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gaussian_unitary.py
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gaussian_unitary.py
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# Copyright 2019 Xanadu Quantum Technologies Inc.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Circuit specifications for the Gaussian simulator backend."""
import numpy as np
from strawberryfields.program_utils import Command
from strawberryfields import ops
from strawberryfields.parameters import par_evaluate
from thewalrus.symplectic import expand_vector, expand, rotation, squeezing, two_mode_squeezing, interferometer, beam_splitter
from .circuit_specs import CircuitSpecs
class GaussianUnitary(CircuitSpecs):
"""Compiler to arrange a Gaussian quantum circuit into the canonical Symplectic form.
This compile specification checks whether the circuit can be implemented as a sequence of
Gaussian operations. If so, it arranges them in the canonical order with displacement at the end.
After compilation, the circuit will consist of at most two operations, a :class:`~.GaussianTransform`
and a :class:`~.Dgate`.
This compiler can be accessed by calling :meth:`.Program.compile` with `'gaussian_unitary'` specified.
**Example:**
Consider the following Strawberry Fields program, compiled using the `'gaussian_unitary'` compiler:
.. code-block:: python3
from strawberryfields.ops import Xgate, Zgate, Sgate, Dgate, Rgate
import strawberryfields as sf
circuit = sf.Program(1)
with circuit.context as q:
Xgate(0.4) | q[0]
Zgate(0.5) | q[0]
Sgate(0.6) | q[0]
Dgate(1.0+2.0j) | q[0]
Rgate(0.3) | q[0]
Sgate(0.6, 1.0) | q[0]
compiled_circuit = circuit.compile("gaussian_unitary")
We can now print the compiled circuit, consisting of one
:class:`~.GaussianTransform` and one :class:`~.Dgate`:
>>> compiled_circuit.print()
GaussianTransform([[ 0.3543 -1.3857]
[-0.0328 2.9508]]) | (q[0])
Dgate(-1.151+3.91j, 0) | (q[0])
"""
short_name = "gaussian_unitary"
modes = None
local = True
remote = True
interactive = True
primitives = {
# meta operations
"All",
"_New_modes",
"_Delete",
# single mode gates
"Dgate",
"Sgate",
"Rgate",
# multi mode gates
"BSgate",
"S2gate",
"Interferometer", # Note that interferometer is accepted as a primitive
"GaussianTransform", # Note that GaussianTransform is accepted as a primitive
}
decompositions = {
"GraphEmbed": {},
"BipartiteGraphEmbed": {},
"Gaussian": {},
"Pgate": {},
"CXgate": {},
"CZgate": {},
"MZgate": {},
"Xgate": {},
"Zgate": {},
"Fouriergate": {},
}
#pylint: disable=too-many-branches
def compile(self, seq, registers):
"""Try to arrange a quantum circuit into the canonical Symplectic form.
This method checks whether the circuit can be implemented as a sequence of Gaussian operations.
If the answer is yes it arranges them in the canonical order with displacement at the end.
Args:
seq (Sequence[Command]): quantum circuit to modify
registers (Sequence[RegRefs]): quantum registers
Returns:
List[Command]: modified circuit
Raises:
CircuitError: the circuit does not correspond to a Gaussian unitary
"""
# Check which modes are actually being used
used_modes = []
for operations in seq:
modes = [modes_label.ind for modes_label in operations.reg]
used_modes.append(modes)
# pylint: disable=consider-using-set-comprehension
used_modes = list(set([item for sublist in used_modes for item in sublist]))
# dictionary mapping the used modes to consecutive non-negative integers
dict_indices = {used_modes[i]: i for i in range(len(used_modes))}
nmodes = len(used_modes)
# This is the identity transformation in phase-space, multiply by the identity and add zero
Snet = np.identity(2 * nmodes)
rnet = np.zeros(2 * nmodes)
# Now we will go through each operation in the sequence `seq` and apply it in quadrature space
# We will keep track of the net transforation in the Symplectic matrix `Snet` and the quadrature
# vector `rnet`.
for operations in seq:
name = operations.op.__class__.__name__
params = par_evaluate(operations.op.p)
modes = [modes_label.ind for modes_label in operations.reg]
if name == "Dgate":
rnet = rnet + expand_vector(
params[0] * (np.exp(1j * params[1])), dict_indices[modes[0]], nmodes
)
else:
if name == "Rgate":
S = expand(rotation(params[0]), dict_indices[modes[0]], nmodes)
elif name == "Sgate":
S = expand(squeezing(params[0], params[1]), dict_indices[modes[0]], nmodes)
elif name == "S2gate":
S = expand(
two_mode_squeezing(params[0], params[1]),
[dict_indices[modes[0]], dict_indices[modes[1]]],
nmodes,
)
elif name == "Interferometer":
S = expand(
interferometer(params[0]), [dict_indices[mode] for mode in modes], nmodes
)
elif name == "GaussianTransform":
S = expand(
params[0], [dict_indices[mode] for mode in modes], nmodes
)
elif name == "BSgate":
S = expand(beam_splitter(params[0], params[1]), [dict_indices[modes[0]], dict_indices[modes[1]]], nmodes)
Snet = S @ Snet
rnet = S @ rnet
# Having obtained the net displacement we simply convert it into complex notation
alphas = 0.5 * (rnet[0:nmodes] + 1j * rnet[nmodes : 2 * nmodes])
# And now we just pass the net transformation as a big Symplectic operation plus displacements
ord_reg = [r for r in list(registers) if r.ind in used_modes]
ord_reg = sorted(list(ord_reg), key=lambda x: x.ind)
if np.allclose(Snet, np.identity(2 * nmodes)):
A = []
else:
A = [Command(ops.GaussianTransform(Snet), ord_reg)]
B = [
Command(ops.Dgate(alphas[i]), ord_reg[i])
for i in range(len(ord_reg))
if not np.allclose(alphas[i], 0.0)
]
return A + B