 2 min). So more than 10 controls in total, counting a gate's implicit ones (ctrl(7) @ c4xis 11), raisesValidationErrorbefore unrolling. A linear-size MCX would be a separate follow-up and could lift the limit.Testing
New
tests/qasm3/test_controlled_gates.pycompares the unrolled operator against matrices written independently in the test, with a check that pyqasm's own unrolling of each bare gate agrees with them:ctrl(n) @ gfor n = 1, 2, 3 on 29 single-qubit gates and aliases, plus n = 5 forx,z,tdg,sx,u;ctrl @oncs,csdg,csx,cy,cz,ch,crx,cp,cu1,cu3,cu(with γ),swap,cswap,c3sx;negctrl,inv @ ctrl @ t, andctrl(2) @ gphase.The comparison allows only a phase on the whole controlled operator, so a wrong relative phase between control states fails. Each unrolled program is also dumped and re-validated.
test_ctrl_too_many_controls_raiseschecks thatctrl(11) @ xandctrl(7) @ c4xare rejected.tests/utils.py: the unitary simulator now handlessxandccx.test_too_many_controls_raises(ctrl @ c4x/ctrl(5) @ x) becomestest_controls_beyond_c4x_realise_mcx, which checks both against the exact 6-qubit MCX.The real failing program, fetched from the production job, now unrolls (202 lines) and re-validates. With reset/delay/measure stripped, its unrolled operator equals Qiskit's import of the original program (
Operator.equiv→True).pytest tests: 1155 passed, 3 skipped. New code is fully covered.tox -e format-check(pylint 10.00, isort, black, mypy, headers) passes, andsphinx-build -Wpasses.Summary by CodeRabbit
uoperations.gphasebehavior.TypeErrorfailures.