Source code for qiskit.circuit.library.grover_operator
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"""The Grover operator."""
from typing import List, Optional, Union
import numpy
from qiskit.circuit import QuantumCircuit, QuantumRegister, AncillaRegister
from qiskit.exceptions import QiskitError
from qiskit.quantum_info import Statevector, Operator, DensityMatrix
from .standard_gates import MCXGate
[docs]class GroverOperator(QuantumCircuit):
r"""The Grover operator.
Grover's search algorithm [1, 2] consists of repeated applications of the so-called
Grover operator used to amplify the amplitudes of the desired output states.
This operator, :math:`\mathcal{Q}`, consists of the phase oracle, :math:`\mathcal{S}_f`,
zero phase-shift or zero reflection, :math:`\mathcal{S}_0`, and an
input state preparation :math:`\mathcal{A}`:
.. math::
\mathcal{Q} = \mathcal{A} \mathcal{S}_0 \mathcal{A}^\dagger \mathcal{S}_f
In the standard Grover search we have :math:`\mathcal{A} = H^{\otimes n}`:
.. math::
\mathcal{Q} = H^{\otimes n} \mathcal{S}_0 H^{\otimes n} \mathcal{S}_f
= D \mathcal{S_f}
The operation :math:`D = H^{\otimes n} \mathcal{S}_0 H^{\otimes n}` is also referred to as
diffusion operator. In this formulation we can see that Grover's operator consists of two
steps: first, the phase oracle multiplies the good states by -1 (with :math:`\mathcal{S}_f`)
and then the whole state is reflected around the mean (with :math:`D`).
This class allows setting a different state preparation, as in quantum amplitude
amplification (a generalization of Grover's algorithm), :math:`\mathcal{A}` might not be
a layer of Hardamard gates [3].
The action of the phase oracle :math:`\mathcal{S}_f` is defined as
.. math::
\mathcal{S}_f: |x\rangle \mapsto (-1)^{f(x)}|x\rangle
where :math:`f(x) = 1` if :math:`x` is a good state and 0 otherwise. To highlight the fact
that this oracle flips the phase of the good states and does not flip the state of a result
qubit, we call :math:`\mathcal{S}_f` a phase oracle.
Note that you can easily construct a phase oracle from a bitflip oracle by sandwiching the
controlled X gate on the result qubit by a X and H gate. For instance
.. parsed-literal::
Bitflip oracle Phaseflip oracle
q_0: ──■── q_0: ────────────■────────────
┌─┴─┐ ┌───┐┌───┐┌─┴─┐┌───┐┌───┐
out: ┤ X ├ out: ┤ X ├┤ H ├┤ X ├┤ H ├┤ X ├
└───┘ └───┘└───┘└───┘└───┘└───┘
There is some flexibility in defining the oracle and :math:`\mathcal{A}` operator. Before the
Grover operator is applied in Grover's algorithm, the qubits are first prepared with one
application of the :math:`\mathcal{A}` operator (or Hadamard gates in the standard formulation).
Thus, we always have operation of the form
:math:`\mathcal{A} \mathcal{S}_f \mathcal{A}^\dagger`. Therefore it is possible to move
bitflip logic into :math:`\mathcal{A}` and leaving the oracle only to do phaseflips via Z gates
based on the bitflips. One possible use-case for this are oracles that do not uncompute the
state qubits.
The zero reflection :math:`\mathcal{S}_0` is usually defined as
.. math::
\mathcal{S}_0 = 2 |0\rangle^{\otimes n} \langle 0|^{\otimes n} - \mathbb{I}_n
where :math:`\mathbb{I}_n` is the identity on :math:`n` qubits.
By default, this class implements the negative version
:math:`2 |0\rangle^{\otimes n} \langle 0|^{\otimes n} - \mathbb{I}_n`, since this can simply
be implemented with a multi-controlled Z sandwiched by X gates on the target qubit and the
introduced global phase does not matter for Grover's algorithm.
Examples:
>>> from qiskit.circuit import QuantumCircuit
>>> from qiskit.circuit.library import GroverOperator
>>> oracle = QuantumCircuit(2)
>>> oracle.z(0) # good state = first qubit is |1>
>>> grover_op = GroverOperator(oracle, insert_barriers=True)
>>> grover_op.decompose().draw()
┌───┐ ░ ┌───┐ ░ ┌───┐ ┌───┐ ░ ┌───┐
state_0: ┤ Z ├─░─┤ H ├─░─┤ X ├───────■──┤ X ├──────░─┤ H ├
└───┘ ░ ├───┤ ░ ├───┤┌───┐┌─┴─┐├───┤┌───┐ ░ ├───┤
state_1: ──────░─┤ H ├─░─┤ X ├┤ H ├┤ X ├┤ H ├┤ X ├─░─┤ H ├
░ └───┘ ░ └───┘└───┘└───┘└───┘└───┘ ░ └───┘
>>> oracle = QuantumCircuit(1)
>>> oracle.z(0) # the qubit state |1> is the good state
>>> state_preparation = QuantumCircuit(1)
>>> state_preparation.ry(0.2, 0) # non-uniform state preparation
>>> grover_op = GroverOperator(oracle, state_preparation)
>>> grover_op.decompose().draw()
┌───┐┌──────────┐┌───┐┌───┐┌───┐┌─────────┐
state_0: ┤ Z ├┤ RY(-0.2) ├┤ X ├┤ Z ├┤ X ├┤ RY(0.2) ├
└───┘└──────────┘└───┘└───┘└───┘└─────────┘
>>> oracle = QuantumCircuit(4)
>>> oracle.z(3)
>>> reflection_qubits = [0, 3]
>>> state_preparation = QuantumCircuit(4)
>>> state_preparation.cry(0.1, 0, 3)
>>> state_preparation.ry(0.5, 3)
>>> grover_op = GroverOperator(oracle, state_preparation,
... reflection_qubits=reflection_qubits)
>>> grover_op.decompose().draw()
┌───┐ ┌───┐
state_0: ──────────────────────■──────┤ X ├───────■──┤ X ├──────────■────────────────
│ └───┘ │ └───┘ │
state_1: ──────────────────────┼──────────────────┼─────────────────┼────────────────
│ │ │
state_2: ──────────────────────┼──────────────────┼─────────────────┼────────────────
┌───┐┌──────────┐┌────┴─────┐┌───┐┌───┐┌─┴─┐┌───┐┌───┐┌────┴────┐┌─────────┐
state_3: ┤ Z ├┤ RY(-0.5) ├┤ RY(-0.1) ├┤ X ├┤ H ├┤ X ├┤ H ├┤ X ├┤ RY(0.1) ├┤ RY(0.5) ├
└───┘└──────────┘└──────────┘└───┘└───┘└───┘└───┘└───┘└─────────┘└─────────┘
>>> mark_state = Statevector.from_label('011')
>>> diffuse_operator = 2 * DensityMatrix.from_label('000') - Operator.from_label('III')
>>> grover_op = GroverOperator(oracle=mark_state, zero_reflection=diffuse_operator)
>>> grover_op.decompose().draw(fold=70)
┌─────────────────┐ ┌───┐ »
state_0: ┤0 ├──────┤ H ├──────────────────────────»
│ │┌─────┴───┴─────┐ ┌───┐ »
state_1: ┤1 UCRZ(0,pi,0,0) ├┤0 ├─────┤ H ├──────────»
│ ││ UCRZ(pi/2,0) │┌────┴───┴────┐┌───┐»
state_2: ┤2 ├┤1 ├┤ UCRZ(-pi/4) ├┤ H ├»
└─────────────────┘└───────────────┘└─────────────┘└───┘»
« ┌─────────────────┐ ┌───┐
«state_0: ┤0 ├──────┤ H ├─────────────────────────
« │ │┌─────┴───┴─────┐ ┌───┐
«state_1: ┤1 UCRZ(pi,0,0,0) ├┤0 ├────┤ H ├──────────
« │ ││ UCRZ(pi/2,0) │┌───┴───┴────┐┌───┐
«state_2: ┤2 ├┤1 ├┤ UCRZ(pi/4) ├┤ H ├
« └─────────────────┘└───────────────┘└────────────┘└───┘
References:
[1]: L. K. Grover (1996), A fast quantum mechanical algorithm for database search,
`arXiv:quant-ph/9605043 <https://arxiv.org/abs/quant-ph/9605043>`_.
[2]: I. Chuang & M. Nielsen, Quantum Computation and Quantum Information,
Cambridge: Cambridge University Press, 2000. Chapter 6.1.2.
[3]: Brassard, G., Hoyer, P., Mosca, M., & Tapp, A. (2000).
Quantum Amplitude Amplification and Estimation.
`arXiv:quant-ph/0005055 <http://arxiv.org/abs/quant-ph/0005055>`_.
"""
def __init__(
self,
oracle: Union[QuantumCircuit, Statevector],
state_preparation: Optional[QuantumCircuit] = None,
zero_reflection: Optional[Union[QuantumCircuit, DensityMatrix, Operator]] = None,
reflection_qubits: Optional[List[int]] = None,
insert_barriers: bool = False,
mcx_mode: str = "noancilla",
name: str = "Q",
) -> None:
r"""
Args:
oracle: The phase oracle implementing a reflection about the bad state. Note that this
is not a bitflip oracle, see the docstring for more information.
state_preparation: The operator preparing the good and bad state.
For Grover's algorithm, this is a n-qubit Hadamard gate and for amplitude
amplification or estimation the operator :math:`\mathcal{A}`.
zero_reflection: The reflection about the zero state, :math:`\mathcal{S}_0`.
reflection_qubits: Qubits on which the zero reflection acts on.
insert_barriers: Whether barriers should be inserted between the reflections and A.
mcx_mode: The mode to use for building the default zero reflection.
name: The name of the circuit.
"""
super().__init__(name=name)
# store inputs
if isinstance(oracle, Statevector):
from qiskit.circuit.library import Diagonal # pylint: disable=cyclic-import
oracle = Diagonal((-1) ** oracle.data)
self._oracle = oracle
if isinstance(zero_reflection, (Operator, DensityMatrix)):
from qiskit.circuit.library import Diagonal # pylint: disable=cyclic-import
zero_reflection = Diagonal(zero_reflection.data.diagonal())
self._zero_reflection = zero_reflection
self._reflection_qubits = reflection_qubits
self._state_preparation = state_preparation
self._insert_barriers = insert_barriers
self._mcx_mode = mcx_mode
# build circuit
self._build()
@property
def reflection_qubits(self):
"""Reflection qubits, on which S0 is applied (if S0 is not user-specified)."""
if self._reflection_qubits is not None:
return self._reflection_qubits
num_state_qubits = self.oracle.num_qubits - self.oracle.num_ancillas
return list(range(num_state_qubits))
@property
def zero_reflection(self) -> QuantumCircuit:
"""The subcircuit implementing the reflection about 0."""
if self._zero_reflection is not None:
return self._zero_reflection
num_state_qubits = self.oracle.num_qubits - self.oracle.num_ancillas
return _zero_reflection(num_state_qubits, self.reflection_qubits, self._mcx_mode)
@property
def state_preparation(self) -> QuantumCircuit:
"""The subcircuit implementing the A operator or Hadamards."""
if self._state_preparation is not None:
return self._state_preparation
num_state_qubits = self.oracle.num_qubits - self.oracle.num_ancillas
hadamards = QuantumCircuit(num_state_qubits, name="H")
# apply Hadamards only on reflection qubits, rest will cancel out
hadamards.h(self.reflection_qubits)
return hadamards
@property
def oracle(self):
"""The oracle implementing a reflection about the bad state."""
return self._oracle
def _build(self):
num_state_qubits = self.oracle.num_qubits - self.oracle.num_ancillas
circuit = QuantumCircuit(QuantumRegister(num_state_qubits, name="state"), name="Q")
num_ancillas = numpy.max(
[
self.oracle.num_ancillas,
self.zero_reflection.num_ancillas,
self.state_preparation.num_ancillas,
]
)
if num_ancillas > 0:
circuit.add_register(AncillaRegister(num_ancillas, name="ancilla"))
circuit.compose(self.oracle, list(range(self.oracle.num_qubits)), inplace=True)
if self._insert_barriers:
circuit.barrier()
circuit.compose(
self.state_preparation.inverse(),
list(range(self.state_preparation.num_qubits)),
inplace=True,
)
if self._insert_barriers:
circuit.barrier()
circuit.compose(
self.zero_reflection, list(range(self.zero_reflection.num_qubits)), inplace=True
)
if self._insert_barriers:
circuit.barrier()
circuit.compose(
self.state_preparation, list(range(self.state_preparation.num_qubits)), inplace=True
)
# minus sign
circuit.global_phase = numpy.pi
self.add_register(*circuit.qregs)
try:
circuit_wrapped = circuit.to_gate()
except QiskitError:
circuit_wrapped = circuit.to_instruction()
self.compose(circuit_wrapped, qubits=self.qubits, inplace=True)
# TODO use the oracle compiler or the bit string oracle
def _zero_reflection(
num_state_qubits: int, qubits: List[int], mcx_mode: Optional[str] = None
) -> QuantumCircuit:
qr_state = QuantumRegister(num_state_qubits, "state")
reflection = QuantumCircuit(qr_state, name="S_0")
num_ancillas = MCXGate.get_num_ancilla_qubits(len(qubits) - 1, mcx_mode)
if num_ancillas > 0:
qr_ancilla = AncillaRegister(num_ancillas, "ancilla")
reflection.add_register(qr_ancilla)
else:
qr_ancilla = []
reflection.x(qubits)
if len(qubits) == 1:
reflection.z(0) # MCX does not allow 0 control qubits, therefore this is separate
else:
reflection.h(qubits[-1])
reflection.mcx(qubits[:-1], qubits[-1], qr_ancilla[:], mode=mcx_mode)
reflection.h(qubits[-1])
reflection.x(qubits)
return reflection