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Move mcx synthesis methods with ancillas to the synthesis library (#1…
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…2904)

* move mcx synthesis method with dirty ancillas to the synthesis library

* minor lint updates

* move mcx synthesis method with several clean ancillas to the synthesis library

* move handling up to 3 controls to the synthesis code

* move handling up to 3 controls to the synthesis code

* handle cyclic imports

* add mcx synthesis method with one clean ancilla to the synthesis library

* update input to synth_mcx functions

* refactor test for mcx method modes

* update circuit names. add references

* reduce num_controls in tests

* revert circuit names to old ones

* refactor functions names

* add docstrings

* update year

* add synthesis functions to API docs

* add release notes

* fix docs

* update docs and release notes following review

* update imports following review
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@ShellyGarion
ShellyGarion authored Aug 11, 2024
1 parent 6aa933c commit c7e7016
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138 changes: 16 additions & 122 deletions qiskit/circuit/library/standard_gates/x.py
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,7 @@
"""X, CX, CCX and multi-controlled X gates."""
from __future__ import annotations
from typing import Optional, Union, Type
from math import ceil, pi
from math import pi
import numpy
from qiskit.circuit.controlledgate import ControlledGate
from qiskit.circuit.singleton import SingletonGate, SingletonControlledGate, stdlib_singleton_key
Expand Down Expand Up @@ -1371,47 +1371,12 @@ def inverse(self, annotated: bool = False):

def _define(self):
"""Define the MCX gate using recursion."""
# pylint: disable=cyclic-import
from qiskit.circuit.quantumcircuit import QuantumCircuit

q = QuantumRegister(self.num_qubits, name="q")
qc = QuantumCircuit(q, name=self.name)
if self.num_qubits == 4:
qc._append(C3XGate(), q[:], [])
self.definition = qc
elif self.num_qubits == 5:
qc._append(C4XGate(), q[:], [])
self.definition = qc
else:
num_ctrl_qubits = len(q) - 1
q_ancilla = q[-1]
q_target = q[-2]
middle = ceil(num_ctrl_qubits / 2)
first_half = [*q[:middle]]
second_half = [*q[middle : num_ctrl_qubits - 1], q_ancilla]

qc._append(
MCXVChain(num_ctrl_qubits=len(first_half), dirty_ancillas=True),
qargs=[*first_half, q_ancilla, *q[middle : middle + len(first_half) - 2]],
cargs=[],
)
qc._append(
MCXVChain(num_ctrl_qubits=len(second_half), dirty_ancillas=True),
qargs=[*second_half, q_target, *q[: len(second_half) - 2]],
cargs=[],
)
qc._append(
MCXVChain(num_ctrl_qubits=len(first_half), dirty_ancillas=True),
qargs=[*first_half, q_ancilla, *q[middle : middle + len(first_half) - 2]],
cargs=[],
)
qc._append(
MCXVChain(num_ctrl_qubits=len(second_half), dirty_ancillas=True),
qargs=[*second_half, q_target, *q[: len(second_half) - 2]],
cargs=[],
)
# pylint: disable=cyclic-import
from qiskit.synthesis.multi_controlled import synth_mcx_1_clean_b95

self.definition = qc
qc = synth_mcx_1_clean_b95(self.num_ctrl_qubits)
self.definition = qc


class MCXVChain(MCXGate):
Expand Down Expand Up @@ -1513,92 +1478,21 @@ def get_num_ancilla_qubits(num_ctrl_qubits: int, mode: str = "v-chain"):

def _define(self):
"""Define the MCX gate using a V-chain of CX gates."""
# pylint: disable=cyclic-import
from qiskit.circuit.quantumcircuit import QuantumCircuit

q = QuantumRegister(self.num_qubits, name="q")
qc = QuantumCircuit(q, name=self.name)
q_controls = q[: self.num_ctrl_qubits]
q_target = q[self.num_ctrl_qubits]
q_ancillas = q[self.num_ctrl_qubits + 1 :]

if self._dirty_ancillas:
if self.num_ctrl_qubits < 3:
qc.mcx(q_controls, q_target)
elif not self._relative_phase and self.num_ctrl_qubits == 3:
qc._append(C3XGate(), [*q_controls, q_target], [])
else:
num_ancillas = self.num_ctrl_qubits - 2
targets = [q_target] + q_ancillas[:num_ancillas][::-1]

for j in range(2):
for i in range(self.num_ctrl_qubits): # action part
if i < self.num_ctrl_qubits - 2:
if targets[i] != q_target or self._relative_phase:
# gate cancelling

# cancel rightmost gates of action part
# with leftmost gates of reset part
if self._relative_phase and targets[i] == q_target and j == 1:
qc.cx(q_ancillas[num_ancillas - i - 1], targets[i])
qc.t(targets[i])
qc.cx(q_controls[self.num_ctrl_qubits - i - 1], targets[i])
qc.tdg(targets[i])
qc.h(targets[i])
else:
qc.h(targets[i])
qc.t(targets[i])
qc.cx(q_controls[self.num_ctrl_qubits - i - 1], targets[i])
qc.tdg(targets[i])
qc.cx(q_ancillas[num_ancillas - i - 1], targets[i])
else:
controls = [
q_controls[self.num_ctrl_qubits - i - 1],
q_ancillas[num_ancillas - i - 1],
]

qc.ccx(controls[0], controls[1], targets[i])
else:
# implements an optimized toffoli operation
# up to a diagonal gate, akin to lemma 6 of arXiv:1501.06911
qc.h(targets[i])
qc.t(targets[i])
qc.cx(q_controls[self.num_ctrl_qubits - i - 2], targets[i])
qc.tdg(targets[i])
qc.cx(q_controls[self.num_ctrl_qubits - i - 1], targets[i])
qc.t(targets[i])
qc.cx(q_controls[self.num_ctrl_qubits - i - 2], targets[i])
qc.tdg(targets[i])
qc.h(targets[i])

break

for i in range(num_ancillas - 1): # reset part
qc.cx(q_ancillas[i], q_ancillas[i + 1])
qc.t(q_ancillas[i + 1])
qc.cx(q_controls[2 + i], q_ancillas[i + 1])
qc.tdg(q_ancillas[i + 1])
qc.h(q_ancillas[i + 1])

if self._action_only:
qc.ccx(q_controls[-1], q_ancillas[-1], q_target)

break
else:
qc.rccx(q_controls[0], q_controls[1], q_ancillas[0])
i = 0
for j in range(2, self.num_ctrl_qubits - 1):
qc.rccx(q_controls[j], q_ancillas[i], q_ancillas[i + 1])
# pylint: disable=cyclic-import
from qiskit.synthesis.multi_controlled import synth_mcx_n_dirty_i15

i += 1

qc.ccx(q_controls[-1], q_ancillas[i], q_target)

for j in reversed(range(2, self.num_ctrl_qubits - 1)):
qc.rccx(q_controls[j], q_ancillas[i - 1], q_ancillas[i])
qc = synth_mcx_n_dirty_i15(
self.num_ctrl_qubits,
self._relative_phase,
self._action_only,
)

i -= 1
else: # use clean ancillas
# pylint: disable=cyclic-import
from qiskit.synthesis.multi_controlled import synth_mcx_n_clean_m15

qc.rccx(q_controls[0], q_controls[1], q_ancillas[i])
qc = synth_mcx_n_clean_m15(self.num_ctrl_qubits)

self.definition = qc
12 changes: 12 additions & 0 deletions qiskit/synthesis/__init__.py
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Expand Up @@ -122,6 +122,13 @@
.. autofunction:: two_qubit_cnot_decompose
Multi Controlled Synthesis
==========================
.. autofunction:: synth_mcx_n_dirty_i15
.. autofunction:: synth_mcx_n_clean_m15
.. autofunction:: synth_mcx_1_clean_b95
"""

from .evolution import (
Expand Down Expand Up @@ -173,3 +180,8 @@
two_qubit_cnot_decompose,
TwoQubitWeylDecomposition,
)
from .multi_controlled.mcx_with_ancillas_synth import (
synth_mcx_n_dirty_i15,
synth_mcx_n_clean_m15,
synth_mcx_1_clean_b95,
)
19 changes: 19 additions & 0 deletions qiskit/synthesis/multi_controlled/__init__.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
# This code is part of Qiskit.
#
# (C) Copyright IBM 2024.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""Module containing multi-controlled circuits synthesis"""

from .mcx_with_ancillas_synth import (
synth_mcx_n_dirty_i15,
synth_mcx_n_clean_m15,
synth_mcx_1_clean_b95,
)
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