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Improve stim.Tableau.from_stabilizers error messages #761

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May 3, 2024
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Improve stim.Tableau.from_stabilizers error messages #761

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ce3f388
Improve stim.Tableau.from_stabilizers error messages
Strilanc May 2, 2024
ad255d2
Merge branch 'main' into anticommerror
Strilanc May 3, 2024
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28 changes: 28 additions & 0 deletions src/stim/stabilizers/tableau_pybind_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -975,3 +975,31 @@ def test_to_circuit_mpp_unsigned_preserves_stabilizers():
for e in reconstructed:
e.sign = +1
assert original == reconstructed


def test_from_stabilizers_error_messages():
with pytest.raises(ValueError, match="anticommute"):
stim.Tableau.from_stabilizers([
stim.PauliString("Z"),
stim.PauliString("X"),
])
with pytest.raises(ValueError, match="anticommute"):
stim.Tableau.from_stabilizers([
stim.PauliString("Z"),
stim.PauliString("X" + "_"*500),
])
with pytest.raises(ValueError, match="contradict"):
stim.Tableau.from_stabilizers([
stim.PauliString("Z_"),
stim.PauliString("-_Z"),
stim.PauliString("Z" + "_"*500 + "X"),
stim.PauliString("ZZ"),
])
with pytest.raises(ValueError, match="redundant"):
stim.Tableau.from_stabilizers([
stim.PauliString("-Z_"),
stim.PauliString("Z" + "_"*500 + "X"),
stim.PauliString("-__Z"),
stim.PauliString("_Z_"),
stim.PauliString("Z_Z"),
])
92 changes: 66 additions & 26 deletions src/stim/util_top/stabilizers_to_tableau.inl
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Expand Up @@ -24,10 +24,58 @@ Tableau<W> stabilizers_to_tableau(

Circuit elimination_instructions;

auto fail_due_to_anticommutation = [&]() {
for (size_t k1 = 0; k1 < stabilizers.size(); k1++) {
for (size_t k2 = k1 + 1; k2 < stabilizers.size(); k2++) {
if (!stabilizers[k1].ref().commutes(stabilizers[k2])) {
std::stringstream ss;
ss << "Some of the given stabilizers anticommute.\n";
ss << "For example:";
ss << "\n stabilizers[" << k1 << "] = " << stabilizers[k1];
ss << "\nanticommutes with";
ss << "\n stabilizers[" << k2 << "] = " << stabilizers[k2];
throw std::invalid_argument(ss.str());
}
}
}
throw std::invalid_argument(
"The given stabilizers commute but the solver failed in a way that suggests they anticommute. Please "
"report this as a bug.");
};

auto print_redundant_z_product_parts = [&](size_t stabilizer_index, std::ostream &out) {
PauliString<W> target = stabilizers[stabilizer_index];
target.ensure_num_qubits(num_qubits, 1.0);
target = target.ref().after(elimination_instructions);
if (num_qubits > 0) {
GateTarget t = GateTarget::qubit(num_qubits - 1);
elimination_instructions.safe_append(CircuitInstruction{GateType::X, {}, &t});
elimination_instructions.safe_append(CircuitInstruction{GateType::X, {}, &t});
}
Tableau<W> inverse = circuit_to_tableau<W>(elimination_instructions, false, false, false, true);
target.ref().for_each_active_pauli([&](size_t q) {
out << "\n ";
for (size_t k = 0; k < stabilizers.size(); k++) {
PauliString<W> s = stabilizers[k];
s.ensure_num_qubits(num_qubits, 1.0);
if (s == inverse.zs[q]) {
out << "stabilizers[" << k << "] = " << stabilizers[k];
return;
}
}
out << inverse.zs[q];
});
};

size_t used = 0;
for (size_t k = 0; k < stabilizers.size(); k++) {
// Find a non-identity term in the Pauli string past the region used by other stabilizers.
size_t pivot;
for (size_t q = 0; q < used; q++) {
if (buf_xs[q][k]) {
fail_due_to_anticommutation();
}
}
for (pivot = used; pivot < num_qubits; pivot++) {
if (buf_xs[pivot][k] || buf_zs[pivot][k]) {
break;
Expand All @@ -37,12 +85,26 @@ Tableau<W> stabilizers_to_tableau(
// Check for incompatible / redundant stabilizers.
if (pivot == num_qubits) {
if (buf_signs[k]) {
throw std::invalid_argument("Some of the given stabilizers contradict each other.");
std::stringstream ss;
ss << "Some of the given stabilizers contradict each other.\n";
ss << "For example:";
ss << "\n stabilizers[" << k << "] = " << stabilizers[k];
ss << "\nis the negation of the product of the following stabilizers: {";
print_redundant_z_product_parts(k, ss);
ss << "\n}";
throw std::invalid_argument(ss.str());
}
if (!allow_redundant) {
throw std::invalid_argument(
"Didn't specify allow_redundant=True but one of the given stabilizers is a product of the others. "
"To allow redundant stabilizers, pass the argument allow_redundant=True.");
std::stringstream ss;
ss << "Some of the given stabilizers are redundant.";
ss<< "\nTo allow redundant stabilizers, pass the argument allow_redundant=True.";
ss << "\n";
ss << "\nFor example:";
ss << "\n stabilizers[" << k << "] = " << stabilizers[k];
ss << "\nis the product of the following stabilizers: {";
print_redundant_z_product_parts(k, ss);
ss << "\n}";
throw std::invalid_argument(ss.str());
}
continue;
}
Expand Down Expand Up @@ -140,28 +202,6 @@ Tableau<W> stabilizers_to_tableau(
used++;
}

// All stabilizers will have been mapped into Z products, if they commuted.
for (size_t q = 0; q < num_qubits; q++) {
if (buf_xs[q].not_zero()) {
for (size_t k1 = 0; k1 < stabilizers.size(); k1++) {
for (size_t k2 = k1 + 1; k2 < stabilizers.size(); k2++) {
if (!stabilizers[k1].ref().commutes(stabilizers[k2])) {
std::stringstream ss;
ss << "Some of the given stabilizers anticommute.\n";
ss << "For example:\n ";
ss << stabilizers[k1];
ss << "\nanticommutes with\n";
ss << stabilizers[k2] << "\n";
throw std::invalid_argument(ss.str());
}
}
}
throw std::invalid_argument(
"The given stabilizers commute but the solver failed in a way that suggests they anticommute. Please "
"report this as a bug.");
}
}

if (used < num_qubits) {
if (!allow_underconstrained) {
throw std::invalid_argument(
Expand Down
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