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Clean up stim.Tableau.from_stabilizers a bit
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- Switch to streaming computations of the gates
- Improve the anticommutation check from O(n^2) additional work to O(n) additional work
- Fix a sizing error and test that it's fixed
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@Strilanc
Strilanc committed Mar 15, 2024
1 parent 835aec1 commit 7fcee69
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Showing 2 changed files with 68 additions and 65 deletions.
120 changes: 55 additions & 65 deletions src/stim/stabilizers/conversions.inl
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Original file line number Diff line number Diff line change
Expand Up @@ -558,10 +558,9 @@ Tableau<W> stabilizers_to_tableau(
num_qubits = std::max(num_qubits, e.num_qubits);
}

simd_bit_table<W> buf_xs(num_qubits, stabilizers.size());
simd_bit_table<W> buf_zs(num_qubits, stabilizers.size());
simd_bit_table<W> buf_xs(stabilizers.size(), num_qubits);
simd_bit_table<W> buf_zs(stabilizers.size(), num_qubits);
simd_bits<W> buf_signs(stabilizers.size());
simd_bits<W> buf_workspace(stabilizers.size());
for (size_t k = 0; k < stabilizers.size(); k++) {
memcpy(buf_xs[k].u8, stabilizers[k].xs.u8, stabilizers[k].xs.num_u8_padded());
memcpy(buf_zs[k].u8, stabilizers[k].zs.u8, stabilizers[k].zs.num_u8_padded());
Expand All @@ -570,19 +569,6 @@ Tableau<W> stabilizers_to_tableau(
buf_xs = buf_xs.transposed();
buf_zs = buf_zs.transposed();

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());
}
}
}
Circuit elimination_instructions;

size_t used = 0;
Expand All @@ -597,11 +583,6 @@ Tableau<W> stabilizers_to_tableau(

// Check for incompatible / redundant stabilizers.
if (pivot == num_qubits) {
for (size_t q = 0; q < num_qubits; q++) {
if (buf_xs[q][k]) {
throw std::invalid_argument("Some of the given stabilizers anticommute.");
}
}
if (buf_signs[k]) {
throw std::invalid_argument("Some of the given stabilizers contradict each other.");
}
Expand All @@ -620,19 +601,21 @@ Tableau<W> stabilizers_to_tableau(
CircuitInstruction instruction{g, {}, &t};
elimination_instructions.safe_append(instruction);
size_t q = pivot;
simd_bits_range_ref<W> xs1 = buf_xs[q];
simd_bits_range_ref<W> zs1 = buf_zs[q];
simd_bits_range_ref<W> ss = buf_signs;
switch (g) {
case GateType::H_YZ:
buf_xs[q] ^= buf_zs[q];
buf_workspace = buf_zs[q];
buf_workspace.invert_bits();
buf_workspace &= buf_xs[q];
buf_signs ^= buf_workspace;
ss.for_each_word(xs1, zs1, [](auto &s, auto &x, auto &z) {
x ^= z;
s ^= z.andnot(x);
});
break;
case GateType::H:
buf_xs[q].swap_with(buf_zs[q]);
buf_workspace = buf_zs[q];
buf_workspace &= buf_xs[q];
buf_signs ^= buf_workspace;
ss.for_each_word(xs1, zs1, [](auto &s, auto &x, auto &z) {
std::swap(x, z);
s ^= x & z;
});
break;
default:
throw std::invalid_argument("Unrecognized gate type.");
Expand All @@ -649,47 +632,34 @@ Tableau<W> stabilizers_to_tableau(
elimination_instructions.safe_append(instruction);
size_t q1 = targets[0].qubit_value();
size_t q2 = targets[1].qubit_value();
simd_bits_range_ref<W> x1 = buf_xs[q1];
simd_bits_range_ref<W> z1 = buf_zs[q1];
simd_bits_range_ref<W> x2 = buf_xs[q2];
simd_bits_range_ref<W> z2 = buf_zs[q2];
simd_bits_range_ref<W> ss = buf_signs;
simd_bits_range_ref<W> xs1 = buf_xs[q1];
simd_bits_range_ref<W> zs1 = buf_zs[q1];
simd_bits_range_ref<W> xs2 = buf_xs[q2];
simd_bits_range_ref<W> zs2 = buf_zs[q2];
switch (g) {
case GateType::XCX:
buf_workspace = x1;
buf_workspace ^= x2;
buf_workspace &= z1;
buf_workspace &= z2;
buf_signs ^= buf_workspace;
x1 ^= z2;
x2 ^= z1;
ss.for_each_word(xs1, zs1, xs2, zs2, [](auto &s, auto &x1, auto &z1, auto &x2, auto &z2) {
s ^= (x1 ^ x2) & z1 & z2;
x1 ^= z2;
x2 ^= z1;
});
break;
case GateType::XCY:
x1 ^= x2;
x1 ^= z2;
x2 ^= z1;
z2 ^= z1;
buf_workspace = x1;
buf_workspace |= x2;
buf_workspace.invert_bits();
buf_workspace &= z1;
buf_workspace &= z2;
buf_signs ^= buf_workspace;
buf_workspace = z2;
buf_workspace.invert_bits();
buf_workspace &= z1;
buf_workspace &= x1;
buf_workspace &= x2;
buf_signs ^= buf_workspace;
ss.for_each_word(xs1, zs1, xs2, zs2, [](auto &s, auto &x1, auto &z1, auto &x2, auto &z2) {
x1 ^= x2 ^ z2;
x2 ^= z1;
z2 ^= z1;
s ^= x1.andnot(z1) & x2.andnot(z2);
s ^= x1 & z1 & z2.andnot(x2);
});
break;
case GateType::XCZ:
z2 ^= z1;
x1 ^= x2;
buf_workspace = z2;
buf_workspace ^= x1;
buf_workspace.invert_bits();
buf_workspace &= x2;
buf_workspace &= z1;
buf_signs ^= buf_workspace;
ss.for_each_word(xs1, zs1, xs2, zs2, [](auto &s, auto &x1, auto &z1, auto &x2, auto &z2) {
z2 ^= z1;
x1 ^= x2;
s ^= (z2 ^ x1).andnot(z1 & x2);
});
break;
default:
throw std::invalid_argument("Unrecognized gate type.");
Expand Down Expand Up @@ -717,6 +687,26 @@ 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
13 changes: 13 additions & 0 deletions src/stim/stabilizers/conversions.test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -663,6 +663,19 @@ TEST_EACH_WORD_SIZE_W(conversions, stabilizer_to_tableau_detect_anticommutation,
ASSERT_THROW({ stabilizers_to_tableau<W>(input_stabilizers, false, false, false); }, std::invalid_argument);
})

TEST_EACH_WORD_SIZE_W(conversions, stabilizer_to_tableau_size_affecting_redundancy, {
std::vector<stim::PauliString<W>> input_stabilizers;
input_stabilizers.push_back(PauliString<W>::from_str("X_"));
input_stabilizers.push_back(PauliString<W>::from_str("_X"));
for (size_t k = 0; k < 150; k++) {
input_stabilizers.push_back(PauliString<W>::from_str("__"));
}
auto t = stabilizers_to_tableau<W>(input_stabilizers, true, true, false);
ASSERT_EQ(t.num_qubits, 2);
ASSERT_EQ(t.zs[0], PauliString<W>::from_str("X_"));
ASSERT_EQ(t.zs[1], PauliString<W>::from_str("_X"));
})

TEST(conversions, independent_to_disjoint_xyz_errors) {
double out_x;
double out_y;
Expand Down

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