def _extract_su2su2_prefactors(U, V):
r"""This function is used for the case of 2 CNOTs and 3 CNOTs. It does something
similar as the 1-CNOT case, but there is no special form for one of the
SO(4) operations.
Suppose U, V are SU(4) matrices for which there exists A, B, C, D such that
(A \otimes B) V (C \otimes D) = U. The problem is to find A, B, C, D in SU(2)
in an analytic and fully differentiable manner.
This decomposition is possible when U and V are in the same double coset of
SU(4), meaning there exists G, H in SO(4) s.t. G (Edag V E) H = (Edag U
E). This is guaranteed here by how V was constructed in both the
_decomposition_2_cnots and _decomposition_3_cnots methods.
Then, we can use the fact that E SO(4) Edag gives us something in SU(2) x
SU(2) to give A, B, C, D.
"""
# A lot of the work here happens in the magic basis. Essentially, we
# don't look explicitly at some U = G V H, but rather at
# E^\dagger U E = G E^\dagger V E H
# so that we can recover
# U = (E G E^\dagger) V (E H E^\dagger) = (A \otimes B) V (C \otimes D).
# There is some math in the paper explaining how when we define U in this way,
# we can simultaneously diagonalize functions of U and V to ensure they are
# in the same coset and recover the decomposition.
u = math.dot(math.cast_like(Edag, V), math.dot(U, math.cast_like(E, V)))
v = math.dot(math.cast_like(Edag, V), math.dot(V, math.cast_like(E, V)))
uuT = math.dot(u, math.T(u))
vvT = math.dot(v, math.T(v))
# Get the p and q in SO(4) that diagonalize uuT and vvT respectively (and
# their eigenvalues). We are looking for a simultaneous diagonalization,
# which we know exists because of how U and V were constructed. Furthermore,
# The way we will do this is by noting that, since uuT/vvT are complex and
# symmetric, so both their real and imaginary parts share a set of
# real-valued eigenvectors, which are also eigenvectors of uuT/vvT
# themselves. So we can use eigh, which orders the eigenvectors, and so we
# are guaranteed that the p and q returned will be "in the same order".
_, p = math.linalg.eigh(math.real(uuT) + math.imag(uuT))
_, q = math.linalg.eigh(math.real(vvT) + math.imag(vvT))
# If determinant of p/q is not 1, it is in O(4) but not SO(4), and has determinant
# We can transform it to SO(4) by simply negating one of the columns.
p = math.dot(p, math.diag([1, 1, 1, math.sign(math.linalg.det(p))]))
q = math.dot(q, math.diag([1, 1, 1, math.sign(math.linalg.det(q))]))
# Now, we should have p, q in SO(4) such that p^T u u^T p = q^T v v^T q.
# Then (v^\dag q p^T u)(v^\dag q p^T u)^T = I.
# So we can set G = p q^T, H = v^\dag q p^T u to obtain G v H = u.
G = math.dot(math.cast_like(p, 1j), math.T(q))
H = math.dot(math.conj(math.T(v)), math.dot(math.T(G), u))
# These are still in SO(4) though - we want to convert things into SU(2) x SU(2)
# so use the entangler. Since u = E^\dagger U E and v = E^\dagger V E where U, V
# are the target matrices, we can reshuffle as in the docstring above,
# U = (E G E^\dagger) V (E H E^\dagger) = (A \otimes B) V (C \otimes D)
# where A, B, C, D are in SU(2) x SU(2).
AB = math.dot(math.cast_like(E, G), math.dot(G, math.cast_like(Edag, G)))
CD = math.dot(math.cast_like(E, H), math.dot(H, math.cast_like(Edag, H)))
# Now, we just need to extract the constituent tensor products.
A, B = _su2su2_to_tensor_products(AB)
C, D = _su2su2_to_tensor_products(CD)
return A, B, C, D
def _decomposition_2_cnots(U, wires):
r"""If 2 CNOTs are required, we can write the circuit as
-╭U- = -A--╭X--RZ(d)--╭X--C-
-╰U- = -B--╰C--RX(p)--╰C--D-
We need to find the angles for the Z and X rotations such that the inner
part has the same spectrum as U, and then we can recover A, B, C, D.
"""
# Compute the rotation angles
u = math.dot(Edag, math.dot(U, E))
gammaU = math.dot(u, math.T(u))
evs, _ = math.linalg.eig(gammaU)
# These choices are based on Proposition III.3 of
# https://arxiv.org/abs/quant-ph/0308045
# There is, however, a special case where the circuit has the form
# -╭U- = -A--╭C--╭X--C-
# -╰U- = -B--╰X--╰C--D-
#
# or some variant of this, where the two CNOTs are adjacent.
#
# What happens here is that the set of evs is -1, -1, 1, 1 and we can write
# -╭U- = -A--╭X--SZ--╭X--C-
# -╰U- = -B--╰C--SX--╰C--D-
# where SZ and SX are square roots of Z and X respectively. (This
# decomposition comes from using Hadamards to flip the direction of the
# first CNOT, and then decomposing them and merging single-qubit gates.) For
# some reason this case is not handled properly with the full algorithm, so
# we treat it separately.
sorted_evs = math.sort(math.real(evs))
if math.allclose(sorted_evs, [-1, -1, 1, 1]):
interior_decomp = [
qml.CNOT(wires=[wires[1], wires[0]]),
qml.S(wires=wires[0]),
qml.SX(wires=wires[1]),
qml.CNOT(wires=[wires[1], wires[0]]),
]
# S \otimes SX
inner_matrix = S_SX
else:
# For the non-special case, the eigenvalues come in conjugate pairs.
# We need to find two non-conjugate eigenvalues to extract the angles.
x = math.angle(evs[0])
y = math.angle(evs[1])
# If it was the conjugate, grab a different eigenvalue.
if math.allclose(x, -y):
y = math.angle(evs[2])
delta = (x + y) / 2
phi = (x - y) / 2
interior_decomp = [
qml.CNOT(wires=[wires[1], wires[0]]),
qml.RZ(delta, wires=wires[0]),
qml.RX(phi, wires=wires[1]),
qml.CNOT(wires=[wires[1], wires[0]]),
]
RZd = qml.RZ(math.cast_like(delta, 1j), wires=0).matrix
RXp = qml.RX(phi, wires=0).matrix
inner_matrix = math.kron(RZd, RXp)
# We need the matrix representation of this interior part, V, in order to
# decompose U = (A \otimes B) V (C \otimes D)
V = math.dot(math.cast_like(CNOT10, U),
math.dot(inner_matrix, math.cast_like(CNOT10, U)))
# Now we find the A, B, C, D in SU(2), and return the decomposition
A, B, C, D = _extract_su2su2_prefactors(U, V)
A_ops = zyz_decomposition(A, wires[0])
B_ops = zyz_decomposition(B, wires[1])
C_ops = zyz_decomposition(C, wires[0])
D_ops = zyz_decomposition(D, wires[1])
return C_ops + D_ops + interior_decomp + A_ops + B_ops
def zyz_decomposition(U, wire):
r"""Recover the decomposition of a single-qubit matrix :math:`U` in terms of
elementary operations.
Diagonal operations will be converted to a single :class:`.RZ` gate, while non-diagonal
operations will be converted to a :class:`.Rot` gate that implements the original operation
up to a global phase in the form :math:`RZ(\omega) RY(\theta) RZ(\phi)`.
Args:
U (tensor): A 2 x 2 unitary matrix.
wire (Union[Wires, Sequence[int] or int]): The wire on which to apply the operation.
Returns:
list[qml.Operation]: A ``Rot`` gate on the specified wire that implements ``U``
up to a global phase, or an equivalent ``RZ`` gate if ``U`` is diagonal.
**Example**
Suppose we would like to apply the following unitary operation:
.. code-block:: python3
U = np.array([
[-0.28829348-0.78829734j, 0.30364367+0.45085995j],
[ 0.53396245-0.10177564j, 0.76279558-0.35024096j]
])
For PennyLane devices that cannot natively implement ``QubitUnitary``, we
can instead recover a ``Rot`` gate that implements the same operation, up
to a global phase:
>>> decomp = zyz_decomposition(U, 0)
>>> decomp
[Rot(-0.24209529417800013, 1.14938178234275, 1.7330581433950871, wires=[0])]
"""
U = _convert_to_su2(U)
# Check if the matrix is diagonal; only need to check one corner.
# If it is diagonal, we don't need a full Rot, just return an RZ.
if math.allclose(U[0, 1], [0.0]):
omega = 2 * math.angle(U[1, 1])
return [qml.RZ(omega, wires=wire)]
# If the top left element is 0, can only use the off-diagonal elements. We
# have to be very careful with the math here to ensure things that get
# multiplied together are of the correct type in the different interfaces.
if math.allclose(U[0, 0], [0.0]):
phi = 0.0
theta = -np.pi
omega = 1j * math.log(U[0, 1] / U[1, 0]) - np.pi
else:
# If not diagonal, compute the angle of the RY
cos2_theta_over_2 = math.abs(U[0, 0] * U[1, 1])
theta = 2 * math.arccos(math.sqrt(cos2_theta_over_2))
el_division = U[0, 0] / U[1, 0]
tan_part = math.cast_like(math.tan(theta / 2), el_division)
omega = 1j * math.log(tan_part * el_division)
phi = -omega - math.cast_like(2 * math.angle(U[0, 0]), omega)
return [
qml.Rot(math.real(phi), math.real(theta), math.real(omega), wires=wire)
]