Source code for strawberryfields.compilers.gaussian_unitary

# 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
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"""This module contains a compiler to arrange a Gaussian quantum circuit into the canonical Symplectic form."""

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 .compiler import Compiler


[docs]class GaussianUnitary(Compiler): """Compiler to arrange a Gaussian quantum circuit into the canonical Symplectic form. This compiler 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(compiler="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" interactive = True primitives = { # meta operations "All", "_New_modes", "_Delete", # single mode gates "Dgate", "Sgate", "Rgate", # multi mode gates "MZgate", "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": {}, "Xgate": {}, "Zgate": {}, "Fouriergate": {}, } # pylint: disable=too-many-branches
[docs] 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, ) elif name == "MZgate": v = np.exp(1j * params[0]) u = np.exp(1j * params[1]) U = 0.5 * np.array([[u * (v - 1), 1j * (1 + v)], [1j * u * (1 + v), 1 - v]]) S = expand( interferometer(U), [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(np.abs(alphas[i]), np.angle(alphas[i])), ord_reg[i]) for i in range(len(ord_reg)) if not np.allclose(alphas[i], 0.0) ] return A + B