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Fix issue when using a wire subset with BasisState #61

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merged 12 commits into from
May 27, 2021
Merged

Fix issue when using a wire subset with BasisState #61

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632c71c
add expand_state
thisac May 26, 2021
845ff57
move basis state checks
thisac May 26, 2021
e19b129
update changelog
thisac May 26, 2021
1a7b49c
Merge branch 'master' into fix-using-wire-subset
thisac May 26, 2021
17c7e75
update apply state vector
thisac May 26, 2021
53d18e8
Merge branch 'fix-using-wire-subset' of github.com:PennyLaneAI/pennyl…
thisac May 26, 2021
4fb594d
add tests
thisac May 26, 2021
ff83b4d
add expand state test
thisac May 26, 2021
d5f2826
remove try-except
thisac May 26, 2021
52269b6
fix single-qubit var tests
thisac May 26, 2021
dc88464
remove import
thisac May 26, 2021
ff90b90
apply changes from code review
thisac May 27, 2021
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5 changes: 5 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,9 @@

## Bug fixes

* Fixed issue when using a subset of wires with `BasisState`.
[(#61)](https://github.com/PennyLaneAI/pennylane-cirq/pull/61)

## Breaking changes

## Documentation
Expand All @@ -14,6 +17,8 @@

This release contains contributions from (in alphabetical order):

Theodor Isacsson

---

# Release 0.15.0
Expand Down
37 changes: 32 additions & 5 deletions pennylane_cirq/simulator_device.py
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Expand Up @@ -32,6 +32,8 @@
----
"""
import math
import itertools as it

import cirq
import numpy as np
import pennylane as qml
Expand Down Expand Up @@ -81,26 +83,47 @@ def _apply_basis_state(self, basis_state_operation):
if not self.shots is None:
raise qml.DeviceError("The operation BasisState is only supported in analytic mode.")

basis_state_array = np.array(basis_state_operation.parameters[0])
wires = basis_state_operation.wires

if len(basis_state_array) != len(self.qubits):
if len(basis_state_operation.parameters[0]) != len(wires):
raise qml.DeviceError(
"For BasisState, the state has to be specified for the correct number of qubits. Got a state for {} qubits, expected {}.".format(
len(basis_state_array), len(self.qubits)
len(basis_state_operation.parameters[0]), len(self.qubits)
)
)

if not np.all(np.isin(basis_state_array, np.array([0, 1]))):
if not np.all(np.isin(basis_state_operation.parameters[0], np.array([0, 1]))):
raise qml.DeviceError(
"Argument for BasisState can only contain 0 and 1. Got {}".format(
basis_state_operation.parameters[0]
)
)

# expand basis state to device wires
basis_state_array = np.zeros(self.num_wires, dtype=int)
basis_state_array[wires] = basis_state_operation.parameters[0]

self._initial_state = np.zeros(2 ** len(self.qubits), dtype=np.complex64)
basis_state_idx = np.sum(2 ** np.argwhere(np.flip(basis_state_array) == 1))
Comment on lines 106 to 107
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Oh, this is a permutation of converting from base2 to base10 that I hadn't seen before!

Interestingly, however, this seems to be a rare case where the native Python approach is still faster:

>>> bs = np.array([1, 1, 0, 1, 1, 0, 0, 1, 1])
>>> int("".join(str(i) for i in bs), 2)
435
>>> np.sum(2 ** np.argwhere(np.flip(bs) == 1))
435
>>> %timeit np.sum(2 ** np.argwhere(np.flip(bs) == 1))
41.4 µs ± 4.82 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)
>>> %timeit int("".join(str(i) for i in bs), 2)
14.4 µs ± 489 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

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Interesting! I feel like the Python solution is somewhat cleaner. The time spent there is almost all in the "".join while the NumPy solution spends a significant amounts of time in np.argwhere, 2 ** and np.sum. I guess there's just more that's going on there. 🤔

self._initial_state[basis_state_idx] = 1.0

def _expand_state(self, state_vector, wires):
"""Expands state vector to more wires"""
basis_states = np.array(list(it.product([0, 1], repeat=len(wires))))

# get basis states to alter on full set of qubits
unravelled_indices = np.zeros((2 ** len(wires), self.num_wires), dtype=int)
unravelled_indices[:, wires] = basis_states

# get indices for which the state is changed to input state vector elements
ravelled_indices = np.ravel_multi_index(unravelled_indices.T, [2] * self.num_wires)

state_vector = self._scatter(ravelled_indices, state_vector, [2 ** self.num_wires])
state_vector = self._reshape(state_vector, [2] * self.num_wires)
state_vector = self._asarray(state_vector, dtype=self.C_DTYPE)

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Same comment here as over on the Qulacs PR, recommend changing this to simply use native NumPy functions :)

return state_vector.flatten()

def _apply_qubit_state_vector(self, qubit_state_vector_operation):
# pylint: disable=missing-function-docstring
if not self.shots is None:
Expand All @@ -109,11 +132,15 @@ def _apply_qubit_state_vector(self, qubit_state_vector_operation):
)

state_vector = np.array(qubit_state_vector_operation.parameters[0], dtype=np.complex64)
wires = self.map_wires(qubit_state_vector_operation.wires)

if len(wires) != self.num_wires or sorted(wires) != wires.tolist():
state_vector = self._expand_state(state_vector, wires)

if len(state_vector) != 2 ** len(self.qubits):
raise qml.DeviceError(
"For QubitStateVector, the state has to be specified for the correct number of qubits. Got a state of length {}, expected {}.".format(
len(state_vector), 2 ** len(self.qubits)
len(state_vector), 2 ** len(wires)
)
)

Expand Down
45 changes: 45 additions & 0 deletions tests/test_apply.py
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Expand Up @@ -105,6 +105,33 @@ def test_basis_state(self, shots, tol):
expected[np.ravel_multi_index(state, [2] * 4)] = 1
assert np.allclose(res, expected, **tol)

@pytest.mark.parametrize(
"state",
[
np.array([0, 0]),
np.array([1, 0]),
np.array([0, 1]),
np.array([1, 1]),
],
)
@pytest.mark.parametrize("device_wires", [3, 4, 5])
@pytest.mark.parametrize("op_wires", [[0, 1], [1, 0], [2, 0]])
def test_basis_state_on_wires_subset(self, state, device_wires, op_wires, tol):
"""Test basis state initialization on a subset of device wires"""
dev = SimulatorDevice(device_wires)

with mimic_execution_for_apply(dev):
dev.apply([qml.BasisState(state, wires=op_wires)])

res = np.abs(dev.state) ** 2
# compute expected probabilities
expected = np.zeros([2 ** len(op_wires)])
expected[np.ravel_multi_index(state, [2] * len(op_wires))] = 1

expected = dev._expand_state(expected, op_wires)

assert np.allclose(res, expected, **tol)

def test_identity_basis_state(self, shots, tol):
"""Test basis state initialization if identity"""
dev = SimulatorDevice(4, shots=shots)
Expand All @@ -119,6 +146,7 @@ def test_identity_basis_state(self, shots, tol):
expected[np.ravel_multi_index(state, [2] * 4)] = 1
assert np.allclose(res, expected, **tol)


def test_qubit_state_vector(self, init_state, shots, tol):
"""Test PauliX application"""
dev = SimulatorDevice(1, shots=shots)
Expand All @@ -131,6 +159,23 @@ def test_qubit_state_vector(self, init_state, shots, tol):
expected = state
assert np.allclose(res, expected, **tol)

@pytest.mark.parametrize("device_wires", [3, 4, 5])
@pytest.mark.parametrize("op_wires", [[0], [2], [0, 1], [1, 0], [2, 0]])
def test_qubit_state_vector_on_wires_subset(
self, init_state, device_wires, op_wires, shots, tol
):
"""Test QubitStateVector application on a subset of device wires"""
dev = SimulatorDevice(device_wires, shots=shots)
state = init_state(len(op_wires))

with mimic_execution_for_apply(dev):
dev.apply([qml.QubitStateVector(state, wires=op_wires)])

res = dev.state
expected = dev._expand_state(state, op_wires)

assert np.allclose(res, expected, **tol)

def test_invalid_qubit_state_vector(self, shots):
"""Test that an exception is raised if the state
vector is the wrong size"""
Expand Down
4 changes: 2 additions & 2 deletions tests/test_mixed_simulator_device.py
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Expand Up @@ -693,7 +693,7 @@ def test_var_single_wire_no_parameters(

simulator_device_1_wire.reset()
simulator_device_1_wire.apply(
[qml.QubitStateVector(np.array(input), wires=[0, 1])],
[qml.QubitStateVector(np.array(input), wires=[0])],
rotations=op.diagonalizing_gates(),
)

Expand Down Expand Up @@ -724,7 +724,7 @@ def test_var_single_wire_with_parameters(

simulator_device_1_wire.reset()
simulator_device_1_wire.apply(
[qml.QubitStateVector(np.array(input), wires=[0, 1])],
[qml.QubitStateVector(np.array(input), wires=[0])],
rotations=op.diagonalizing_gates(),
)

Expand Down
27 changes: 24 additions & 3 deletions tests/test_simulator_device.py
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Expand Up @@ -46,7 +46,7 @@ def test_custom_simulator(self):
def circuit():
qml.PauliX(0)
return qml.expval(qml.PauliX(0))

assert circuit() == 0.0


Expand Down Expand Up @@ -485,6 +485,27 @@ def test_qubit_state_vector_not_at_beginning_error(self, simulator_device_1_wire
)


@pytest.mark.parametrize(
"state, device_wires, op_wires, expected",
[
(np.array([1, 0]), 2, [0], [1, 0, 0, 0]),
(np.array([0, 1]), 2, [0], [0, 0, 1, 0]),
(np.array([1, 1]) / np.sqrt(2), 2, [1], np.array([1, 1, 0, 0]) / np.sqrt(2)),
(np.array([1, 1]) / np.sqrt(2), 3, [0], np.array([1, 0, 0, 0, 1, 0, 0, 0]) / np.sqrt(2)),
(np.array([1, 2, 3, 4]) / np.sqrt(48), 3, [0, 1], np.array([1, 0, 2, 0, 3, 0, 4, 0]) / np.sqrt(48)),
(np.array([1, 2, 3, 4]) / np.sqrt(48), 3, [1, 0], np.array([1, 0, 3, 0, 2, 0, 4, 0]) / np.sqrt(48)),
(np.array([1, 2, 3, 4]) / np.sqrt(48), 3, [0, 2], np.array([1, 2, 0, 0, 3, 4, 0, 0]) / np.sqrt(48)),
(np.array([1, 2, 3, 4]) / np.sqrt(48), 3, [1, 2], np.array([1, 2, 3, 4, 0, 0, 0, 0]) / np.sqrt(48)),
],
)
@pytest.mark.parametrize("shots", [None])
def test_expand_state(state, op_wires, device_wires, expected, tol):
"""Test that the expand_state method works as expected."""
dev = SimulatorDevice(device_wires)
res = dev._expand_state(state, op_wires)

assert np.allclose(res, expected, **tol)
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great tests!


@pytest.mark.parametrize("shots", [1000])
class TestStatePreparationErrorsNonAnalytic:
"""Tests state preparation errors that occur for non-analytic devices."""
Expand Down Expand Up @@ -690,7 +711,7 @@ def test_var_single_wire_no_parameters(

simulator_device_1_wire.reset()
simulator_device_1_wire.apply(
[qml.QubitStateVector(np.array(input), wires=[0, 1])],
[qml.QubitStateVector(np.array(input), wires=[0])],
rotations=op.diagonalizing_gates(),
)

Expand Down Expand Up @@ -721,7 +742,7 @@ def test_var_single_wire_with_parameters(

simulator_device_1_wire.reset()
simulator_device_1_wire.apply(
[qml.QubitStateVector(np.array(input), wires=[0, 1])],
[qml.QubitStateVector(np.array(input), wires=[0])],
rotations=op.diagonalizing_gates(),
)

Expand Down