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thomashaenerdamiansteiger
thomashaener
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damiansteiger
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Added collapse_wavefunction to simulator. (ProjectQ-Framework#122)
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‎projectq/backends/_sim/_cppkernels/simulator.hpp‎

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Original file line numberDiff line numberDiff line change
@@ -365,6 +365,36 @@ class Simulator{
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vec_[i] = wavefunction[i];
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}
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void collapse_wavefunction(std::vector<unsigned> const& ids, std::vector<bool> const& values){
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run();
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assert(ids.size() == values.size());
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if (!check_ids(ids))
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throw(std::runtime_error("collapse_wavefunction(): Unknown qubit id(s) provided. Try calling eng.flush() before invoking this function."));
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std::size_t mask = 0, val = 0;
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for (unsigned i = 0; i < ids.size(); ++i){
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mask |= (1UL << map_[ids[i]]);
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val |= ((values[i]?1UL:0UL) << map_[ids[i]]);
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}
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// set bad entries to 0 and compute probability of outcome to renormalize
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calc_type N = 0.;
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#pragma omp parallel for reduction(+:N) schedule(static)
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for (std::size_t i = 0; i < vec_.size(); ++i){
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if ((i & mask) == val)
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N += std::norm(vec_[i]);
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}
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if (N < 1.e-12)
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throw(std::runtime_error("collapse_wavefunction(): Invalid collapse! Probability is ~0."));
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// re-normalize (if possible)
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N = 1./std::sqrt(N);
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#pragma omp parallel for schedule(static)
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for (std::size_t i = 0; i < vec_.size(); ++i){
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if ((i & mask) != val)
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vec_[i] = 0.;
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else
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vec_[i] *= N;
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}
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}
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void run(){
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if (fused_gates_.size() < 1)
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return;

‎projectq/backends/_sim/_cppsim.cpp‎

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Original file line numberDiff line numberDiff line change
@@ -55,6 +55,7 @@ PYBIND11_PLUGIN(_cppsim) {
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.def("get_probability", &Simulator::get_probability)
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.def("get_amplitude", &Simulator::get_amplitude)
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.def("set_wavefunction", &Simulator::set_wavefunction)
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.def("collapse_wavefunction", &Simulator::collapse_wavefunction)
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.def("run", &Simulator::run)
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.def("cheat", &Simulator::cheat)
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;

‎projectq/backends/_sim/_pysim.py‎

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Original file line numberDiff line numberDiff line change
@@ -406,6 +406,44 @@ def set_wavefunction(self, wavefunction, ordering):
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self._state = _np.array(wavefunction)
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self._map = {ordering[i]: i for i in range(len(ordering))}
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def collapse_wavefunction(self, ids, values):
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"""
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Collapse a quantum register onto a classical basis state.
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Args:
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ids (list[int]): Qubit IDs to collapse.
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values (list[bool]): Measurement outcome for each of the qubit IDs
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in `ids`.
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Raises:
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RuntimeError: If probability of outcome is ~0 or unknown qubits
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are provided.
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"""
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assert len(ids) == len(values)
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# all qubits must have been allocated before
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if not all([Id in self._map for Id in ids]):
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raise RuntimeError("collapse_wavefunction(): Unknown qubit id(s)"
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" provided. Try calling eng.flush() before "
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"invoking this function.")
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mask = 0
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val = 0
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for i in range(len(ids)):
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pos = self._map[ids[i]]
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mask |= (1 << pos)
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val |= (int(values[i]) << pos)
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nrm = 0.
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for i in range(len(self._state)):
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if (mask & i) == val:
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nrm += _np.abs(self._state[i]) ** 2
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if nrm < 1.e-12:
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raise RuntimeError("collapse_wavefunction(): Invalid collapse! "
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"Probability is ~0.")
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inv_nrm = 1. / _np.sqrt(nrm)
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for i in range(len(self._state)):
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if (mask & i) != val:
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self._state[i] = 0.
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else:
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self._state[i] *= inv_nrm
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def run(self):
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"""
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Dummy function to implement the same interface as the c++ simulator.

‎projectq/backends/_sim/_simulator.py‎

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Original file line numberDiff line numberDiff line change
@@ -196,6 +196,27 @@ def set_wavefunction(self, wavefunction, qureg):
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self._simulator.set_wavefunction(wavefunction,
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[qb.id for qb in qureg])
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def collapse_wavefunction(self, qureg, values):
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"""
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Collapse a quantum register onto a classical basis state.
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Args:
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qureg (Qureg|list[Qubit]): Qubits to collapse.
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values (list[bool]): Measurement outcome for each of the qubits
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in `qureg`.
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Raises:
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RuntimeError: If an outcome has probability (approximately) 0 or
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if unknown qubits are provided (see note).
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Note:
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Make sure all previous commands have passed through the
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compilation chain (call main_engine.flush() to make sure).
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"""
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return self._simulator.collapse_wavefunction([qb.id for qb in qureg],
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[bool(v) for v in
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values])
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def cheat(self):
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"""
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Access the ordering of the qubits and the state vector directly.

‎projectq/backends/_sim/_simulator_test.py‎

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -377,6 +377,33 @@ def test_simulator_set_wavefunction(sim):
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Measure | qubits
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379379

380+
def test_simulator_collapse_wavefunction(sim):
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eng = MainEngine(sim)
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qubits = eng.allocate_qureg(4)
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# unknown qubits: raises
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with pytest.raises(RuntimeError):
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eng.backend.collapse_wavefunction(qubits, [0] * 4)
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eng.flush()
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eng.backend.collapse_wavefunction(qubits, [0] * 4)
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assert pytest.approx(eng.backend.get_probability([0] * 4, qubits)) == 1.
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All(H) | qubits[1:]
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eng.flush()
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assert pytest.approx(eng.backend.get_probability([0] * 4, qubits)) == .125
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# impossible outcome: raises
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with pytest.raises(RuntimeError):
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eng.backend.collapse_wavefunction(qubits, [1] + [0] * 3)
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eng.backend.collapse_wavefunction(qubits[:-1], [0, 1, 0])
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assert (pytest.approx(eng.backend.get_probability([0, 1, 0, 1], qubits))
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== .5)
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eng.backend.set_wavefunction([1.] + [0.] * 15, qubits)
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H | qubits[0]
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CNOT | (qubits[0], qubits[1])
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eng.flush()
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eng.backend.collapse_wavefunction([qubits[0]], [1])
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assert (pytest.approx(eng.backend.get_probability([1, 1], qubits[0:2]))
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== 1.)
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406+
380407
def test_simulator_no_uncompute_exception(sim):
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eng = MainEngine(sim, [])
382409
qubit = eng.allocate_qubit()

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