Source code for qiskit.extensions.quantum_initializer.uc
# This code is part of Qiskit.
#
# (C) Copyright IBM 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.
# The structure of the code is based on Emanuel Malvetti's semester thesis at
# ETH in 2018, which was supervised by Raban Iten and Prof. Renato Renner.
# pylint: disable=invalid-name
# pylint: disable=missing-param-doc
# pylint: disable=missing-type-doc
"""
Uniformly controlled gates (also called multiplexed gates).
These gates can have several control qubits and a single target qubit.
If the k control qubits are in the state |i> (in the computational basis),
a single-qubit unitary U_i is applied to the target qubit.
This gate is represented by a block-diagonal matrix, where each block is a
2x2 unitary:
[[U_0, 0, ...., 0],
[0, U_1, ...., 0],
.
.
[0, 0, ...., U_(2^k-1)]]
"""
import cmath
import math
import numpy as np
from qiskit.circuit.gate import Gate
from qiskit.circuit.library.standard_gates.h import HGate
from qiskit.quantum_info.operators.predicates import is_unitary_matrix
from qiskit.circuit.quantumregister import QuantumRegister
from qiskit.circuit.quantumcircuit import QuantumCircuit
from qiskit.circuit.exceptions import CircuitError
from qiskit.exceptions import QiskitError
from qiskit.quantum_info.synthesis import OneQubitEulerDecomposer
_EPS = 1e-10 # global variable used to chop very small numbers to zero
_DECOMPOSER1Q = OneQubitEulerDecomposer("U3")
class UCGate(Gate):
"""Uniformly controlled gate (also called multiplexed gate).
The decomposition is based on: https://arxiv.org/pdf/quant-ph/0410066.pdf.
"""
def __init__(self, gate_list, up_to_diagonal=False):
"""UCGate Gate initializer.
Args:
gate_list (list[ndarray]): list of two qubit unitaries [U_0,...,U_{2^k-1}],
where each single-qubit unitary U_i is given as a 2*2 numpy array.
up_to_diagonal (bool): determines if the gate is implemented up to a diagonal.
or if it is decomposed completely (default: False).
If the UCGate u is decomposed up to a diagonal d, this means that the circuit
implements a unitary u' such that d.u'=u.
Raises:
QiskitError: in case of bad input to the constructor
"""
# check input format
if not isinstance(gate_list, list):
raise QiskitError("The single-qubit unitaries are not provided in a list.")
for gate in gate_list:
if not gate.shape == (2, 2):
raise QiskitError("The dimension of a controlled gate is not equal to (2,2).")
if not gate_list:
raise QiskitError("The gate list cannot be empty.")
# Check if number of gates in gate_list is a positive power of two
num_contr = math.log2(len(gate_list))
if num_contr < 0 or not num_contr.is_integer():
raise QiskitError(
"The number of controlled single-qubit gates is not a " "non-negative power of 2."
)
# Check if the single-qubit gates are unitaries
for gate in gate_list:
if not is_unitary_matrix(gate, _EPS):
raise QiskitError("A controlled gate is not unitary.")
# Create new gate.
super().__init__("multiplexer", int(num_contr) + 1, gate_list)
self.up_to_diagonal = up_to_diagonal
def inverse(self):
"""Return the inverse.
This does not re-compute the decomposition for the multiplexer with the inverse of the
gates but simply inverts the existing decomposition.
"""
inverse_gate = Gate(
name=self.name + "_dg", num_qubits=self.num_qubits, params=[]
) # remove parameters since array is deprecated as parameter
inverse_gate.definition = QuantumCircuit(*self.definition.qregs)
inverse_gate.definition._data = [
(inst.inverse(), qargs, []) for inst, qargs, _ in reversed(self._definition)
]
return inverse_gate
def _get_diagonal(self):
# Important: for a control list q_controls = [q[0],...,q_[k-1]] the
# diagonal gate is provided in the computational basis of the qubits
# q[k-1],...,q[0],q_target, decreasingly ordered with respect to the
# significance of the qubit in the computational basis
_, diag = self._dec_ucg()
return diag
def _define(self):
ucg_circuit, _ = self._dec_ucg()
self.definition = ucg_circuit
def _dec_ucg(self):
"""
Call to create a circuit that implements the uniformly controlled gate. If
up_to_diagonal=True, the circuit implements the gate up to a diagonal gate and
the diagonal gate is also returned.
"""
diag = np.ones(2 ** self.num_qubits).tolist()
q = QuantumRegister(self.num_qubits)
q_controls = q[1:]
q_target = q[0]
circuit = QuantumCircuit(q)
# If there is no control, we use the ZYZ decomposition
if not q_controls:
theta, phi, lamb = _DECOMPOSER1Q.angles(self.params[0])
circuit.u(theta, phi, lamb, q)
return circuit, diag
# If there is at least one control, first,
# we find the single qubit gates of the decomposition.
(single_qubit_gates, diag) = self._dec_ucg_help()
# Now, it is easy to place the C-NOT gates and some Hadamards and Rz(pi/2) gates
# (which are absorbed into the single-qubit unitaries) to get back the full decomposition.
for i, gate in enumerate(single_qubit_gates):
# Absorb Hadamards and Rz(pi/2) gates
if i == 0:
squ = HGate().to_matrix().dot(gate)
elif i == len(single_qubit_gates) - 1:
squ = gate.dot(UCGate._rz(np.pi / 2)).dot(HGate().to_matrix())
else:
squ = (
HGate()
.to_matrix()
.dot(gate.dot(UCGate._rz(np.pi / 2)))
.dot(HGate().to_matrix())
)
# Add single-qubit gate
circuit.squ(squ, q_target)
# The number of the control qubit is given by the number of zeros at the end
# of the binary representation of (i+1)
binary_rep = np.binary_repr(i + 1)
num_trailing_zeros = len(binary_rep) - len(binary_rep.rstrip("0"))
q_contr_index = num_trailing_zeros
# Add C-NOT gate
if not i == len(single_qubit_gates) - 1:
circuit.cx(q_controls[q_contr_index], q_target)
if not self.up_to_diagonal:
# Important: the diagonal gate is given in the computational basis of the qubits
# q[k-1],...,q[0],q_target (ordered with decreasing significance),
# where q[i] are the control qubits and t denotes the target qubit.
circuit.diagonal(diag.tolist(), q)
return circuit, diag
def _dec_ucg_help(self):
"""
This method finds the single qubit gate arising in the decomposition of UCGates given in
https://arxiv.org/pdf/quant-ph/0410066.pdf.
"""
single_qubit_gates = [gate.astype(complex) for gate in self.params]
diag = np.ones(2 ** self.num_qubits, dtype=complex)
num_contr = self.num_qubits - 1
for dec_step in range(num_contr):
num_ucgs = 2 ** dec_step
# The decomposition works recursively and the following loop goes over the different
# UCGates that arise in the decomposition
for ucg_index in range(num_ucgs):
len_ucg = 2 ** (num_contr - dec_step)
for i in range(int(len_ucg / 2)):
shift = ucg_index * len_ucg
a = single_qubit_gates[shift + i]
b = single_qubit_gates[shift + len_ucg // 2 + i]
# Apply the decomposition for UCGates given in equation (3) in
# https://arxiv.org/pdf/quant-ph/0410066.pdf
# to demultiplex one control of all the num_ucgs uniformly-controlled gates
# with log2(len_ucg) uniform controls
v, u, r = self._demultiplex_single_uc(a, b)
# replace the single-qubit gates with v,u (the already existing ones
# are not needed any more)
single_qubit_gates[shift + i] = v
single_qubit_gates[shift + len_ucg // 2 + i] = u
# Now we decompose the gates D as described in Figure 4 in
# https://arxiv.org/pdf/quant-ph/0410066.pdf and merge some of the gates
# into the UCGates and the diagonal at the end of the circuit
# Remark: The Rz(pi/2) rotation acting on the target qubit and the Hadamard
# gates arising in the decomposition of D are ignored for the moment (they will
# be added together with the C-NOT gates at the end of the decomposition
# (in the method dec_ucg()))
if ucg_index < num_ucgs - 1:
# Absorb the Rz(pi/2) rotation on the control into the UC-Rz gate and
# merge the UC-Rz rotation with the following UCGate,
# which hasn't been decomposed yet.
k = shift + len_ucg + i
single_qubit_gates[k] = single_qubit_gates[k].dot(
UCGate._ct(r)
) * UCGate._rz(np.pi / 2).item((0, 0))
k = k + len_ucg // 2
single_qubit_gates[k] = single_qubit_gates[k].dot(r) * UCGate._rz(
np.pi / 2
).item((1, 1))
else:
# Absorb the Rz(pi/2) rotation on the control into the UC-Rz gate and merge
# the trailing UC-Rz rotation into a diagonal gate at the end of the circuit
for ucg_index_2 in range(num_ucgs):
shift_2 = ucg_index_2 * len_ucg
k = 2 * (i + shift_2)
diag[k] = (
diag[k]
* UCGate._ct(r).item((0, 0))
* UCGate._rz(np.pi / 2).item((0, 0))
)
diag[k + 1] = (
diag[k + 1]
* UCGate._ct(r).item((1, 1))
* UCGate._rz(np.pi / 2).item((0, 0))
)
k = len_ucg + k
diag[k] *= r.item((0, 0)) * UCGate._rz(np.pi / 2).item((1, 1))
diag[k + 1] *= r.item((1, 1)) * UCGate._rz(np.pi / 2).item((1, 1))
return single_qubit_gates, diag
def _demultiplex_single_uc(self, a, b):
"""
This method implements the decomposition given in equation (3) in
https://arxiv.org/pdf/quant-ph/0410066.pdf.
The decomposition is used recursively to decompose uniformly controlled gates.
a,b = single qubit unitaries
v,u,r = outcome of the decomposition given in the reference mentioned above
(see there for the details).
"""
# The notation is chosen as in https://arxiv.org/pdf/quant-ph/0410066.pdf.
x = a.dot(UCGate._ct(b))
det_x = np.linalg.det(x)
x11 = x.item((0, 0)) / cmath.sqrt(det_x)
phi = cmath.phase(det_x)
r1 = cmath.exp(1j / 2 * (np.pi / 2 - phi / 2 - cmath.phase(x11)))
r2 = cmath.exp(1j / 2 * (np.pi / 2 - phi / 2 + cmath.phase(x11) + np.pi))
r = np.array([[r1, 0], [0, r2]], dtype=complex)
d, u = np.linalg.eig(r.dot(x).dot(r))
# If d is not equal to diag(i,-i), then we put it into this "standard" form
# (see eq. (13) in https://arxiv.org/pdf/quant-ph/0410066.pdf) by interchanging
# the eigenvalues and eigenvectors.
if abs(d[0] + 1j) < _EPS:
d = np.flip(d, 0)
u = np.flip(u, 1)
d = np.diag(np.sqrt(d))
v = d.dot(UCGate._ct(u)).dot(UCGate._ct(r)).dot(b)
return v, u, r
@staticmethod
def _ct(m):
return np.transpose(np.conjugate(m))
@staticmethod
def _rz(alpha):
return np.array([[np.exp(1j * alpha / 2), 0], [0, np.exp(-1j * alpha / 2)]])
def validate_parameter(self, parameter):
"""Uniformly controlled gate parameter has to be an ndarray."""
if isinstance(parameter, np.ndarray):
return parameter
else:
raise CircuitError(
"invalid param type {} in gate " "{}".format(type(parameter), self.name)
)
def uc(self, gate_list, q_controls, q_target, up_to_diagonal=False):
"""Attach a uniformly controlled gates (also called multiplexed gates) to a circuit.
The decomposition was introduced by Bergholm et al. in
https://arxiv.org/pdf/quant-ph/0410066.pdf.
Args:
gate_list (list[ndarray]): list of two qubit unitaries [U_0,...,U_{2^k-1}],
where each single-qubit unitary U_i is a given as a 2*2 array
q_controls (QuantumRegister|list[(QuantumRegister,int)]): list of k control qubits.
The qubits are ordered according to their significance in the computational basis.
For example if q_controls=[q[1],q[2]] (with q = QuantumRegister(2)),
the unitary U_0 is performed if q[1] and q[2] are in the state zero, U_1 is
performed if q[2] is in the state zero and q[1] is in the state one, and so on
q_target (QuantumRegister|(QuantumRegister,int)): target qubit, where we act on with
the single-qubit gates.
up_to_diagonal (bool): If set to True, the uniformly controlled gate is decomposed up
to a diagonal gate, i.e. a unitary u' is implemented such that there exists a
diagonal gate d with u = d.dot(u'), where the unitary u describes the uniformly
controlled gate
Returns:
QuantumCircuit: the uniformly controlled gate is attached to the circuit.
Raises:
QiskitError: if the list number of control qubits does not correspond to the provided
number of single-qubit unitaries; if an input is of the wrong type
"""
if isinstance(q_controls, QuantumRegister):
q_controls = q_controls[:]
if isinstance(q_target, QuantumRegister):
q_target = q_target[:]
if len(q_target) == 1:
q_target = q_target[0]
else:
raise QiskitError(
"The target qubit is a QuantumRegister containing more than" " one qubit."
)
# Check if q_controls has type "list"
if not isinstance(q_controls, list):
raise QiskitError(
"The control qubits must be provided as a list"
" (also if there is only one control qubit)."
)
# Check if gate_list has type "list"
if not isinstance(gate_list, list):
raise QiskitError("The single-qubit unitaries are not provided in a list.")
# Check if number of gates in gate_list is a positive power of two
num_contr = math.log2(len(gate_list))
if num_contr < 0 or not num_contr.is_integer():
raise QiskitError(
"The number of controlled single-qubit gates is not a non negative" " power of 2."
)
# Check if number of control qubits does correspond to the number of single-qubit rotations
if num_contr != len(q_controls):
raise QiskitError(
"Number of controlled gates does not correspond to the number of" " control qubits."
)
return self.append(UCGate(gate_list, up_to_diagonal), [q_target] + q_controls)
QuantumCircuit.uc = uc