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Source code for qiskit.circuit.library.generalized_gates.mcmt

# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020.
#
# 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.

"""Multiple-Control, Multiple-Target Gate."""

import warnings
from typing import Union, Callable, List, Tuple, Optional

from qiskit.circuit import ControlledGate, Gate, Instruction, Qubit, QuantumRegister, QuantumCircuit
from qiskit.exceptions import QiskitError
from ..standard_gates import XGate, YGate, ZGate, HGate, TGate, TdgGate, SGate, SdgGate


[docs]class MCMT(QuantumCircuit): """The multi-controlled multi-target gate, for an arbitrary singly controlled target gate. For example, the H gate controlled on 3 qubits and acting on 2 target qubit is represented as: .. parsed-literal:: ───■──── ───■──── ───■──── ┌──┴───┐ ┤0 ├ │ 2-H │ ┤1 ├ └──────┘ This default implementations requires no ancilla qubits, by broadcasting the target gate to the number of target qubits and using Qiskit's generic control routine to control the broadcasted target on the control qubits. If ancilla qubits are available, a more efficient variant using the so-called V-chain decomposition can be used. This is implemented in :class:`~qiskit.circuit.library.MCMTVChain`. """ def __init__( self, gate: Union[Gate, Callable[[QuantumCircuit, Qubit, Qubit], Instruction]], num_ctrl_qubits: int, num_target_qubits: int, label: Optional[str] = None, ) -> None: """Create a new multi-control multi-target gate. Args: gate: The gate to be applied controlled on the control qubits and applied to the target qubits. Can be either a Gate or a circuit method. If it is a callable, it will be casted to a Gate. num_ctrl_qubits: The number of control qubits. num_target_qubits: The number of target qubits. label: The name for the controlled circuit block. If None, set to `C-name` where `name` is `gate.name`. Raises: AttributeError: If the gate cannot be casted to a controlled gate. AttributeError: If the number of controls or targets is 0. """ if label is not None: warnings.warn( "The MCMT 'label' kwarg is deprecated as of qiskit-terra 0.19.0 and " "will be removed no earlier than 3 months after the release date. " "For details, see https://github.com/Qiskit/qiskit-terra/issues/6934.", DeprecationWarning, 2, ) if num_ctrl_qubits == 0 or num_target_qubits == 0: raise AttributeError("Need at least one control and one target qubit.") # set the internal properties and determine the number of qubits self.gate = self._identify_gate(gate) self.num_ctrl_qubits = num_ctrl_qubits self.num_target_qubits = num_target_qubits num_qubits = num_ctrl_qubits + num_target_qubits + self.num_ancilla_qubits # initialize the circuit object super().__init__(num_qubits, name="mcmt") if label is None: self._label = f"{num_target_qubits}-{self.gate.name.capitalize()}" else: self._label = label # build the circuit self._build() def _build(self): """Define the MCMT gate without ancillas.""" if self.num_target_qubits == 1: # no broadcasting needed (makes for better circuit diagrams) broadcasted_gate = self.gate else: broadcasted = QuantumCircuit(self.num_target_qubits, name=self._label) for target in list(range(self.num_target_qubits)): broadcasted.append(self.gate, [target], []) broadcasted_gate = broadcasted.to_gate() mcmt_gate = broadcasted_gate.control(self.num_ctrl_qubits) self.append(mcmt_gate, self.qubits, []) @property def label(self): """Get label.""" warnings.warn( "The MCMT 'label' property is deprecated as of qiskit-terra 0.19.0 and " "will be removed no earlier than 3 months after the release date. " "For details, see https://github.com/Qiskit/qiskit-terra/issues/6934.", DeprecationWarning, 2, ) return self._label @label.setter def label(self, label): """Set label.""" warnings.warn( "The MCMT 'label' property is deprecated as of qiskit-terra 0.19.0 and " "will be removed no earlier than 3 months after the release date. " "For details, see https://github.com/Qiskit/qiskit-terra/issues/6934.", DeprecationWarning, 2, ) self._label = label @property def num_ancilla_qubits(self): """Return the number of ancillas.""" return 0 def _identify_gate(self, gate): """Case the gate input to a gate.""" valid_gates = { "ch": HGate(), "cx": XGate(), "cy": YGate(), "cz": ZGate(), "h": HGate(), "s": SGate(), "sdg": SdgGate(), "x": XGate(), "y": YGate(), "z": ZGate(), "t": TGate(), "tdg": TdgGate(), } if isinstance(gate, ControlledGate): base_gate = gate.base_gate elif isinstance(gate, Gate): if gate.num_qubits != 1: raise AttributeError("Base gate must act on one qubit only.") base_gate = gate elif isinstance(gate, QuantumCircuit): if gate.num_qubits != 1: raise AttributeError( "The circuit you specified as control gate can only have one qubit!" ) base_gate = gate.to_gate() # raises error if circuit contains non-unitary instructions else: if callable(gate): # identify via name of the passed function name = gate.__name__ elif isinstance(gate, str): name = gate else: raise AttributeError(f"Invalid gate specified: {gate}") base_gate = valid_gates[name] return base_gate
[docs] def control(self, num_ctrl_qubits=1, label=None, ctrl_state=None): """Return the controlled version of the MCMT circuit.""" if label is not None: warnings.warn( "The MCMT.control 'label' kwarg is deprecated as of qiskit-terra 0.19.0 and " "will be removed no earlier than 3 months after the release date. " "For details, see https://github.com/Qiskit/qiskit-terra/issues/6934.", DeprecationWarning, 2, ) if ctrl_state is None: # TODO add ctrl state implementation by adding X gates return MCMT( self.gate, self.num_ctrl_qubits + num_ctrl_qubits, self.num_target_qubits, label ) return super().control(num_ctrl_qubits, label, ctrl_state)
[docs] def inverse(self): """Return the inverse MCMT circuit, which is itself.""" return MCMT(self.gate, self.num_ctrl_qubits, self.num_target_qubits)
[docs]class MCMTVChain(MCMT): """The MCMT implementation using the CCX V-chain. This implementation requires ancillas but is decomposed into a much shallower circuit than the default implementation in :class:`~qiskit.circuit.library.MCMT`. **Expanded Circuit:** .. jupyter-execute:: :hide-code: from qiskit.circuit.library import MCMTVChain, ZGate import qiskit.tools.jupyter circuit = MCMTVChain(ZGate(), 2, 2) %circuit_library_info circuit.decompose() **Examples:** >>> from qiskit.circuit.library import HGate >>> MCMTVChain(HGate(), 3, 2).draw() q_0: ──■────────────────────────■── │ │ q_1: ──■────────────────────────■── │ │ q_2: ──┼────■──────────────■────┼── │ │ ┌───┐ │ │ q_3: ──┼────┼──┤ H ├───────┼────┼── │ │ └─┬─┘┌───┐ │ │ q_4: ──┼────┼────┼──┤ H ├──┼────┼── ┌─┴─┐ │ │ └─┬─┘ │ ┌─┴─┐ q_5: ┤ X ├──■────┼────┼────■──┤ X ├ └───┘┌─┴─┐ │ │ ┌─┴─┐└───┘ q_6: ─────┤ X ├──■────■──┤ X ├───── └───┘ └───┘ """ def _build(self): """Define the MCMT gate.""" control_qubits = self.qubits[: self.num_ctrl_qubits] target_qubits = self.qubits[ self.num_ctrl_qubits : self.num_ctrl_qubits + self.num_target_qubits ] ancilla_qubits = self.qubits[self.num_ctrl_qubits + self.num_target_qubits :] if len(ancilla_qubits) > 0: master_control = ancilla_qubits[-1] else: master_control = control_qubits[0] self._ccx_v_chain_rule(control_qubits, ancilla_qubits, reverse=False) for qubit in target_qubits: self.append(self.gate.control(), [master_control, qubit], []) self._ccx_v_chain_rule(control_qubits, ancilla_qubits, reverse=True) @property def num_ancilla_qubits(self): """Return the number of ancilla qubits required.""" return max(0, self.num_ctrl_qubits - 1) def _ccx_v_chain_rule( self, control_qubits: Union[QuantumRegister, List[Qubit]], ancilla_qubits: Union[QuantumRegister, List[Qubit]], reverse: bool = False, ) -> List[Tuple[Gate, List[Qubit], List]]: """Get the rule for the CCX V-chain. The CCX V-chain progressively computes the CCX of the control qubits and puts the final result in the last ancillary qubit. Args: control_qubits: The control qubits. ancilla_qubits: The ancilla qubits. reverse: If True, compute the chain down to the qubit. If False, compute upwards. Returns: The rule for the (reversed) CCX V-chain. Raises: QiskitError: If an insufficient number of ancilla qubits was provided. """ if len(ancilla_qubits) == 0: return if len(ancilla_qubits) < len(control_qubits) - 1: raise QiskitError("Insufficient number of ancilla qubits.") iterations = list(enumerate(range(2, len(control_qubits)))) if not reverse: self.ccx(control_qubits[0], control_qubits[1], ancilla_qubits[0]) for i, j in iterations: self.ccx(control_qubits[j], ancilla_qubits[i], ancilla_qubits[i + 1]) else: for i, j in reversed(iterations): self.ccx(control_qubits[j], ancilla_qubits[i], ancilla_qubits[i + 1]) self.ccx(control_qubits[0], control_qubits[1], ancilla_qubits[0])
[docs] def inverse(self): return MCMTVChain(self.gate, self.num_ctrl_qubits, self.num_target_qubits)