Source code for strawberryfields.compilers.gbs

# 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

# 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.
"""Compiler for the general Gaussian Boson Sampling class of circuits."""

from strawberryfields.program_utils import CircuitError, Command, group_operations
import strawberryfields.ops as ops

from .gaussian import Gaussian

[docs]class GBS(Gaussian):
"""Compiler for the general GBS class of circuits."""

short_name = "gbs"
primitives = {
# meta operations
"All",
"_New_modes",
"_Delete",
# state preparations
"Vacuum",
"Coherent",
"Squeezed",
"DisplacedSqueezed",
"Thermal",
"Gaussian",
# measurements
"MeasureHomodyne",
"MeasureHeterodyne",
"MeasureFock",
"MeasureThreshold",
# channels
"LossChannel",
"ThermalLossChannel",
# single mode gates
"Dgate",
"Sgate",
"Rgate",
"Fouriergate",
"BSgate",
}
decompositions = {"Xgate": {}, "Zgate": {}, "Fouriergate": {}, "S2gate": {}}

[docs]    def compile(self, seq, registers):
"""Try to arrange a quantum circuit into a form suitable for Gaussian boson sampling.

This method checks whether the circuit can be implemented as a Gaussian boson sampling
problem, i.e., if it is equivalent to a circuit A+B, where the sequence A only contains
Gaussian operations, and B only contains Fock measurements.

If the answer is yes, the circuit is arranged into the A+B order, and all the Fock
measurements are combined into a single :class:MeasureFock operation.

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 GBS
"""
A, B, C = group_operations(seq, lambda x: isinstance(x, ops.MeasureFock))

# C should be empty
if C:
raise CircuitError("Operations following the Fock measurements.")

# A should only contain Gaussian operations
# (but this is already guaranteed by group_operations() and our primitive set)

# without Fock measurements GBS is pointless
if not B:
raise CircuitError("GBS circuits must contain Fock measurements.")

# there should be only Fock measurements in B
measured = set()
for cmd in B:
if not isinstance(cmd.op, ops.MeasureFock):
raise CircuitError("The Fock measurements are not consecutive.")

# combine the Fock measurements
temp = set(cmd.reg)
if measured & temp:
raise CircuitError("Measuring the same mode more than once.")
measured |= temp

# replace B with a single Fock measurement
B = [Command(ops.MeasureFock(), sorted(list(measured), key=lambda x: x.ind))]
return super().compile(A + B, registers)